title: Color of turbulence

abstract:
  In this paper, we address the problem of how to account for second-order
statistics of turbulent flows using low-complexity stochastic dynamical models
based on the linearized Navier-Stokes equations. The complexity is quantified
by the number of degrees of freedom in the linearized evolution model that are
directly influenced by stochastic excitation sources. For the case where only a
subset of velocity correlations are known, we develop a framework to complete
unavailable second-order statistics in a way that is consistent with
linearization around turbulent mean velocity. In general, white-in-time
stochastic forcing is not sufficient to explain turbulent flow statistics. We
develop models for colored-in-time forcing using a maximum entropy formulation
together with a regularization that serves as a proxy for rank minimization. We
show that colored-in-time excitation of the Navier-Stokes equations can also be
interpreted as a low-rank modification to the generator of the linearized
dynamics. Our method provides a data-driven refinement of models that originate
from first principles and captures complex dynamics of turbulent flows in a way
that is tractable for analysis, optimization, and control design.

categories: physics.flu-dyn math.DS math.OC physics.data-an


title:
Protection of heterogeneous architectures on FPGAs: An approach based on
  hardware firewalls

abstract:
  Embedded systems are parts of our daily life and used in many fields. They
can be found in smartphones or in modern cars including GPS, light/rain sensors
and other electronic assistance mechanisms. These systems may handle sensitive
data (such as credit card numbers, critical information about the host system
and so on) which must be protected against external attacks as these data may
be transmitted through a communication link where attackers can connect to
extract sensitive information or inject malicious code within the system. This
work presents an approach to protect communications in multiprocessor
architectures. This approach is based on hardware security enhancements acting
as firewalls. These firewalls filter all data going through the system
communication bus and an additional flexible cryptographic block aims to
protect external memory from attacks. Benefits of our approach are demonstrated
using a case study and some custom software applications implemented in a
Field-Programmable Gate Array (FPGA). Firewalls implemented in the target
architecture allow getting a low-latency security layer with flexible
cryptographic features. To illustrate the benefit of such a solution,
implementations are discussed for different MPSoCs implemented on Xilinx
Virtex-6 FPGAs. Results demonstrate a reduction up to 33% in terms of latency
overhead compared to existing efforts.


title:
Suppression of Space Charge Induced Beam Halo in Nonlinear Focusing
  Channel

abstract:
  An intense non-uniform particle beam exhibits strong emittance growth and
halo formation in focusing channels due to nonlinear space charge forces of the
beam. This phenomenon limits beam brightness and results in particle losses.
The problem is connected with irreversible distortion of phase space volume of
the beam in conventional focusing structures due to filamentation in phase
space. Emittance growth is accompanied by halo formation in real space, which
results in inevitable particle losses. A new approach for solving a
self-consistent problem for a matched non-uniform beam in two-dimensional
geometry is discussed. The resulting solution is applied to the problem of beam
transport, while avoiding emittance growth and halo formation by the use of
nonlinear focusing field. Conservation of a beam distribution function is
demonstrated analytically and by particle-in-cell simulation for a beam with a
realistic beam distribution.


title:
The close circumstellar environment of Betelgeuse. IV. VLTI/PIONIER
  interferometric monitoring of the photosphere

abstract:
  Context. The mass-loss mechanism of cool massive evolved stars is poorly
understood. The proximity of Betelgeuse makes it an appealing target to study
its atmosphere, map the shape of its envelope, and follow the structure of its
wind from the photosphere out to the interstellar medium. Aims. A link is
suspected between the powerful convective motions in Betelgeuse and its mass
loss. We aim to constrain the spatial structure and temporal evolution of the
convective pattern on the photosphere and to search for evidence of this link.
Methods. We report new interferometric observations in the infrared H band
using the VLTI/PIONIER instrument. We monitored the photosphere of Betelgeuse
between 2012 January and 2014 November to look for evolutions that may trigger
the outflow. Results. Our interferometric observations at low spatial
frequencies are compatible with the presence of a hot spot on the photosphere
that has a characteristic width of one stellar radius. It appears to be
superposed on the smaller scale convective pattern. In the higher spatial
frequency domain, we observe a significant difference between the observations
and the predictions of 3D hydrodynamical simulations. Conclusions. We bring new
evidence for the presence of a convective pattern in the photosphere of red
supergiants. The inferred hot spot is probably the top of a giant convection
cell although an asymmetric extension of the star cannot be excluded by these
interferometric observations alone. The properties of the observed surface
features show a stronger contrast and inhomogeneity as predicted by 3D
radiative hydrodynamical simulations. We propose that the large observed
feature is modifying the signature of the convective pattern at the surface of
the star in a way that simulations cannot reproduce.


title: Towards an Efficient Smart Space Architecture

abstract:
  A smart space offers entirely new opportunities for end users by adapting
services accordingly to make life easy. A number of architectural designs have
been proposed to design context awareness systems and adaptation behavior.
However, the quality of the system depends on the degree of satisfaction of the
initials needs. In this paper, we discuss three main indicators of quality
design for smart spaces that are strongly related to the context modules and
reasoning process: functionality, reusability and changeability. A general
layered architecture system is presented to define the principal components
that should constitute any context aware adaptive system.


title: Generating images with recurrent adversarial networks

abstract:
  Gatys et al. (2015) showed that optimizing pixels to match features in a
convolutional network with respect reference image features is a way to render
images of high visual quality. We show that unrolling this gradient-based
optimization yields a recurrent computation that creates images by
incrementally adding onto a visual "canvas". We propose a recurrent generative
model inspired by this view, and show that it can be trained using adversarial
training to generate very good image samples. We also propose a way to
quantitatively compare adversarial networks by having the generators and
discriminators of these networks compete against each other.


title:
Spectral theory of extended Harper's model and a question by Erd\H{o}s
  and Szekeres

abstract:
  The extended Harper's model, proposed by D.J. Thouless in 1983, generalizes
the famous almost Mathieu operator, allowing for a wider range of lattice
geometries (parametrized by three coupling parameters) by permitting 2D
electrons to hop to both nearest and next nearest neighboring (NNN) lattice
sites, while still exhibiting its characteristic symmetry (Aubry duality).
Previous understanding of the spectral theory of this model was restricted to
two dual regions of the parameter space, one of which is characterized by the
positivity of the Lyapunov exponent. In this paper, we complete the picture
with a description of the spectral measures over the entire remaining
(self-dual) region, for all irrational values of the frequency parameter (the
magnetic flux in the model). Most notably, we prove that in the entire interior
of this regime, the model exhibits a collapse from purely ac spectrum to purely
sc spectrum when the NNN interaction becomes symmetric. In physics literature,
extensive numerical analysis had indicated such "spectral collapse," however so
far not even a heuristic argument for this phenomenon could be provided. On the
other hand, in the remaining part of the self-dual region, the spectral
measures are singular continuous irrespective of such symmetry. The analysis
requires some rather delicate number theoretic estimates, which ultimately
depend on the solution of a problem posed by Erd\H{o}s and Szekeres.


title:
Patient Flow Prediction via Discriminative Learning of
  Mutually-Correcting Processes

abstract:
  Over the past decade the rate of care unit (CU) use in the United States has
been increasing. With an aging population and ever-growing demand for medical
care, effective management of patients' transitions among different care
facilities will prove indispensible for shortening the length of hospital
stays, improving patient outcomes, allocating critical care resources, and
reducing preventable re-admissions. In this paper, we focus on an important
problem of predicting the so-called "patient flow" from longitudinal electronic
health records (EHRs), which has not been explored via existing machine
learning techniques. By treating a sequence of transition events as a point
process, we develop a novel framework for modeling patient flow through various
CUs and jointly predicting patients' destination CUs and duration days. Instead
of learning a generative point process model via maximum likelihood estimation,
we propose a novel discriminative learning algorithm aiming at improving the
prediction of transition events in the case of sparse data. By parameterizing
the proposed model as a mutually-correcting process, we formulate the
estimation problem via generalized linear models, which lends itself to
efficient learning based on alternating direction method of multipliers (ADMM).
Furthermore, we achieve simultaneous feature selection and learning by adding a
group-lasso regularizer to the ADMM algorithm. Additionally, for suppressing
the negative influence of data imbalance on the learning of model, we
synthesize auxiliary training data for the classes with extremely few samples,
and improve the robustness of our learning method accordingly. Testing on
real-world data, we show that our method obtains superior performance in terms
of accuracy of predicting the destination CU transition and duration of each CU
occupancy.


title: Parameter Synthesis for Markov Models: Faster Than Ever

abstract:
  We propose a simple technique for verifying probabilistic models whose
transition probabilities are parametric. The key is to replace parametric
transitions by nondeterministic choices of extremal values. Analysing the
resulting parameter-free model using off-the-shelf means yields (refinable)
lower and upper bounds on probabilities of regions in the parameter space. The
technique outperforms the existing analysis of parametric Markov chains by
several orders of magnitude regarding both run-time and scalability. Its beauty
is its applicability to various probabilistic models. It in particular provides
the first sound and feasible method for performing parameter synthesis of
Markov decision processes.


title:
Evolution of The Proton Velocity Distribution due to Stochastic Heating
  in the Near-Sun Solar Wind

abstract:
  We investigate how the proton distribution function evolves when the protons
undergo stochastic heating by strong, low-frequency, Alfv\'en-wave turbulence
under the assumption that $\beta$ is small. We apply our analysis to protons
undergoing stochastic heating in the supersonic fast solar wind and obtain
proton distributions at heliocentric distances ranging from 4 to 30 solar
radii. We find that the proton distribution develops non-Gaussian structure
with a flat core and steep tail. For $r >5 \ R_{\rm S}$, the proton
distribution is well approximated by a modified Moyal distribution. Comparisons
with future measurements from \emph{Solar Probe Plus} could be used to test
whether stochastic heating is occurring in the solar-wind acceleration region.


title:
Exact solution of gradient echo memory and analytical treatment of
  gradient frequency comb

abstract:
  Gradient echo memory (GEM) stores and retrieves photon wave packet in forward
direction with high efficiency and fidelity using photon-echo mechanism. It is
an important technique for quantum memory applications. By breaking the
continuity of the gradient absorption structure, the scheme becomes gradient
frequency comb (GFC), which is a hybrid of GEM and atomic frequency comb (AFC).
To elucidate the non-trivial field-atom evolution of gradient echo, we derive
its exact analytical solution in a medium of arbitrary optical thickness
subjecting to any linear gradient of transition frequency, and discuss its
physical processes. Based on this solution, we further suggest and analysis two
types of GFC, one with stepwise gradient and the other with discontinuous
gradient.


