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Title: Radio-flaring Ultracool Dwarf Population Synthesis,
Abstract: Over a dozen ultracool dwarfs (UCDs), low-mass objects of spectral types
$\geq$M7, are known to be sources of radio flares. These typically
several-minutes-long radio bursts can be up to 100\% circularly polarized and
have high brightness temperatures, consistent with coherent emission via the
electron cyclotron maser operating in $\sim$kG magnetic fields. Recently, the
statistical properties of the bulk physical parameters that describe these UCDs
have become adequately described to permit synthesis of the population of
radio-flaring objects. For the first time, I construct a Monte Carlo simulator
to model the population of these radio-flaring UCDs. This simulator is powered
by Intel Secure Key (ISK)- a new processor technology that uses a local entropy
source to improve random number generation that has heretofore been used to
improve cryptography. The results from this simulator indicate that only
$\sim$5% of radio-flaring UCDs within the local interstellar neighborhood
($<$25 pc away) have been discovered. I discuss a number of scenarios which may
explain this radio-flaring fraction, and suggest that the observed behavior is
likely a result of several factors. The performance of ISK as compared to other
pseudorandom number generators is also evaluated, and its potential utility for
other astrophysical codes briefly described. | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Computer Science"
] |
Title: A New Taxonomy for Symbiotic EM Sensors,
Abstract: It is clear that the EM spectrum is now rapidly reaching saturation,
especially for frequencies below 10~GHz. Governments, who influence the
regulatory authorities around the world, have resorted to auctioning the use of
spectrum, in a sense to gauge the importance of a particular user. Billions of
USD are being paid for modest bandwidths.
The earth observation, astronomy and similar science driven communities
cannot compete financially with such a pressure system, so this is where
governments have to step in and assess /regulate the situation.
It has been a pleasure to see a situation where the communications and
broadcast communities have come together to formulate sharing of an important
part of the spectrum (roughly, 50 MHz to 800 MHz) in an IEEE standard,
IEEE802.22. This standard (known as the "TV White Space Network" (built on
lower level standards) shows a way that fixed and mobile users can collaborate
in geographically widespread regions, using cognitive radio and geographic
databases of users. This White Space (WS) standard is well described in the
literature and is not the major topic of this short paper.
We wish to extend the idea of the WS concept to include the idea of EM
sensors (such as Radar) adopting this approach to spectrum sharing, providing a
quantum leap in access to spectrum. We postulate that networks of sensors,
using the tools developed by the WS community, can replace and enhance our
present set of EM sensors.
We first define what Networks of Sensors entail (with some history), and then
go on to define, based on a Taxonomy of Symbiosis defined by de
Bary\cite{symb}, how these sensors and other users (especially communications)
can co-exist. This new taxonomy is important for understanding, and should
replace somewhat outdated terminologies from the radar world. | [
1,
0,
0,
0,
0,
0
] | [
"Physics",
"Computer Science"
] |
Title: Asymmetry-Induced Synchronization in Oscillator Networks,
Abstract: A scenario has recently been reported in which in order to stabilize complete
synchronization of an oscillator network---a symmetric state---the symmetry of
the system itself has to be broken by making the oscillators nonidentical. But
how often does such behavior---which we term asymmetry-induced synchronization
(AISync)---occur in oscillator networks? Here we present the first general
scheme for constructing AISync systems and demonstrate that this behavior is
the norm rather than the exception in a wide class of physical systems that can
be seen as multilayer networks. Since a symmetric network in complete synchrony
is the basic building block of cluster synchronization in more general
networks, AISync should be common also in facilitating cluster synchronization
by breaking the symmetry of the cluster subnetworks. | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Mathematics"
] |
Title: When Simpler Data Does Not Imply Less Information: A Study of User Profiling Scenarios with Constrained View of Mobile HTTP(S) Traffic,
Abstract: The exponential growth in smartphone adoption is contributing to the
availability of vast amounts of human behavioral data. This data enables the
development of increasingly accurate data-driven user models that facilitate
the delivery of personalized services which are often free in exchange for the
use of its customers' data. Although such usage conventions have raised many
privacy concerns, the increasing value of personal data is motivating diverse
entities to aggressively collect and exploit the data. In this paper, we unfold
profiling scenarios around mobile HTTP(S) traffic, focusing on those that have
limited but meaningful segments of the data. The capability of the scenarios to
profile personal information is examined with real user data, collected
in-the-wild from 61 mobile phone users for a minimum of 30 days. Our study