title: Adjoint master equation for Quantum Brownian Motion

abstract:
  Quantum brownian motion is a fundamental model for a proper understanding of
open quantum systems in different contexts such as chemistry, condensed matter
physics, bio-physics and opto- mechamics. In this paper we propose a novel
approach to describe this model. We provide an exact and analytic equation for
the time evolution of the operators, and we show that the corresponding
equation for the states is equivalent to well-known results in literature. The
dynamics is expressed in terms of the spectral density, regardless the strength
of the coupling between the system and the bath. Our allows to compute the time
evolution of physically relevant quantities in a much easier way than previous
formulations allow to. An example is explicitly studied.


title: Inequalities for the Psi and $k$-Gamma Functions

abstract:
  In this paper, the authors establish some inequalities involving the Psi and
$k$-Gamma functions. The procedure utilizes some monotonicity properties of
some functions associated with the Psi and $k$-Gamma functions.


title:
Suppression of collisionless magnetic reconnection in asymmetric current
  sheets

abstract:
  Using fully kinetic simulations, we study the suppression of asymmetric
reconnection in the limit where the diamagnetic drift speed >> Alfven speed and
the magnetic shear angle is moderate. We demonstrate that the slippage between
electrons and the magnetic flux facilitates reconnection, and can even result
in fast reconnection that lacks one of the outflow jets. Through comparing a
case where the diamagnetic drift is supported by the temperature gradient with
a companion case that has a density gradient instead, we identify a robust
suppression mechanism. The drift of the x-line is slowed down locally by the
asymmetric nature of the current sheet and the resulting tearing modes, then
the x-line is run over and swallowed by the faster-moving following flux.

categories: physics.plasm-ph astro-ph.EP astro-ph.SR physics.space-ph


title: Parameter Switching Synchronization

abstract:
  In this paper we show how the Parameter Switching algorithm, utilized
initially to approximate attractors of a general class of nonlinear dynamical
systems, can be utilized also as a synchronization-induced method. Two
illustrative examples are considered: the Lorenz system and the
Rabinovich-Fabrikant system.


title:
Transversal symmetry breaking and axial spreading modification for
  Gaussian optical beams

abstract:
  For a long time it was believed there was no reason to include the
geometrical phase in studying the propagation of gaussian optical beams through
dielectric blocks. This can be justified by the fact that the first order term
in the Taylor expansion of this phase is responsible for the lateral shift of
the optical beam which is also predicted by ray optics. From this point of
view, the geometrical phase can be seen as a purely auxiliary concept. In this
paper, we show how the second order term in the Taylor expansion accounts for
the symmetry breaking of the transversal spatial distribution and acts as an
axial spreading modifier. These new effects clearly shows the importance of the
geometrical phase in describing the correct behavior of light. To test our
theoretical predictions, we briefly discuss a possible experimental
implementation.


title:
Fragile singlet ground state magnetism in pyrochlore osmates R2Os2O7
  (R=Y and Ho)

abstract:
  The singlet ground state magnetism in pyrochlore osmates Y$_2$Os$_2$O$_7$ and
Ho$_2$Os$_2$O$_7$ is studied by DC and AC susceptibility, specific heat, and
neutron powder diffraction measurements. Despite the expected non-magnetic
singlet in the strong spin-orbit coupling (SOC) limit for Os$^{4+}$ ($5d^4$),
Y$_2$Os$_2$O$_7$ exhibits a spin-glass (SG) ground state below 4 K with weak
magnetism, suggesting possible proximity to a quantum phase transition between
the non-magnetic state in the strong SOC limit and the magnetic state in the
strong superexchange limit. Ho$_2$Os$_2$O$_7$ has the same structural
distortion as occurs in Y$_2$Os$_2$O$_7$. However, the Os sublattice in
Ho$_2$Os$_2$O$_7$ shows long-range magnetic ordering below 36\,K. The sharp
difference of the magnetic ground state between Y$_2$Os$_2$O$_7$ and
Ho$_2$Os$_2$O$_7$ signals the singlet ground state magnetism in
$R_2$Os$_2$O$_7$ is fragile and can be disturbed by the weak $4f-5d$
interactions.


title:
An $a_0$ resonance in strongly coupled $\pi \eta$, $K\overline{K}$
  scattering from lattice QCD

abstract:
  We present the first calculation of coupled-channel meson-meson scattering in
the isospin $=1$, $G$-parity negative sector, with channels $\pi \eta$,
$K\overline{K}$ and $\pi \eta'$, in a first-principles approach to QCD. From
the discrete spectrum of eigenstates in three volumes extracted from lattice
QCD correlation functions we determine the energy dependence of the $S$-matrix,
and find that the $S$-wave features a prominent cusp-like structure in $\pi
\eta \to \pi \eta$ close to $K\overline{K}$ threshold coupled with a rapid turn
on of amplitudes leading to the $K\overline{K}$ final-state. This behavior is
traced to an $a_0(980)$-like resonance, strongly coupled to both $\pi \eta$ and
$K\overline{K}$, which is identified with a pole in the complex energy plane,
appearing on only a single unphysical Riemann sheet. Consideration of $D$-wave
scattering suggests a narrow tensor resonance at higher energy.


title: Surface Lifshits tails for random quantum Hamiltonians

abstract:
  We consider Schr\"{o}dinger operators on $L^{2}({\mathbb R}^{d})\otimes
L^{2}({\mathbb R}^{\ell})$ of the form $ H_{\omega}~=~H_{\perp}\otimes
I_{\parallel} + I_{\perp} \otimes {H_\parallel} + V_{\omega}$, where
$H_{\perp}$ and $H_{\parallel}$ are Schr\"{o}dinger operators on
$L^{2}({\mathbb R}^{d})$ and $L^{2}({\mathbb R}^{\ell})$ respectively, and $
V_\omega(x,y)$ : = $\sum_{\xi \in {\mathbb Z}^{d}} \lambda_\xi(\omega) v(x -
\xi, y)$, $x \in {\mathbb R}^d$, $y \in {\mathbb R}^\ell$, is a random 'surface
potential'. We investigate the behavior of the integrated density of surface
states of $H_{\omega}$ near the bottom of the spectrum and near internal band
edges. The main result of the current paper is that, under suitable
assumptions, the behavior of the integrated density of surface states of
$H_{\omega}$ can be read off from the integrated density of states of a reduced
Hamiltonian $H_{\perp}+W_{\omega}$ where $W_{\omega}$ is a quantum mechanical
average of $V_{\omega}$ with respect to $y \in {\mathbb R}^\ell$. We are
particularly interested in cases when $H_{\perp}$ is a magnetic Schr\"{o}dinger
operator, but we also recover some of the results from [24] for non-magnetic
$H_{\perp}$.


title: Practical Introduction to Clustering Data

abstract:
  Data clustering is an approach to seek for structure in sets of complex data,
i.e., sets of "objects". The main objective is to identify groups of objects
which are similar to each other, e.g., for classification. Here, an
introduction to clustering is given and three basic approaches are introduced:
the k-means algorithm, neighbour-based clustering, and an agglomerative
clustering method. For all cases, C source code examples are given, allowing
for an easy implementation.

categories: physics.data-an astro-ph.IM cond-mat.stat-mech cs.LG


title: Locally Stationary Functional Time Series

abstract:
  The literature on time series of functional data has focused on processes of
which the probabilistic law is either constant over time or constant up to its
second-order structure. Especially for long stretches of data it is desirable
to be able to weaken this assumption. This paper introduces a framework that
will enable meaningful statistical inference of functional data of which the
dynamics change over time. We put forward the concept of local stationarity in
the functional setting and establish a class of processes that have a
functional time-varying spectral representation. Subsequently, we derive
conditions that allow for fundamental results from nonstationary multivariate
time series to carry over to the function space. In particular, time-varying
functional ARMA processes are investigated and shown to be functional locally
stationary according to the proposed definition. As a side-result, we establish
a Cram\'er representation for an important class of weakly stationary
functional processes. Important in our context is the notion of a time-varying
spectral density operator of which the properties are studied and uniqueness is
derived. Finally, we provide a consistent nonparametric estimator of this
operator and show it is asymptotically Gaussian using a weaker tightness
criterion than what is usually deemed necessary.


title: Poisson-Lie T-duality as a boundary phenomenon of Chern-Simons theory

abstract:
  We give a "holographic" explanation of Poisson-Lie T-duality in terms of
Chern-Simons theory (or, more generally, in terms of Courant sigma-models) with
appropriate boundary conditions.


title: A Harmonic Extension Approach for Collaborative Ranking

abstract:
  We present a new perspective on graph-based methods for collaborative ranking
for recommender systems. Unlike user-based or item-based methods that compute a
weighted average of ratings given by the nearest neighbors, or low-rank
approximation methods using convex optimization and the nuclear norm, we
formulate matrix completion as a series of semi-supervised learning problems,
and propagate the known ratings to the missing ones on the user-user or
item-item graph globally. The semi-supervised learning problems are expressed
as Laplace-Beltrami equations on a manifold, or namely, harmonic extension, and
can be discretized by a point integral method. We show that our approach does
not impose a low-rank Euclidean subspace on the data points, but instead
minimizes the dimension of the underlying manifold. Our method, named LDM (low
dimensional manifold), turns out to be particularly effective in generating
rankings of items, showing decent computational efficiency and robust ranking
quality compared to state-of-the-art methods.


title: Interacting Particle Markov Chain Monte Carlo

abstract:
  We introduce interacting particle Markov chain Monte Carlo (iPMCMC), a PMCMC
method based on an interacting pool of standard and conditional sequential
Monte Carlo samplers. Like related methods, iPMCMC is a Markov chain Monte
Carlo sampler on an extended space. We present empirical results that show
significant improvements in mixing rates relative to both non-interacting PMCMC
samplers, and a single PMCMC sampler with an equivalent memory and
computational budget. An additional advantage of the iPMCMC method is that it
is suitable for distributed and multi-core architectures.


title: What is the Observer generated information process?

abstract:
  Up to now both information and its connection to reality have not
scientifically conclusive definitions neither implicit origin. They emerge in
observing multiple impulses interactive yes-no actions modeling information
Bits. The observed information process connects reality, information, and
creates Observer. Interaction is fundamental reality building structure of
Universe from quantum physics up to brain neurons interactive actions. Merging
action and reaction joins probabilistic prior and posterior actions on edge of
the observed predictability and begins microprocess. Its time of entanglement
starts space interval composing two qubits or Bit of reversible logic in the
emerging information process. The impulse interacting action curves impulse
geometry creating asymmetrical logic Bit as logical Maxwell demon. With
approaching probability one, the attracting interaction captures energy
memorizing asymmetrical logic in information certain Bit. Such Bit is naturally
extracted at minimal quality energy equivalent ln2 working as Maxwell Demon.
The memorized impulse Bit and its free information self-organizes multiple Bits
in triplets sructuring macroprocess. Memorized information binds reversible
microprocess with irreversible information macroprocess along multidimensional
observing process. The macroprocess self-forming triplet units UP attract new
UP through free Information. Multiple UP adjoin hierarchical network whose free
information produces new UP at higher level node and encodes triplets in
multi-levels hierarchical organization. The interactive information dynamics
assemble geometrical and information structures of cognition and intelligence
in double spiral rotating code. Information Path Functional integrates multiple
interactive dynamics in finite bits which observe and measure reality. The time
and space of reality exists only as units of information as Wheeler predicted.


title:
Risk Aversion in Finite Markov Decision Processes Using Total Cost
  Criteria and Average Value at Risk

abstract:
  In this paper we present an algorithm to compute risk averse policies in
Markov Decision Processes (MDP) when the total cost criterion is used together
with the average value at risk (AVaR) metric. Risk averse policies are needed
when large deviations from the expected behavior may have detrimental effects,
and conventional MDP algorithms usually ignore this aspect. We provide
conditions for the structure of the underlying MDP ensuring that approximations
for the exact problem can be derived and solved efficiently. Our findings are
novel inasmuch as average value at risk has not previously been considered in
association with the total cost criterion. Our method is demonstrated in a
rapid deployment scenario, whereby a robot is tasked with the objective of
reaching a target location within a temporal deadline where increased speed is
associated with increased probability of failure. We demonstrate that the
proposed algorithm not only produces a risk averse policy reducing the
probability of exceeding the expected temporal deadline, but also provides the
statistical distribution of costs, thus offering a valuable analysis tool.


title:
Occupation times of alternating renewal processes with L\'evy
  applications

abstract:
  This paper presents a set of results relating to the occupation time
$\alpha(t)$ of a process $X(\cdot)$. The first set of results concerns exact
characterizations of $\alpha(t)$ for $t\geq0$, e.g., in terms of its transform
up to an exponentially distributed epoch. In addition we establish a central
limit theorem (entailing that a centered and normalized version of
$\alpha(t)/t$ converges to a zero-mean Normal random variable as
$t\rightarrow\infty$) and the tail asymptotics of $P(\alpha(t)/t\geq q)$. We
apply our findings to spectrally positive L\'evy processes reflected at the
infimum and establish various new occupation time results for the corresponding
model.


title: The structure of matroids with a spanning clique or projective geometry

abstract:
  Let $s,n \ge 2$ be integers. We give a qualitative structural description of
every matroid $M$ that is spanned by a frame matroid of a complete graph and
has no $U_{s,2s}$-minor and no rank-$n$ projective geometry minor, showing that
every such matroid is `close' to a frame matroid. We also give a similar
description of every matroid $M$ with a spanning projective geometry over a
field GF$(q)$ as a restriction and with no $U_{s,2s}$-minor and no
PG$(n,q')$-minor for any $q' > q$, showing that such an $M$ is `close' to a
GF$(q)$-representable matroid.


title: Thermodynamics of Inozemtsev's Elliptic Spin Chain

abstract:
  We study the thermodynamic behaviour of Inozemtsev's long-range elliptic spin
chain using the Bethe ansatz equations describing the spectrum of the model in
the infinite-length limit. We classify all solutions of these equations in that
limit and argue which of these solutions determine the spectrum in the
thermodynamic limit. Interestingly, some of the solutions are not
selfconjugate, which puts the model in sharp contrast to one of the model's
limiting cases, the Heisenberg xxx spin chain. Invoking the string hypothesis
we derive the thermodynamic Bethe ansatz equations (TBA-equations) from which
we determine the Helmholtz free energy in thermodynamic equilibrium and derive
the associated Y-system. We corroborate our results by comparing numerical
solutions of the TBA-equations to a direct computation of the free energy for
the finite-length hamiltonian. In addition we confirm numerically the
interesting conjecture put forward by Finkel and Gonz\'alez-L\'opez that the
original and supersymmetric versions of Inozemtsev's elliptic spin chain are
equivalent in the thermodynamic limit.


title: Inertial Sensor-Based Humanoid Joint State Estimation

abstract:
  This work presents methods for the determination of a humanoid robot's joint
velocities and accelerations directly from link-mounted Inertial Measurement
Units (IMUs) each containing a three-axis gyroscope and a three-axis
accelerometer. No information about the global pose of the floating base or its
links is required and precise knowledge of the link IMU poses is not necessary
due to presented calibration routines. Additionally, a filter is introduced to
fuse gyroscope angular velocities with joint position measurements and
compensate the computed joint velocities for time-varying gyroscope biases. The
resulting joint velocities are subject to less noise and delay than filtered
velocities computed from numerical differentiation of joint potentiometer
signals, leading to superior performance in joint feedback control as
demonstrated in experiments performed on a SARCOS hydraulic humanoid.


title:
The limitations of model-based experimental design and parameter
  estimation in sloppy systems

abstract:
  We explore the relationship among model fidelity, experimental design, and
parameter estimation in sloppy models. We show that the approximate nature of
mathematical models poses challenges for experimental design in sloppy models.
In many models of complex biological processes it is unknown what are the
relevant physics that must be included to explain collective behaviors. As a
consequence, models are often overly complex, with many practically
unidentifiable parameters. Furthermore, which details are relevant/irrelevant
vary among potential experiments. By selecting complementary experiments,
experimental design may inadvertently make details that were ommitted from the
model become relevant. When this occurs, the model will fail to give a good fit
to the data. We use a simple hyper-model of model error to quantify a model's
inadequacy and apply it to two models of complex biological processes (EGFR
signaling and DNA repair) with optimally selected experiments. We find that
although parameters may be accurately estimated, the error in the model renders
it less predictive than it was in the sloppy regime where model error is small.
We introduce the concept of a \emph{sloppy system}--a sequence of models of
increasing complexity that become sloppy in the limit of microscopic accuracy.
We explore the limits of accurate parameter estimation in sloppy systems and
argue that system identification better approached by considering a hierarchy
of models of varying detail rather than focusing parameter estimation in a
single model.


title: Exploration of Faulty Hamiltonian Graphs

abstract:
  We consider the problem of exploration of networks, some of whose edges are
faulty. A mobile agent, situated at a starting node and unaware of which edges
are faulty, has to explore the connected fault-free component of this node by
visiting all of its nodes. The cost of the exploration is the number of edge
traversals. For a given network and given starting node, the overhead of an
exploration algorithm is the worst-case ratio (taken over all fault
configurations) of its cost to the cost of an optimal algorithm which knows
where faults are situated. An exploration algorithm, for a given network and
given starting node, is called perfectly competitive if its overhead is the
smallest among all exploration algorithms not knowing the location of faults.
We design a perfectly competitive exploration algorithm for any ring, and show
that, for networks modeled by hamiltonian graphs, the overhead of any DFS
exploration is at most 10/9 times larger than that of a perfectly competitive
algorithm. Moreover, for hamiltonian graphs of size at least 24, this overhead
is less than 6% larger than that of a perfectly competitive algorithm.


title:
Lepton flavor violation and scalar dark matter in a radiative model of
  neutrino masses

abstract:
  We consider a simple extension of the Standard Model that can account for the
dark matter and explain the existence of neutrino masses. The model includes a
vector-like doublet of SU(2), a singlet fermion, and two scalar singlets, all
of them odd under a new Z$_2$ symmetry. Neutrino masses are generated
radiatively by one-loop processes involving the new fields, while the dark
matter candidate is the lightest neutral particle among them. We focus
specifically on the case where the dark matter particle is one of the scalars
and its relic density is determined by its Yukawa interactions. The
phenomenology of this setup, including neutrino masses, dark matter and lepton
flavor violation, is analyzed in some detail. We find that the dark matter mass
must be below $500$ GeV to satisfy the relic density constraint. Lepton flavor
violating processes are shown to provide the most promising way to test this
scenario. Future $\mu\to 3e$ and $\mu$-$e$ conversion experiments, in
particular, have the potential to probe the entire viable parameter space of
this model.


title: Quantum anomalies in superconducting Weyl metals

abstract:
  We theoretically study the quantum anomalies in the superconducting Weyl
metals based on the topological field theory. It is demonstrated that the Fermi
arc and the surface Andreev bound state, characteristic of the superconducting
Weyl metals, are the manifestations of two underlying phenomenon, namely the
chiral anomaly and the parity-like anomaly, respectively. The first anomaly is
inherited from the Berry curvature around the original Weyl points, while the
second is the result of the superconductivity. We show that, all the
fascinating topological behavior of the superconducting Weyl metals, either
intranode FFLO or the internode BCS pairing state, can be satisfactorily
described and predicted by our topological field theory.


title: JSJ decompositions of groups

abstract:
  This is an account of the theory of JSJ decompositions of finitely generated
groups, as developed in the last twenty years or so.
  We give a simple general definition of JSJ decompositions (or rather of their
Bass-Serre trees), as maximal universally elliptic trees. In general, there is
no preferred JSJ decomposition, and the right object to consider is the whole
set of JSJ decompositions, which forms a contractible space: the JSJ
deformation space (analogous to Outer Space).
  We prove that JSJ decompositions exist for any finitely presented group,
without any assumption on edge groups. When edge groups are slender, we
describe flexible vertices of JSJ decompositions as quadratically hanging
extensions of 2-orbifold groups.
  Similar results hold in the presence of acylindricity, in particular for
splittings of torsion-free CSA groups over abelian groups, and splittings of
relatively hyperbolic groups over virtually cyclic or parabolic subgroups.
Using trees of cylinders, we obtain canonical JSJ trees (which are invariant
under automorphisms).
  We introduce a variant in which the property of being universally elliptic is
replaced by the more restrictive and rigid property of being universally
compatible. This yields a canonical compatibility JSJ tree, not just a
deformation space. We show that it exists for any finitely presented group.
  We give many examples, and we work throughout with relative decompositions
(restricting to trees where certain subgroups are elliptic).