attempts to model heterogeneous user traits and interests, including
personality, boredom proneness, demographics, and shopping interests. Based on
our modeling results, we discuss various implications to personalization,
privacy, and personal data rights. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Statistics"
] |
Title: A model of electrical impedance tomography on peripheral nerves for a neural-prosthetic control interface,
Abstract: Objective: A model is presented to evaluate the viability of using electrical
impedance tomography (EIT) with a nerve cuff to record neural activity in
peripheral nerves. Approach: Established modelling approaches in neural-EIT are
expanded on to be used, for the first time, on myelinated fibres which are
abundant in mammalian peripheral nerves and transmit motor commands. Main
results: Fibre impedance models indicate activity in unmyelinated fibres can be
screened out using operating frequencies above 100 Hz. At 1 kHz and 10 mm
electrode spacing, impedance magnitude of inactive intra-fascicle tissue and
the fraction changes during neural activity are estimated to be 1,142
{\Omega}.cm and -8.8x10-4, respectively, with a transverse current, and 328
{\Omega}.cm & -0.30, respectively with a longitudinal current. We show that a
novel EIT drive and measurement electrode pattern which utilises longitudinal
current and longitudinal differential boundary voltage measurements could
distinguish activity in different fascicles of a three-fascicle mammalian nerve
using pseudo-experimental data synthesised to replicate real operating
conditions. Significance: The results of this study provide an estimate of the
transient change in impedance of intra-fascicle tissue during neural activity
in mammalian nerve, and present a viable EIT electrode pattern, both of which
are critical steps towards implementing EIT in a nerve cuff for neural
prosthetics interfaces. | [
0,
1,
0,
0,
0,
0
] | [
"Quantitative Biology",
"Physics"
] |
Title: Isolated resonances and nonlinear damping,
Abstract: We analyze isolated resonance curves (IRCs) in a single-degree-of-freedom
system with nonlinear damping. The adopted procedure exploits singularity
theory in conjunction with the harmonic balance method. The analysis unveils a
geometrical connection between the topology of the damping force and IRCs.
Specifically, we demonstrate that extremas and zeros of the damping force
correspond to the appearance and merging of IRCs. | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Mathematics"
] |
Title: Crowd Science: Measurements, Models, and Methods,
Abstract: The increasing practice of engaging crowds, where organizations use IT to
connect with dispersed individuals for explicit resource creation purposes, has
precipitated the need to measure the precise processes and benefits of these
activities over myriad different implementations. In this work, we seek to
address these salient and non-trivial considerations by laying a foundation of
theory, measures, and research methods that allow us to test crowd-engagement
efficacy across organizations, industries, technologies, and geographies. To do
so, we anchor ourselves in the Theory of Crowd Capital, a generalizable
framework for studying IT-mediated crowd-engagement phenomena, and put forth an
empirical apparatus of testable measures and generalizable methods to begin to
unify the field of crowd science. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Statistics"
] |
Title: X-Shooter study of accretion in Chamaeleon I: II. A steeper increase of accretion with stellar mass for very low mass stars?,
Abstract: The dependence of the mass accretion rate on the stellar properties is a key
constraint for star formation and disk evolution studies. Here we present a
study of a sample of stars in the Chamaeleon I star forming region carried out
using the VLT/X-Shooter spectrograph. The sample is nearly complete down to
M~0.1Msun for the young stars still harboring a disk in this region. We derive
the stellar and accretion parameters using a self-consistent method to fit the
broad-band flux-calibrated medium resolution spectrum. The correlation between
the accretion luminosity to the stellar luminosity, and of the mass accretion
rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3,
respectively. These slopes and the accretion rates are consistent with previous
results in various star forming regions and with different theoretical
frameworks. However, we find that a broken power-law fit, with a steeper slope
for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller
than ~ 0.3 Msun, is slightly preferred according to different statistical
tests, but the single power-law model is not excluded. The steeper relation for
lower mass stars can be interpreted as a faster evolution in the past for
accretion in disks around these objects, or as different accretion regimes in
different stellar mass ranges. Finally, we find two regions on the mass
accretion versus stellar mass plane empty of objects. One at high mass
accretion rates and low stellar masses, which is related to the steeper
dependence of the two parameters we derived. The second one is just above the
observational limits imposed by chromospheric emission. This empty region is
located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be
effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with
the one expected for photoevaporation to rapidly dissipate the inner disk. | [
0,
1,
0,
0,
0,
0
] | [
"Astrophysics"
] |
Title: A General Model for Robust Tensor Factorization with Unknown Noise,
Abstract: Because of the limitations of matrix factorization, such as losing spatial
structure information, the concept of low-rank tensor factorization (LRTF) has
been applied for the recovery of a low dimensional subspace from high
dimensional visual data. The low-rank tensor recovery is generally achieved by
minimizing the loss function between the observed data and the factorization
representation. The loss function is designed in various forms under different
noise distribution assumptions, like $L_1$ norm for Laplacian distribution and
$L_2$ norm for Gaussian distribution. However, they often fail to tackle the
real data which are corrupted by the noise with unknown distribution. In this
paper, we propose a generalized weighted low-rank tensor factorization method
(GWLRTF) integrated with the idea of noise modelling. This procedure treats the
target data as high-order tensor directly and models the noise by a Mixture of
Gaussians, which is called MoG GWLRTF. The parameters in the model are
estimated under the EM framework and through a new developed algorithm of
weighted low-rank tensor factorization. We provide two versions of the
algorithm with different tensor factorization operations, i.e., CP
factorization and Tucker factorization. Extensive experiments indicate the
respective advantages of this two versions in different applications and also
demonstrate the effectiveness of MoG GWLRTF compared with other competing
methods. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics",
"Statistics"
] |
Title: Observation of topological valley transport of sound in sonic crystals,
Abstract: Valley pseudospin, labeling quantum states of energy extrema in momentum
space, is attracting tremendous attention1-13 because of its potential in
constructing new carrier of information. Compared with the non-topological bulk
valley transport realized soon after predictions1-5, the topological valley
transport in domain walls6-13 is extremely challenging owing to the
inter-valley scattering inevitably induced by atomic scale imperfectness, until
the recent electronic signature observed in bilayer graphene12,13. Here we
report the first experimental observation of topological valley transport of
sound in sonic crystals. The macroscopic nature of sonic crystals permits the
flexible and accurate design of domain walls. In addition to a direct
visualization of the valley-selective edge modes through spatial scanning of
sound field, reflection immunity is observed in sharply curved interfaces. The
topologically protected interface transport of sound, strikingly different from
that in traditional sound waveguides14,15, may serve as the basis of designing
devices with unconventional functions. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Test results of a prototype device to calibrate the Large Size Telescope camera proposed for the Cherenkov Telescope Array,
Abstract: A Large Size air Cherenkov Telescope (LST) prototype, proposed for the
Cherenkov Telescope Array (CTA), is under construction at the Canary Island of
La Palma (Spain) this year. The LST camera, which comprises an array of about
500 photomultipliers (PMTs), requires a precise and regular calibration over a
large dynamic range, up to $10^3$ photo-electrons (pe's), for each PMT. We
present a system built to provide the optical calibration of the camera
consisting of a pulsed laser (355 nm wavelength, 400 ps pulse width), a set of
filters to guarantee a large dynamic range of photons on the sensors, and a
diffusing sphere to uniformly spread the laser light, with flat fielding within
3%, over the camera focal plane 28 m away. The prototype of the system
developed at INFN is hermetically closed and filled with dry air to make the
system completely isolated from the external environment. In the paper we
present the results of the tests for the evaluation of the photon density at
the camera plane, the system isolation from the environment, and the shape of
the signal as detected by the PMTs. The description of the communication of the
system with the rest of detector is also given. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: The Physics of Eccentric Binary Black Hole Mergers. A Numerical Relativity Perspective,
Abstract: Gravitational wave observations of eccentric binary black hole mergers will
provide unequivocal evidence for the formation of these systems through
dynamical assembly in dense stellar environments. The study of these
astrophysically motivated sources is timely in view of electromagnetic
observations, consistent with the existence of stellar mass black holes in the
globular cluster M22 and in the Galactic center, and the proven detection
capabilities of ground-based gravitational wave detectors. In order to get
insights into the physics of these objects in the dynamical, strong-field
gravity regime, we present a catalog of 89 numerical relativity waveforms that
describe binary systems of non-spinning black holes with mass-ratios $1\leq q
\leq 10$, and initial eccentricities as high as $e_0=0.18$ fifteen cycles
before merger. We use this catalog to provide landmark results regarding the
loss of energy through gravitational radiation, both for quadrupole and
higher-order waveform multipoles, and the astrophysical properties, final mass
and spin, of the post-merger black hole as a function of eccentricity and