title: Short Gamma-Ray Bursts from the Merger of Two Black Holes

abstract:
  Short Gamma-Ray Bursts (GRBs) are explosions of cosmic origin believed to be
associated with the merger of two compact objects, either two neutron stars, or
a neutron star and a black hole. The presence of at least one neutron star has
long been thought to be an essential element of the model: its tidal disruption
provides the needed baryonic material whose rapid accretion onto the
post-merger black hole powers the burst. The recent tentative detection by the
Fermi satellite of a short GRB in association with the gravitational wave
signal GW150914 produced by the merger of two black holes has challenged this
standard paradigm. Here we show that the evolution of two high-mass,
low-metallicity stars with main sequence rotational speeds a few tens of
percent of the critical speed eventually undergoing a weak supernova explosion
{\em can} produce a short gamma-ray burst. The outer layers of the envelope of
the last exploding star remain bound and circularize at large radii. With time,
the disk cools and becomes neutral, suppressing the magneto-rotational
instability, and hence the viscosity. The disk remains 'long-lived dead' until
tidal torques and shocks during the pre-merger phase heat it up and re-ignite
accretion, rapidly consuming the disk and powering the short gamma-ray burst.


title:
Anomalous coupling, top-mass and parton-shower effects in ${W^+W^-}$
  production

abstract:
  We calculate the process $pp\to W^+W^-\to e^+ \nu_e \mu^-\bar{\nu}_\mu$ at
NLO QCD, including also effective field theory (EFT) operators mediating the
$ggW^+W^-$ interaction, which first occur at dimension eight. We further
combine the NLO and EFT matrix elements produced by GoSam with the
HERWIG7/MATCHBOX framework, which offers the possibility to study the impact of
a parton shower. We assess the effects of the anomalous couplings by comparing
them to top-mass effects as well as uncertainties related to variations of the
renormalisation, factorisation and hard shower scales.


title: Data Science at Udemy: Agile Experimentation with Algorithms

abstract:
  In this paper, we describe the data science framework at Udemy, which
currently supports the recommender and search system. We explain the
motivations behind the framework and review the approach, which allows multiple
individual data scientists to all become 'full stack', taking control of their
own destinies from the exploration and research phase, through algorithm
development, experiment setup, and deep experiment analytics. We describe
algorithms tested and deployed in 2015, as well as some key insights obtained
from experiments leading to the launch of the new recommender system at Udemy.
Finally, we outline the current areas of research, which include search,
personalization, and algorithmic topic generation.


title: A Preliminary Jupiter Model

abstract:
  In anticipation of new observational results for Jupiter's axial moment of
inertia and gravitational zonal harmonic coefficients from the forthcoming Juno
orbiter, we present a number of preliminary Jupiter interior models. We combine
results from ab initio computer simulations of hydrogen-helium mixtures,
including immiscibility calculations, with a new nonperturbative calculation of
Jupiter's zonal harmonic coefficients, to derive a self-consistent model for
the planet's external gravity and moment of inertia. We assume helium rain
modified the interior temperature and composition profiles. Our calculation
predicts zonal harmonic values to which measurements can be compared. Although
some models fit the observed (pre-Juno) second- and fourth-order zonal
harmonics to within their error bars, our preferred reference model predicts a
fourth-order zonal harmonic whose absolute value lies above the pre-Juno error
bars. This model has a dense core of about 12 Earth masses, and a
hydrogen-helium-rich envelope with approximately 3 times solar metallicity.


title:
Perpendicular laser cooling with a rotating wall potential in a Penning
  trap

abstract:
  We investigate the impact of a rotating wall potential on perpendicular laser
cooling in a Penning ion trap. By including energy exchange with the rotating
wall, we extend previous Doppler laser cooling theory and show that low
perpendicular temperatures are more readily achieved with a rotating wall than
without. Detailed numerical studies determine optimal operating parameters for
producing low temperature, stable 2-dimensional crystals, important for quantum
information processing experiments employing Penning traps.


title:
Electric field enhancement with plasmonic colloidal nanoantennas excited
  by a silicon nitride waveguide

abstract:
  We investigate the feasibility of CMOS-compatible optical structures to
develop novel integrated spectroscopy systems. We show that local field
enhancement is achievable utilizing dimers of plasmonic nanospheres that can be
assembled from colloidal solutions on top of a CMOS-compatible optical
waveguide. The resonant dimer nanoantennas are excited by modes guided in the
integrated silicon nitride waveguide. Simulations show that 100 fold electric
field enhancement builds up in the dimer gap as compared to the waveguide
evanescent field amplitude at the same location. We investigate how the field
enhancement depends on dimer location, orientation, distance and excited
waveguide modes.


title:
Asymptotic expansions of the hypergeometric function with two large
  parameters $-$ application to the partition function of a lattice gas in a
  field of traps

abstract:
  The canonical partition function of a two-dimensional lattice gas in a field
of randomly placed traps, like many other problems in physics, evaluates to the
Gauss hypergeometric function ${}_2F_1(a,b;c;z)$ in the limit when one or more
of its parameters become large. This limit is difficult to compute from first
principles, and finding the asymptotic expansions of the hypergeometric
function is therefore an important task. While some possible cases of the
asymptotic expansions of ${}_2F_1(a,b;c;z)$ have been provided in the
literature, they are all limited by a narrow domain of validity, either in the
complex plane of the variable or in the parameter space. Overcoming this
restriction, we provide new asymptotic expansions for the hypergeometric
function with two large parameters, which are valid for the entire complex
plane of $z$ except for a few specific points. We show that these expansions
work well even when we approach the possible singularity of ${}_2F_1(a,b;c;z)$,
$|z|=1$, where the current expansions typically fail. Using our results we
determine asymptotically the partition function of a lattice gas in a field of
traps in the different possible physical limits of few/many particles and
few/many traps, illustrating the applicability of the derived asymptotic
expansions of the HGF in physics.


title:
An Integrated Tantalum Sulfide - Boron Nitride - Graphene Oscillator: A
  Charge-Density-Wave Device Operating at Room Temperature

abstract:
  The charge-density-wave (CDW) phase is a macroscopic quantum state consisting
of a periodic modulation of the electronic charge density accompanied by a
periodic distortion of the atomic lattice in quasi-1D or layered 2D metallic
crystals. Several layered transition metal dichalcogenides, such as 1T-TaSe2,
1T-TaS2 and 1T-TiSe2, exhibit unusually high transition temperatures to
different CDW symmetry-reducing phases. These transitions can be affected by
environmental conditions, film thickness and applied electric bias. However,
device applications of these intriguing systems at room temperature or their
integration with other 2D materials have not been explored. Here we show that
in 2D CDW 1T-TaS2, the abrupt change in the electrical conductivity and
hysteresis at the transition point between nearly-commensurate and
incommensurate charge-density-wave phases can be used for constructing an
oscillator that operates at room temperature. The hexagonal boron nitride was
capped on 1T-TaS2 thin film to provide protection from oxidation, and an
integrated graphene transistor provides a voltage tunable, matched,
low-resistance load enabling precise voltage control of the oscillator
frequency. The integration of these three disparate two-dimensional materials,
in a way that exploits the unique properties of each, yields a simple,
miniaturized, voltage-controlled oscillator device. Theoretical considerations
suggest that the upper limit of oscillation frequency to be in the THz regime.


title:
Mixtures of quasi-two and three dimensional hybrid organic-inorganic
  semiconducting perovskites for single layer LED

abstract:
  New blends of simply synthesized quasi two-dimensional (quasi-2D) hydrophobic
perovskite semiconductors, employed in high performance light emitting diodes
(LEDs) which function due to excitonic energy transfer effects, are reported.
These materials are self-assembled blends of 2D, quasi-2D and three-dimensional
(3D) hybrid organic-inorganic semiconductors (HOIS). Moreover, shown for the
first time, crude mixing of 3D perovskite and unprotonated amines provides
similar semiconductors. HOIS reported here are based on the organic cations
CH3NH3+, CH3(CH2)7CH=CH(CH2)8NH3+ or C6H5CH2CH2NH3+ and inorganic networks
formed out of PbX42- anions (X=I, Br, Cl). HOIS exhibit strong bound excitonic
states with increased oscillator strength at room temperature, tunable via
simple halide substitution. HOIS blends manifest energy transfer effects, where
adjacent nanoparticles of different band gap energies (Eg) transfer optical
energy to those with the lowest Eg; the suggested light emission mechanism
here. LED fabrication is attained via a single deposition of the hydrophobic
mixture, reducing device complexity, cost and degradability. LED's diodic
behavior is observed even under light albeit its photovoltaic and
photoconductive quasi-2D and 3D components. LED devices exposed for over than
four months under adverse laboratory conditions, showed stable light emission.
Further research on this class of quasi-2D/3D HOIS mixtures is expected to lead
to novel quantum electronic devices.


title: Parallel Bayesian Global Optimization of Expensive Functions

abstract:
  We consider parallel global optimization of derivative-free
expensive-to-evaluate functions, and propose an efficient method based on
stochastic approximation for implementing a conceptual Bayesian optimization
algorithm proposed by Ginsbourger et al. (2007). At the heart of this algorithm
is maximizing the information criterion called the "multi-points expected
improvement'', or the q-EI. To accomplish this, we use infinitessimal
perturbation analysis (IPA) to construct a stochastic gradient estimator and
show that this estimator is unbiased. We also show that the stochastic gradient
ascent algorithm using the constructed gradient estimator converges to a
stationary point of the q-EI surface, and therefore, as the number of multiple
starts of the gradient ascent algorithm and the number of steps for each start
grow large, the one-step Bayes optimal set of points is recovered. We show in
numerical experiments that our method for maximizing the q-EI is faster than
methods based on closed-form evaluation using high-dimensional integration,
when considering many parallel function evaluations, and is comparable in speed
when considering few. We also show that the resulting one-step Bayes optimal
algorithm for parallel global optimization finds high-quality solutions with
fewer evaluations than a heuristic based on approximately maximizing the q-EI.
A high-quality open source implementation of this algorithm is available in the
open source Metrics Optimization Engine (MOE).


title: Approximation and Hardness for Token Swapping

abstract:
  Given a graph $G=(V,E)$ with $V=\{1,\ldots,n\}$, we place on every vertex a
token $T_1,\ldots,T_n$. A swap is an exchange of tokens on adjacent vertices.
We consider the algorithmic question of finding a shortest sequence of swaps
such that token $T_i$ is on vertex $i$. We are able to achieve essentially
matching upper and lower bounds, for exact algorithms and approximation
algorithms. For exact algorithms, we rule out any $2^{o(n)}$ algorithm under
the ETH. This is matched with a simple $2^{O(n\log n)}$ algorithm based on a
breadth-first search in an auxiliary graph. We show one general
$4$-approximation and show APX-hardness. Thus, there is a small constant
$\delta>1$ such that every polynomial time approximation algorithm has
approximation factor at least $\delta$. Our results also hold for a generalized
version, where tokens and vertices are colored. In this generalized version
each token must go to a vertex with the same color.


title: Branching Bisimilarity of Normed BPA Processes as a Rational Monoid

abstract:
  The paper presents an elaborated and simplified version of the structural
result for branching bisimilarity on normed BPA (Basic Process Algebra)
processes that was the crux of a conference paper by Czerwinski and Jancar
(arxiv 7/2014 and LiCS 2015). That paper focused on the computational
complexity, and a NEXPTIME-upper bound has been derived; the authors built on
the ideas by Fu (ICALP 2013), and strengthened his decidability result. Later
He and Huang announced the EXPTIME-completeness of this problem (arxiv 1/2015,
and LiCS 2015), giving a technical proof for the EXPTIME membership. He and
Huang indirectly acknowledge the decomposition ideas by Czerwinski and Jancar
on which they also built, but it is difficult to separate their starting point
from their new ideas. One aim here is to present the previous decomposition
result of Czerwinski and Jancar in a technically new framework, noting that
branching bisimulation equivalence on normed BPA processes corresponds to a
rational monoid (in the sense of [Sakarovitch, 1987]); in particular it is
shown that the mentioned equivalence can be decided by normal-form computing
deterministic finite transducers. Another aim is to provide a complete
description, including an informal overview, that should also make clear how
Fu's ideas were used, and to give all proofs in a form that should be readable
and easily verifiable.


title:
Order-to-chaos transition in the hardness of random Boolean
  satisfiability problems

abstract:
  Transient chaos is an ubiquitous phenomenon characterizing the dynamics of
phase space trajectories evolving towards a steady state attractor in physical
systems as diverse as fluids, chemical reactions and condensed matter systems.
Here we show that transient chaos also appears in the dynamics of certain
efficient algorithms searching for solutions of constraint satisfaction
problems that include scheduling, circuit design, routing, database problems or
even Sudoku. In particular, we present a study of the emergence of hardness in
Boolean satisfiability ($k$-SAT), a canonical class of constraint satisfaction
problems, by using an analog deterministic algorithm based on a system of
ordinary differential equations. Problem hardness is defined through the escape
rate $\kappa$, an invariant measure of transient chaos of the dynamical system
corresponding to the analog algorithm, and it expresses the rate at which the
trajectory approaches a solution.We show that for a given density of
constraints and fixed number of Boolean variables $N$, the hardness of formulas
in random $k$-SAT ensembles has a wide variation, approximable by a lognormal
distribution. We also show that when increasing the density of constraints
$\alpha$, hardness appears through a second-order phase transition at
$\alpha_{\chi}$ in the random 3-SAT ensemble where dynamical trajectories
become transiently chaotic. A similar behavior is found in 4-SAT as well,
however, such transition does not occur for 2-SAT. This behavior also implies a
novel type of transient chaos in which the escape rate has an
exponential-algebraic dependence on the critical parameter $\kappa \sim
N^{B|\alpha - \alpha_{\chi}|^{1-\gamma}}$ with $0< \gamma < 1$. We demonstrate
that the transition is generated by the appearance of metastable basins in the
solution space as the density of constraints $\alpha$ is increased.


title: Non-symmetric Macdonald polynomials and Demazure-Lusztig operators

abstract:
  We extend the family non-symmetric Macdonald polynomials and define
general-basement Macdonald polynomials. We show that these also satisfy a
triangularity property with respect to the monomials bases and behave well
under the Demazure-Lusztig operators.
  The symmetric Macdonald polynomials $J_\lambda$ are expressed as a sum of
general-basement Macdonald polynomials via an explicit formula.
  By letting $q=0$, we obtain $t$-deformations of key polynomials and Demazure
atoms and we show that the Hall--Littlewood polynomials expand positively into
these. This generalizes a result by Haglund, Luoto, Mason and van Willigenburg.
  As a corollary, we prove that Schur polynomials decompose with non-negative
coefficients into $t$-deformations of general Demazure atoms and thus
generalizing the $t=0$ case which was previously known. This gives a unified
formula for the classical expansion of Schur polynomials in Hall-Littlewood
polynomials and the expansion of Schur polynomials into Demazure atoms.


title: MCP-based Photodetectors for Cryogenic Applications

abstract:
  The Argonne MCP-based photo detector is an offshoot of the Large Area
Pico-second Photo Detector (LAPPD) project, wherein 6 cm x 6 cm sized detectors
are made at Argonne National Laboratory. We have successfully built and tested
our first detectors for pico-second timing and few mm spatial resolution. We
discuss our efforts to customize these detectors to operate in a cryogenic
environment. Initial plans aim to operate in liquid argon. We are also
exploring ways to mitigate wave length shifting requirements and also
developing bare-MCP photodetectors to operate in a gaseous cryogenic
environment.


title:
A Dirichlet Process Functional Approach to Heteroscedastic-Consistent
  Covariance Estimation

abstract:
  The mixture of Dirichlet process (MDP) defines a flexible prior distribution
on the space of probability measures. This study shows that ordinary
least-squares (OLS) estimator, as a functional of the MDP posterior
distribution, has posterior mean given by weighted least-squares (WLS), and has
posterior covariance matrix given by the (weighted) heteroscedastic-consistent
sandwich estimator. This is according to a pairs bootstrap distribution
approximation of the posterior, using a P\'olya urn scheme. Also, when the MDP
prior baseline distribution is specified as a product of independent
probability measures, this WLS solution provides a new type of generalized
ridge regression estimator which can handle multicollinear or singular design
matrices even when the number of covariates exceeds the sample size, and which
shrinks the coefficient estimates of irrelevant covariates towards zero, thus
useful for nonlinear regressions. Also, this MDP/OLS functional methodology can
be extended to methods for analyzing the sensitivity of the
heteroscedasticity-consistent causal effect size over a range of hidden biases
due to missing covariates omitted from the regression, and more generally
extended to a Vibration of Effects analysis. The methodology is illustrated
through the analysis of simulated and real data sets. Overall, this study
establishes new connections between Dirichlet process functional inference, the
bootstrap, consistent sandwich covariance estimation, ridge shrinkage
regression, WLS, and sensitivity analysis, to provide regression methodology
useful for inferences of the mean dependent response.


title: On Crossed Modules in Modified Categories of Interest

abstract:
  We introduce some algebraic structures such as singularity, commutators and
central extension in modified categories of interest. Additionally, we
introduce the cat$^{1}$-objects with their connection to crossed modules in
these categories which gives rise to unify many notions about (pre)crossed
modules in various algebras of categories.


title: A Ranking Algorithm for Re-finding

abstract:
  Re-finding files from a personal computer is a frequent demand to users. When
encountered a difficult re-finding task, people may not recall the attributes
used by conventional re-finding methods, such as a file's path, file name,
keywords etc., the re-finding would fail.
  We proposed a method to support difficult re-finding tasks. By asking the
user a list of questions about the target, such as a document's pages, author
numbers, accumulated reading time, last reading location etc. Then use the
user's answers to filter out the target.
  After the user answered a list of questions about the target file, we
evaluate the user's familiar degree about the target file based on the answers.
We devise a ranking algorithm which sorts the candidates by comparing the
user's familiarity degree about the target and the candidates.
  We also propose a method to generate re-finding tasks artificially based on
the user's own document corpus.


title: Measuring Light Echos in NGC 4051

abstract:
  Five archived X-ray observations of NGC 4051, taken using the NuSTAR
observatory, have been analysed, revealing lags between flux variations in
bands covering a wide range of X-ray photon energy. In all pairs of bands
compared, the harder band consistently lags the softer band by at least 1000s,
at temporal frequencies ~5E-5 Hz. In addition, soft-band lags up to 400s are
measured at frequencies ~2E-4 Hz. Light echos from an excess of soft band
emission in the inner accretion disk cannot explain the lags in these data, as
they are seen in cross-correlations with energy bands where the softer band is
expected to have no contribution from reflection. The basic properties of the
time delays have been parameterised by fitting a top hat response function that
varies with photon energy, taking fully into account the covariance between
measured time lag values. The low-frequency hard-band lags and the transition
to soft-band lags are consistent with time lags arising as reverberation delays
from circumnuclear scattering of X-rays, although greater model complexity is
required to explain the entire spectrum of lags. The scattered fraction
increases with increasing photon energy as expected, and the scattered fraction
is high, indicating the reprocessor to have a global covering fraction ~50%
around the continuum source. Circumnuclear material, possibly associated with a
disk wind at a few hundred gravitational radii from the primary X-ray source,
may provide suitable reprocessing.


title:
The FeII emission in active galactic nuclei: excitation mechanisms and
  location of the emitting region

abstract:
  We present a study of FeII emission in the near-infrared region (NIR) for 25
active galactic nuclei (AGNs) to obtain information about the excitation
mechanisms that power it and the location where it is formed. We employ a NIR
FeII template derived in the literature and found that it successfully
reproduces the observed FeII spectrum. The FeII bump at 9200 Angstroms detected
in all objects studied confirms that Lyman-alpha fluorescence is always present
in AGNs. The The correlation found between the flux of the 9200 Angstroms bump,
the 1 micron lines and the optical FeII imply that Lyman-alpha fluorescence
plays an important role in the FeII production. We determined that at least 18%
of the optical FeII is due to this process while collisional excitation
dominates the production of the observed FeII. The line profiles of FeII 10502,
OI 11287, CaII 8664 and Paschen-beta were compared to gather information about
the most likely location where they are emitted. We found that FeII, OI and
CaII have similar widths and are, on average, 30% narrower than Paschen-beta.
Assuming that the clouds emitting the lines are virialized, we show that the
FeII is emitted in a region twice as far from the central source than
Paschen-beta. The distance though strongly varies: from 8.5 light-days for
NGC4051 to 198.2 light-days for Mrk509. Our results reinforce the importance of
the FeII in the NIR to constrain critical parameters that drive its physics and
the underlying AGN kinematics as well as more accurate models aimed at
reproducing this complex emission.