mass-ratio. We discuss the implications of these results for gravitational wave
source modeling, and the design of algorithms to search for and identify the
complex signatures of these events in realistic detection scenarios. | [
1,
0,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Cost-Effective Cache Deployment in Mobile Heterogeneous Networks,
Abstract: This paper investigates one of the fundamental issues in cache-enabled
heterogeneous networks (HetNets): how many cache instances should be deployed
at different base stations, in order to provide guaranteed service in a
cost-effective manner. Specifically, we consider two-tier HetNets with
hierarchical caching, where the most popular files are cached at small cell
base stations (SBSs) while the less popular ones are cached at macro base
stations (MBSs). For a given network cache deployment budget, the cache sizes
for MBSs and SBSs are optimized to maximize network capacity while satisfying
the file transmission rate requirements. As cache sizes of MBSs and SBSs affect
the traffic load distribution, inter-tier traffic steering is also employed for
load balancing. Based on stochastic geometry analysis, the optimal cache sizes
for MBSs and SBSs are obtained, which are threshold-based with respect to cache
budget in the networks constrained by SBS backhauls. Simulation results are
provided to evaluate the proposed schemes and demonstrate the applications in
cost-effective network deployment. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science"
] |
Title: Experimental demonstration of a Josephson magnetic memory cell with a programmable π-junction,
Abstract: We experimentally demonstrate the operation of a Josephson magnetic random
access memory unit cell, built with a Ni_80Fe_20/Cu/Ni pseudo spin-valve
Josephson junction with Nb electrodes and an integrated readout SQUID in a
fully planarized Nb fabrication process. We show that the parallel and
anti-parallel memory states of the spin-valve can be mapped onto a junction
equilibrium phase of either zero or pi by appropriate choice of the ferromagnet
thicknesses, and that the magnetic Josephson junction can be written to either
a zero-junction or pi-junction state by application of write fields of
approximately 5 mT. This work represents a first step towards a scalable,
dense, and power-efficient cryogenic memory for superconducting
high-performance digital computing. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Adaptive Mesh Refinement in Analog Mesh Computers,
Abstract: The call for efficient computer architectures has introduced a variety of
application-specific compute engines to the heterogeneous computing landscape.
One particular engine, the analog mesh computer, has been well received due to
its ability to efficiently solve partial differential equations by eliminating
the iterative stages common to numerical solvers. This article introduces an
implementation of refinement for analog mesh computers. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Physics"
] |
Title: On the local view of atmospheric available potential energy,
Abstract: The possibility of constructing Lorenz's concept of available potential
energy (APE) from a local principle has been known for some time, but has
received very little attention so far. Yet, the local APE framework offers the
advantage of providing a positive definite local form of potential energy,
which like kinetic energy can be transported, converted, and created/dissipated
locally. In contrast to Lorenz's definition, which relies on the exact from of
potential energy, the local APE theory uses the particular form of potential
energy appropriate to the approximations considered. In this paper, this idea
is illustrated for the dry hydrostatic primitive equations, whose relevant form
of potential energy is the specific enthalpy. The local APE density is
non-quadratic in general, but can nevertheless be partitioned exactly into mean
and eddy components regardless of the Reynolds averaging operator used.
This paper introduces a new form of the local APE that is easily computable
from atmospheric datasets. The advantages of using the local APE over the
classical Lorenz APE are highlighted. The paper also presents the first
calculation of the three-dimensional local APE in observation-based atmospheric
data. Finally, it illustrates how the eddy and mean components of the local APE
can be used to study regional and temporal variability in the large-scale
circulation. It is revealed that advection from high latitudes is necessary to
supply APE into the storm track regions, and that Greenland and Ross Sea, which
have suffered from rapid land ice and sea ice loss in recent decades, are
particularly susceptible to APE variability. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Towards Optimally Decentralized Multi-Robot Collision Avoidance via Deep Reinforcement Learning,
Abstract: Developing a safe and efficient collision avoidance policy for multiple
robots is challenging in the decentralized scenarios where each robot generate
its paths without observing other robots' states and intents. While other
distributed multi-robot collision avoidance systems exist, they often require
extracting agent-level features to plan a local collision-free action, which
can be computationally prohibitive and not robust. More importantly, in
practice the performance of these methods are much lower than their centralized
counterparts.