title: Liquid dispensing and writing by a nano-grooved pin

abstract:
  Liquid dispensing and writing in the extremely small size regime are
important for applications in many current technologies, such as micro/nano
fabrication, biological/chemical patterning and analysis, and drug discovery.
Most of current dispensing/writing methods can be sorted into a category of
liquid flowing through tiny tubes or nozzles that requires inputting an impulse
energy, which leads to complex procedures, expensive equipment and narrow
material applicability, especially for biomaterials. Here, we report a method
that may lead to a new category: liquid flows over the tapered surface of a pin
with longitudinal nano grooves on the surface to uninterruptedly perform
droplet dispensing and direct writing. The dispensed droplet diameters were
controllable from several microns down to 150 nm, and the written line heights
were as low as 5 nm. The mechanism underlying automatic liquid storage on
conical surface and spontaneous liquid transport through nano grooves is
revealed and well modeled by a simple relationship. Furthermore, the
nano-grooved pins are much simpler and cheaper in fabrication than nanoscale
tubes and nozzles, and pins have much depressed clogging problems that are
typically troublesome for tubes and nozzles. Our new strategy may constitute a
basis for creating liquid dispensing/writing technologies that are
simultaneously smaller, simpler, faster and applicable for more types of
materials.


title:
Fast Learning Requires Good Memory: A Time-Space Lower Bound for Parity
  Learning

abstract:
  We prove that any algorithm for learning parities requires either a memory of
quadratic size or an exponential number of samples. This proves a recent
conjecture of Steinhardt, Valiant and Wager and shows that for some learning
problems a large storage space is crucial.
  More formally, in the problem of parity learning, an unknown string $x \in
\{0,1\}^n$ was chosen uniformly at random. A learner tries to learn $x$ from a
stream of samples $(a_1, b_1), (a_2, b_2) \ldots$, where each~$a_t$ is
uniformly distributed over $\{0,1\}^n$ and $b_t$ is the inner product of $a_t$
and $x$, modulo~2. We show that any algorithm for parity learning, that uses
less than $\frac{n^2}{25}$ bits of memory, requires an exponential number of
samples.
  Previously, there was no non-trivial lower bound on the number of samples
needed, for any learning problem, even if the allowed memory size is $O(n)$
(where $n$ is the space needed to store one sample).
  We also give an application of our result in the field of bounded-storage
cryptography. We show an encryption scheme that requires a private key of
length $n$, as well as time complexity of $n$ per encryption/decription of each
bit, and is provenly and unconditionally secure as long as the attacker uses
less than $\frac{n^2}{25}$ memory bits and the scheme is used at most an
exponential number of times. Previous works on bounded-storage cryptography
assumed that the memory size used by the attacker is at most linear in the time
needed for encryption/decription.


title: A Bayes interpretation of stacking for M-complete and M-open settings

abstract:
  In M-open problems where no true model can be conceptualized, it is common to
back off from modeling and merely seek good prediction. Even in M-complete
problems, taking a predictive approach can be very useful. Stacking is a model
averaging procedure that gives a composite predictor by combining individual
predictors from a list of models using weights that optimize a cross-validation
criterion. We show that the stacking weights also asymptotically minimize a
posterior expected loss. Hence we formally provide a Bayesian justification for
cross-validation. Often the weights are constrained to be positive and sum to
one. For greater generality, we omit the positivity constraint and relax the
`sum to one' constraint.
  A key question is `What predictors should be in the average?' We first verify
that the stacking error depends only on the span of the models. Then we propose
using bootstrap samples from the data to generate empirical basis elements that
can be used to form models. We use this in two computed examples to give
stacking predictors that are (i) data driven, (ii) optimal with respect to the
number of component predictors, and (iii) optimal with respect to the weight
each predictor gets.


title: Composable Industrial Internet Applications for Tiered Architectures

abstract:
  A single vendor cannot provide complete IIoT end-to-end solutions because
cooperation is required from multiple parties. Interoperability is a key
architectural quality. Composability of capabilities, information and
configuration is the prerequisite for interoperability, supported by a data
storage infrastructure and defined set of interfaces to build applications.
Secure collection, transport and storage of data and algorithms are
expectations for collaborative participation in any IIoT solution. Participants
require control of their data ownership and confidentiality. We propose an
Internet of Things, Services and People (IoTSP) application development and
management framework which includes components for data storage, algorithm
design and packaging, and computation execution. Applications use clusters of
platform services, organized in tiers, and local access to data to reduce
complexity and enhance reliable data exchange. Since communication is less
reliable across tiers, data is synchronized between storage replicas when
communication is available. The platform services provide a common ecosystem to
exchange data uniting data storage, applications, and components that process
the data. Configuration and orchestration of the tiers are managed using shared
tools and facilities. The platform promotes the data storage components to be
peers of the applications where each data owner is in control of when and how
much information is shared with a service provider. The service components and
applications are securely integrated using local event and data exchange
communication channels. This tiered architecture reduces the cyber attack
surface and enables individual tiers to operate autonomously, while addressing
interoperability concerns. We present our framework using predictive
maintenance as an example, and evaluate compatibility of our vision with an
emerging set of standards.


title:
The non-existence of sharply $2$-transitive sets of permutations in
  $\mathrm{Sp}(2d,2)$ of degree $2^{2d-1} \pm 2^{d-1}$

abstract:
  We use M\"uller and Nagy's method of contradicting subsets to give a new
proof for the non-existence of sharply $2$-transitive subsets of the symplectic
groups $\mathrm{Sp}(2d,2)$ in their doubly-transitive actions of degrees
$2^{2d-1}\pm 2^{d-1}$. The original proof by Grundh\"ofer and M\"uller was
rather complicated and used some results from modular representation theory,
whereas our new proof requires only simple counting arguments.


title: Fast Radio Bursts: Searches, Sensitivities & Implications

abstract:
  Fast radio bursts (FRBs) are millisecond-duration transient signals
discovered over the past decade. Here we describe the scientific usefulness of
FRBs, consider ongoing work at the Parkes telescope, and examine some relevant
search sensitivity and completeness considerations. We also look ahead to the
results from ongoing and future planned studies in the field.


title: Bose Gases at Positive Temperature and Non-Linear Gibbs Measures

abstract:
  We summarize recent results on positive temperature equilibrium states of
large bosonic systems. The emphasis will be on the connection between bosonic
grand-canonical thermal states and the (semi-) classical Gibbs measures on
one-body quantum states built using the corresponding mean-field energy
functionals. An illustrative comparison with the case of "distinguishable"
particles (boltzons) is provided.

categories: math-ph cond-mat.quant-gas math.AP math.MP


title: The economic basis of periodic enzyme dynamics

abstract:
  Periodic enzyme activities can improve the metabolic performance of cells. As
an adaptation to periodic environments or by driving metabolic cycles that can
shift fluxes and rearrange metabolic processes in time to increase their
efficiency. To study what benefits can ensue from rhythmic gene expression or
posttranslational modification of enzymes, I propose a theory of optimal enzyme
rhythms in periodic or static environments. The theory is based on kinetic
metabolic models with predefined metabolic objectives, scores the effects of
harmonic enzyme oscillations, and determines amplitudes and phase shifts that
maximise cell fitness. In an expansion around optimal steady states, the
optimal enzyme profiles can be computed by solving a quadratic optimality
problem. The formulae show how enzymes can increase their efficiency by
oscillating in phase with their substrates and how cells can benefit from
adapting to external rhythms and from spontaneous, intrinsic enzyme rhythms.
Both types of behaviour may occur different parameter regions of the same
model. Optimal enzyme profiles are not passively adapted to existing substrate
rhythms, but shape them actively to create opportunities for further fitness
advantage: in doing so, they reflect the dynamic effects that enzymes can exert
in the network. The proposed theory combines the dynamics and economics of
metabolic systems and shows how optimal enzyme profiles are shaped by network
structure, dynamics, external rhythms, and metabolic objectives. It covers
static enzyme adaptation as a special case, reveals the conditions for
beneficial metabolic cycles, and predicts optimally combinations of gene
expression and posttranslational modification for creating enzyme rhythms.


title: An Approach for Noise Removal on Depth Images

abstract:
  Image based rendering is a fundamental problem in computer vision and
graphics. Modern techniques often rely on depth image for the 3D construction.
However for most of the existing depth cameras, the large and unpredictable
noises can be problematic, which can cause noticeable artifacts in the rendered
results. In this paper, we proposed an efficacious method for depth image noise
removal that can be applied for most RGBD systems. The proposed solution will
benefit many subsequent vision problems such as 3D reconstruction, novel view
rendering, object recognition. Our experimental results demonstrate the
efficacy and accuracy.


title:
Rainbow perfect matchings and Hamilton cycles in the random geometric
  graph

abstract:
  Given a graph on $n$ vertices and an assignment of colours to the edges, a
rainbow Hamilton cycle is a cycle of length $n$ visiting each vertex once and
with pairwise different colours on the edges. Similarly (for even $n$) a
rainbow perfect matching is a collection of $n/2$ independent edges with
pairwise different colours. In this note we show that if we randomly colour the
edges of a random geometric graph with sufficiently many colours, then a.a.s.
the graph contains a rainbow perfect matching (rainbow Hamilton cycle) if and
only if the minimum degree is at least $1$ (respectively, at least $2$). More
precisely, consider $n$ points (i.e. vertices) chosen independently and
uniformly at random from the unit $d$-dimensional cube for any fixed $d\ge2$.
Form a sequence of graphs on these $n$ vertices by adding edges one by one
between each possible pair of vertices. Edges are added in increasing order of
lengths (measured with respect to the $\ell_p$ norm, for any fixed
$1<p\le\infty$). Each time a new edge is added, it receives a random colour
chosen uniformly at random and with repetition from a set of $\lceil Kn\rceil$
colours, where $K=K(d)$ is a sufficiently large fixed constant. Then, a.a.s.
the first graph in the sequence with minimum degree at least $1$ must contain a
rainbow perfect matching (for even $n$), and the first graph with minimum
degree at least $2$ must contain a rainbow Hamilton cycle.


title: Applied Logic in Engineering

abstract:
  Logic not only helps to solve complicated and safety-critical problems, but
also disciplines the mind and helps to develop abstract thinking, which is very
important for any area of Engineering. In this technical report, we present an
overview of common challenges in teaching of formal methods and discuss our
experiences from the course Applied Logic in Engineering. This course was
taught at TU Munich, Germany, in Winter Semester 2012/2013.


title: The time-dependent Hartree-Fock-Bogoliubov equations for Bosons

abstract:
  In this article, we use quasifree reduction to derive the time-dependent
Hartree-Fock-Bogoliubov (HFB) equations describing the dynamics of quantum
fluctuations around a Bose-Einstein condensate in $\mathbb R^d$. We prove
global well-posedness for the HFB equations for sufficiently regular pair
interaction potentials, and establish key conservation laws. Moreover, we show
that the solutions to the HFB equations exhibit a symplectic structure, and
have a form reminiscent of a Hamiltonian system. In particular, this is used to
relate the HFB equations to the HFB eigenvalue equations encountered in the
physics literature. Furthermore, we construct the Gibbs states at positive
temperature associated with the HFB equations, and establish criteria for the
emergence of Bose-Einstein condensation.


title: Summability Condition and Rigidity for Finite Type Maps

abstract:
  We extend a series of results due to Makienko, Dominguez and Sienra on the
rigidity of some holomorphic dynamical systems with summable critical values to
the setting of finite type maps. We also recover a shorter proof of a
transversality theorem of Levin. Our methods are based on the deformation
theory introduced by Epstein.


title: Unimodularity unified

abstract:
  Unimodularity is localized to a complete stationary type, and its properties
are analysed. Some variants of unimodularity for definable and type-definable
sets are introduced, and the relationship between these different notions is
studied. In particular, it is shown that all notions coincide for
non-multidimensional theories where the dimensions are associated to strongly
minimal types.


title:
Spatial geometry of charged rotating and non-rotating rings in rotating
  and non-rotating frames

abstract:
  Spatial geometry of charged thin rotating and non-rotating rings in a
rotating frame is investigated. It is shown, on an example of interaction
between a charged probe and two positive charged non-rotating and negative
charged rotating rings that the spatial geometry of the rotating ring in the
rotating frame has to be different to the spatial geometry of the rotating
frame. In the absent of direct relation between the spatial geometry rotating
frame and the spatial geometry of the rotating ring in that frame the
possibility of a non-flat spatial geometry of rotating electron rings in
tokamak plasma is discussed.


title:
A simple, general result for the variance of substitution number in
  molecular evolution

abstract:
  The number of substitutions (of nucleotides, amino acids, ...) that take
place during the evolution of a sequence is a stochastic variable of
fundamental importance in the field of molecular evolution. Although the mean
number of substitutions during molecular evolution of a sequence can be
estimated for a given substitution model, no simple solution exists for the
variance of this random variable. We show in this article that the computation
of the variance is as simple as that of the mean number of substitutions for
both short and long times. Apart from its fundamental importance, this result
can be used to investigate the dispersion index R , i.e. the ratio of the
variance to the mean substitution number, which is of prime importance in the
neutral theory of molecular evolution. By investigating large classes of
substitution models, we demonstrate that although R\ge1 , to obtain R
significantly larger than unity necessitates in general additional hypotheses
on the structure of the substitution model.

categories: q-bio.PE physics.bio-ph q-bio.GN q-bio.QM


title:
The inner edge of the habitable zone for synchronously rotating planets
  around low-mass stars using general circulation models

abstract:
  Terrestrial planets at the inner edge of the habitable zone of late-K and
M-dwarf stars are expected to be in synchronous rotation, as a consequence of
strong tidal interactions with their host stars. Previous global climate model
(GCM) studies have shown that, for slowly-rotating planets, strong convection
at the substellar point can create optically thick water clouds, increasing the
planetary albedo, and thus stabilizing the climate against a thermal runaway.
However these studies did not use self-consistent orbital/rotational periods
for synchronously rotating planets placed at different distances from the host
star. Here we provide new estimates of the inner edge of the habitable zone for
synchronously rotating terrestrial planets around late-K and M-dwarf stars
using a 3-D Earth-analog GCM with self-consistent relationships between stellar
metallicity, stellar effective temperature, and the planetary
orbital/rotational period. We find that both atmospheric dynamics and the
efficacy of the substellar cloud deck are sensitive to the precise rotation
rate of the planet. Around mid-to-late M-dwarf stars with low metallicity,
planetary rotation rates at the inner edge of the HZ become faster, and the
inner edge of the habitable zone is farther away from the host stars than in
previous GCM studies. For an Earth-sized planet, the dynamical regime of the
substellar clouds begins to transition as the rotation rate approaches ~10
days. These faster rotation rates produce stronger zonal winds that encircle
the planet and smear the substellar clouds around it, lowering the planetary
albedo, and causing the onset of the water-vapor greenhouse climatic
instability to occur at up to ~25% lower incident stellar fluxes than found in
previous GCM studies. For mid-to-late M-dwarf stars with high metallicity and
for mid-K to early-M stars, we agree with previous studies.


title:
Sampling of conformational ensemble for virtual screening using
  molecular dynamics simulations and normal mode analysis

abstract:
  Aim: Molecular dynamics simulations and normal mode analysis are
well-established approaches to generate receptor conformational ensembles
(RCEs) for ligand docking and virtual screening. Here, we report new fast
molecular dynamics-based and normal mode analysis-based protocols combined with
conformational pocket classifications to efficiently generate RCEs. Materials
\& methods: We assessed our protocols on two well-characterized protein targets
showing local active site flexibility, dihydrofolate reductase and large
collective movements, CDK2. The performance of the RCEs was validated by
distinguishing known ligands of dihydrofolate reductase and CDK2 among a
dataset of diverse chemical decoys. Results \& discussion: Our results show
that different simulation protocols can be efficient for generation of RCEs
depending on different kind of protein flexibility.


title: Cosmic Dawn Intensity Mapper

abstract:
  Cosmic Dawn Intensity Mapper is a "Probe Class" mission concept for
reionization studies of the universe. It will be capable of spectroscopic
imaging observations between 0.7 to 6-7 microns in the near-Infrared. The
primary observational objective is pioneering observations of spectral emission
lines of interest throughout the cosmic history, but especially from the first
generation of distant, faint galaxies when the universe was less than 800
million years old. With spectro-imaging capabilities, using a set of linear
variable filters (LVFs), CDIM will produce a three-dimensional tomographic view
of the epoch of reionization (EoR). CDIM will also study galaxy formation over
more than 90% of the cosmic history and will move the astronomical community
from broad-band astronomical imaging to low-resolution (R=200-300)
spectro-imaging of the universe.


title:
Equilibrium Propagation: Bridging the Gap Between Energy-Based Models
  and Backpropagation

abstract:
  We introduce Equilibrium Propagation, a learning framework for energy-based
models. It involves only one kind of neural computation, performed in both the
first phase (when the prediction is made) and the second phase of training
(after the target or prediction error is revealed). Although this algorithm
computes the gradient of an objective function just like Backpropagation, it
does not need a special computation or circuit for the second phase, where
errors are implicitly propagated. Equilibrium Propagation shares similarities
with Contrastive Hebbian Learning and Contrastive Divergence while solving the
theoretical issues of both algorithms: our algorithm computes the gradient of a
well defined objective function. Because the objective function is defined in
terms of local perturbations, the second phase of Equilibrium Propagation
corresponds to only nudging the prediction (fixed point, or stationary
distribution) towards a configuration that reduces prediction error. In the
case of a recurrent multi-layer supervised network, the output units are
slightly nudged towards their target in the second phase, and the perturbation
introduced at the output layer propagates backward in the hidden layers. We
show that the signal 'back-propagated' during this second phase corresponds to
the propagation of error derivatives and encodes the gradient of the objective
function, when the synaptic update corresponds to a standard form of
spike-timing dependent plasticity. This work makes it more plausible that a
mechanism similar to Backpropagation could be implemented by brains, since
leaky integrator neural computation performs both inference and error
back-propagation in our model. The only local difference between the two phases
is whether synaptic changes are allowed or not.


title:
A dissipative quantum reservoir for microwave light using a mechanical
  oscillator

abstract:
  Isolation of a system from its environment is often desirable, from precision
measurements to control of individual quantum systems; however, dissipation can
also be a useful resource. Remarkably, engineered dissipation enables the
preparation of quantum states of atoms, ions or superconducting qubits as well
as their stabilization. This is achieved by a suitably engineered coupling to a
dissipative cold reservoir formed by electromagnetic modes. Similarly, in the
field of cavity electro- and optomechanics, the control over mechanical
oscillators utilizes the inherently cold, dissipative nature of the
electromagnetic degree of freedom. Breaking from this paradigm, recent
theoretical work has considered the opposite regime in which the dissipation of
the mechanical oscillator dominates and provides a cold, dissipative reservoir
to an electromagnetic mode. Here we realize this reversed dissipation regime in
a microwave cavity optomechanical system and realize a quasi-instantaneous,
cold reservoir for microwave light. Coupling to this reservoir enables to
manipulate the susceptibility of the microwave cavity, corresponding to
dynamical backaction control of the microwave field. Additionally, we observe
the onset of parametric instability, i.e. the stimulated emission of microwaves
(masing). Equally important, the reservoir can function as a useful quantum
resource. We evidence this by employing the engineered cold reservoir to
implement a large gain (above 40 dB) phase preserving microwave amplifier that
operates 0.87 quanta above the limit of added noise imposed by quantum
mechanics. Such a dissipative cold reservoir forms the basis of microwave
entanglement schemes, the study of dissipative quantum phase transitions,
amplifiers with unlimited gain-bandwidth product and non-reciprocal devices,
thereby extending the available toolbox of quantum-limited microwave
manipulation techniques.


title: A Simple Condition for the Existence of Transversals

abstract:
  Hall's Theorem is a basic result in Combinatorics which states that the
obvious necesssary condition for a finite family of sets to have a transversal
is also sufficient. We present a sufficient (but not necessary) condition on
the sizes of the sets in the family and the sizes of their intersections so
that a transversal exists. Using this, we prove that in a bipartite graph $G$
(bipartition $\{A, B\}$), without 4-cycles, if $\deg(v) \geq \sqrt{2e|A|}$ for
all $v \in A$, then $G$ has a matching of size $|A|$.


title: Five subsets of permutations enumerated as weak sorting permutations

abstract:
  We show that the number of members of S_n avoiding any one of five specific
triples of 4-letter patterns is given by sequence A111279 in OEIS, which is
known to count weak sorting permutations. By numerical evidence, there are no
other (non-trivial) triples of 4-letter patterns giving rise to this sequence.
We make use of a variety of methods in proving our result, including
recurrences, the kernel method, direct counting, and bijections.


title:
A note and a correction on measuring cognitive distance in multiple
  dimensions

abstract:
  In a previous article (Rahman, Guns, Rousseau, and Engels, 2015) we described
several approaches to determine the cognitive distance between two units. One
of these approaches was based on what we called barycenters in N dimensions.
The present note corrects this terminology and introduces the more adequate
term 'similarity-adapted publication vectors'. Furthermore, we correct an error
in normalization and explain the importance of scale invariance in determining
cognitive distance. We also consider weighted cosine similarity as an
alternative approach to determine cognitive (dis)similarity. Overall, we find
that the three approaches (distance between barycenters, distance between
similarity-adapted publication vectors, and weighted cosine similarity) yield
very similar results.


title: On the difference between the Szeged and Wiener index

abstract:
  We prove a conjecture of Nadjafi-Arani, Khodashenas and Ashrafi on the
difference between the Szeged and Wiener index of a graph. Namely, if $G$ is a
2-connected non-complete graph on $n$ vertices, then $Sz(G)-W(G)\ge 2n-6$.
Furthermore, the equality is obtained if and only if $G$ is the complete graph
$K_{n-1}$ with an extra vertex attached to either $2$ or $n-2$ vertices of
$K_{n-1}$. We apply our method to strengthen some known results on the
difference between the Szeged and Wiener index of bipartite graphs, graphs of
girth at least five, and the difference between the revised Szeged and Wiener
index. We also propose a stronger version of the aforementioned conjecture.


title:
Spicules and their on-disk counterparts, the main driver for solar
  chromospheric heating?

abstract:
  The question how the outer solar atmosphere is heated from solar photospheric
temperatures of about 5800K up to solar chromospheric and coronal temperatures
of about 20.000K and millions of degrees respectively, remained without any
satisfying answer for centuries. On 4 May 2005, I recorded several time series
of Halpha line scans with the GREGOR Fabry-Perot Interferometer, still deployed
at the German Vacuum Tower Telescope (VTT), for different solar limb and
on-disk positions as well as for quiet sun at solar disk center. The spatially
and temporally highly resolved time series of Halpha line parameters reveal the
entire and detailed complexity as well as the overwhelming dynamics of spicules
covering the entire solar disk, thus apparently confirming spicules as the
potential driver of chromospheric heating for both the Sun and sun-like stars,
with an expected mass flux larger than 100 times that of the solar wind.
Spicules seem to be the result of the interaction of the highly dynamic
photospheric quiet-sun or active-region small-scale magnetic field, which is
dominated by convective processes and is predominantly located in intergranular
lanes and at meso- or supergranular scales.


title:
Four-wave mixing in perovskite photovoltaic materials reveals long
  dephasing times and weaker many-body interactions than GaAs

abstract:
  Perovksite semiconductors have shown promise for low-cost solar cells, lasers
and photodetectors, yet their fundamental photophysical properties are not well
understood. Recent observations of a low ($\sim$few meV) exciton binding energy
and evidence of hot phonon effects in the room temperature phase suggest that
perovskites are much closer to inorganic semiconductors than the absorber
layers in traditional organic photovoltaics, signaling the need for experiments
that shed light on the placement of perovskite materials within the spectrum of
semiconductors used in optoelectronics and photovoltaics. Here we use four-wave
mixing (FWM) to contrast the coherent optical response of CH$_3$NH$_3$PbI$_3$
thin films and crystalline GaAs. At carrier densities relevant for solar cell
operation, our results show that carriers interact surprisingly weakly via the
Coulomb interaction in perovskite, much weaker than in inorganic
semiconductors. These weak many-body effects lead to a dephasing time in
CH$_3$NH$_3$PbI$_3$ $\sim$3 times longer than in GaAs. Our results also show
that the strong enhancement of the exciton FWM signal tied to
excitation-induced dephasing in GaAs and other III-V semiconductors does not
occur in perovskite due to weak exciton-carrier scattering interactions.


title: $2$-local automorphisms on finite-dimensional Lie algebras

abstract:
  We prove that every $2$-local automorphism on a finite-dimensional
semi-simple Lie algebra $\mathcal{L}$ over an algebraically closed field of
characteristic zero is an automorphism. We also show that each
finite-dimensional nilpotent Lie algebra $\mathcal{L}$ with $\dim
\mathcal{L}\geq 2$ admits a $2$-local automorphism which is not an
automorphism.


title: Simple Models for All Topological Phases

abstract:
  We construct simple models for all topological phases of free fermions. These
explicit models can realize all the nontrivial topological phases (with any
possible topological invariant) of the periodic table. Many well known models
for topological insulators and superconductors are special cases of our general
constructions.

categories: cond-mat.mes-hall cond-mat.supr-con math-ph math.MP


title: A Central Limit Theorem for Punctuated Equilibrium

abstract:
  Current evolutionary biology models usually assume that a phenotype undergoes
gradual change. This is in stark contrast to biological intuition, which
indicates that change can also be punctuated-the phenotype can jump. Such a
jump could especially occur at speciation, i.e. dramatic change occurs that
drives the species apart. Here we derive a Central Limit Theorem for punctuated
equilibrium. We show that, if adaptation is fast, for weak convergence to
normality to hold, the variability in the occurrence of change has to disappear
with time.


title: The Massive Stellar Population of W49: A Spectroscopic Survey

abstract:
  Massive stars form on different scales ranging from large, dispersed OB
associations to compact, dense starburst clusters. The complex structure of
regions of massive star formation, and the involved short timescales provide a
challenge for our understanding of their birth and early evolution. As one of
the most massive and luminous star-forming region in our Galaxy, W49 is the
ideal place to study the formation of the most massive stars. By classifying
the massive young stars deeply embedded into the molecular cloud of W49, we aim
to investigate and trace the star formation history of this region. We analyse
near-infrared $K$-band spectroscopic observations of W49 from LBT/LUCI combined
with $JHK$ images obtained with NTT/SOFI and LBT/LUCI. Based on $JHK$-band
photometry and K-band spectroscopy the massive stars are placed in a
Hertzsprung Russell diagram. By comparison with evolutionary models, their age
and hence the star formation history of W49 can be investigated. Fourteen O
type stars as well as two young stellar objects (YSOs) are identified by our
spectroscopic survey. Eleven O-stars are main sequence stars with subtypes
ranging from O3 to O9.5, with masses ranging from $\sim 20$ $M_{\odot}$ to
$\sim 120$ $M_{\odot}$. Three of the O-stars show strong wind features, and are
considered to be Of-type supergiants with masses beyond 100 $M_{\odot}$ . The
two YSOs show CO emission, indicative for the presence of circumstellar disks
in the central region of the massive cluster. The age of the cluster is
estimated as $\sim1.5$ Myr, with star formation still ongoing in different
parts of the region. The ionising photons from the central massive stars have
not yet cleared the molecular cocoon surrounding the cluster. W49 is comparable
to extragalactic star-forming regions and provides us with an unique
possibility to study a starburst in detail.


title: The slowest spinning X-ray pulsar in an extragalactic globular cluster

abstract:
  Neutron stars are thought to be born rapidly rotating and then exhibit a
phase of a rotation-powered pulsations as they slow down to 1-10 s periods. The
significant population of millisecond pulsars observed in our Galaxy is
explained by the recycling concept: during an epoch of accretion from a donor
star in a binary system, the neutron star is spun up to millisecond periods.
However, only a few pulsars are observed during this recycling process, with
relatively high rotational frequencies. Here we report the detection of an
X-ray pulsar with $P_{\rm spin} = 1.20$ s in the globular cluster B091D in the
Andromeda galaxy, the slowest pulsar ever found in a globular cluster. This
bright (up-to 30% of the Eddington luminosity) spinning-up pulsar, persistent
over the 12 years of observations, must have started accreting less than 1 Myr
ago and has not yet had time to accelerate to hundreds of Hz. The neutron star
in this unique wide binary with an orbital period $P_{\rm orb} = 30.5$ h in a
12 Gyr old, metal rich star cluster, accretes from a low mass, slightly evolved
post-main sequence companion. We argue that we are witnessing a binary formed
at relatively recent epoch by getting a $\sim 0.8M_\odot$ star in a dynamical
interaction -- a viable scenario in a massive dense globular cluster like B091D
with high global and specific stellar encounter rates. This intensively
accreting non-recycled X-ray pulsar provides therefore a long-sought missing
piece in the standard pulsar recycling picture.


title: Indirect Detection Imprint of a CP Violating Dark Sector

abstract:
  We introduce a simple scenario involving fermionic dark matter ($\chi$) and
singlet scalar mediators that may account for the Galactic Center GeV
$\gamma$-ray excess while satisfying present direct detection constraints.
CP-violation in the scalar potential leads to mixing between the Standard Model
Higgs boson and the scalar singlet, resulting in three scalars $h_{1,2,3}$ of
indefinite CP-transformation properties. This mixing enables s-wave
$\chi{\bar\chi}$ annihilation into di-scalar states, followed by decays into
four fermion final states. The observed $\gamma$-ray spectrum can be fitted
while respecting present direct detection bounds and Higgs boson properties for
$m_{\chi} = 60 \sim 80 $ GeV, and $m_{h_3} \sim m_{\chi}$. Searches for the
Higgs exotic decay channel $h_1 \to h_3 h_3$ at the 14 TeV LHC should be able
to further probe the parameter region favored by the $\gamma$-ray excess.


title:
Suppression of quadrupole and octupole modes in red giants observed by
  Kepler

abstract:
  The asteroseismology of red giant stars has continued to yield surprises
since the onset of high-precision photometry from space-based observations. An
exciting new theoretical result shows that the previously observed suppression
of dipole oscillation modes in red giants can be used to detect strong magnetic
fields in the stellar cores. A fundamental facet of the theory is that nearly
all the mode energy leaking into the core is trapped by the magnetic greenhouse
effect. This results in clear predictions for how the mode visibility changes
as a star evolves up the red giant branch, and how that depends on stellar
mass, spherical degree, and mode lifetime. Here, we investigate the validity of
these predictions with a focus on the visibility of different spherical
degrees. We find that mode suppression weakens for higher degree modes with an
average reduction in the quadrupole mode visibility of up to 49% for the least
evolved stars in our sample, and no detectable suppression of octupole modes,
in agreement with the theoretical predictions. We furthermore find evidence for
the influence of increasing mode lifetimes on the measured visibilities along
the red giant branch, in agreement with previous independent observations.
These results support the theory that strong internal magnetic fields are
responsible for the observed suppression of non-radial modes in red giants. We
also find preliminary evidence that stars with suppressed dipole modes on
average have slightly lower metallicity than normal stars.