We present a decentralized sensor-level collision avoidance policy for
multi-robot systems, which directly maps raw sensor measurements to an agent's
steering commands in terms of movement velocity. As a first step toward
reducing the performance gap between decentralized and centralized methods, we
present a multi-scenario multi-stage training framework to find an optimal
policy which is trained over a large number of robots on rich, complex
environments simultaneously using a policy gradient based reinforcement
learning algorithm. We validate the learned sensor-level collision avoidance
policy in a variety of simulated scenarios with thorough performance
evaluations and show that the final learned policy is able to find time
efficient, collision-free paths for a large-scale robot system. We also
demonstrate that the learned policy can be well generalized to new scenarios
that do not appear in the entire training period, including navigating a
heterogeneous group of robots and a large-scale scenario with 100 robots.
Videos are available at this https URL | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Robotics"
] |
Title: The Robustness of LWPP and WPP, with an Application to Graph Reconstruction,
Abstract: We show that the counting class LWPP [FFK94] remains unchanged even if one
allows a polynomial number of gap values rather than one. On the other hand, we
show that it is impossible to improve this from polynomially many gap values to
a superpolynomial number of gap values by relativizable proof techniques.
The first of these results implies that the Legitimate Deck Problem (from the
study of graph reconstruction) is in LWPP (and thus low for PP, i.e., $\rm
PP^{\mbox{Legitimate Deck}} = PP$) if the weakened version of the
Reconstruction Conjecture holds in which the number of nonisomorphic preimages
is assumed merely to be polynomially bounded. This strengthens the 1992 result
of Köbler, Schöning, and Torán [KST92] that the Legitimate Deck
Problem is in LWPP if the Reconstruction Conjecture holds, and provides
strengthened evidence that the Legitimate Deck Problem is not NP-hard.
We additionally show on the one hand that our main LWPP robustness result
also holds for WPP, and also holds even when one allows both the rejection- and
acceptance- gap-value targets to simultaneously be polynomial-sized lists; yet
on the other hand, we show that for the #P-based analog of LWPP the behavior
much differs in that, in some relativized worlds, even two target values
already yield a richer class than one value does. Despite that nonrobustness
result for a #P-based class, we show that the #P-based "exact counting" class
$\rm C_{=}P$ remains unchanged even if one allows a polynomial number of target
values for the number of accepting paths of the machine. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics"
] |
Title: Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques,
Abstract: Current-induced spin-orbit torques (SOTs) represent one of the most effective
ways to manipulate the magnetization in spintronic devices. The orthogonal
torque-magnetization geometry, the strong damping, and the large domain wall
velocities inherent to materials with strong spin-orbit coupling make SOTs
especially appealing for fast switching applications in nonvolatile memory and
logic units. So far, however, the timescale and evolution of the magnetization
during the switching process have remained undetected. Here, we report the
direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO$_x$ dots
during current pulse injection. Time-resolved x-ray images with 25 nm spatial
and 100 ps temporal resolution reveal that switching is achieved within the
duration of a sub-ns current pulse by the fast nucleation of an inverted domain
at the edge of the dot and propagation of a tilted domain wall across the dot.
The nucleation point is deterministic and alternates between the four dot
quadrants depending on the sign of the magnetization, current, and external
field. Our measurements reveal how the magnetic symmetry is broken by the
concerted action of both damping-like and field-like SOT and show that
reproducible switching events can be obtained for over $10^{12}$ reversal
cycles. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: To Pool or Not To Pool? Revisiting an Old Pattern,
Abstract: We revisit the well-known object-pool design pattern in Java. In the last
decade, the pattern has attracted a lot of criticism regarding its validity
when used for light-weight objects that are only meant to hold memory rather
than any other resources (database connections, sockets etc.) and in fact,
common opinion holds that is an anti-pattern in such cases. Nevertheless, we
show through several experiments in different systems that the use of this
pattern for extremely short-lived and light-weight memory objects can in fact
significantly reduce the response time of high-performance multi-threaded
applications, especially in memory-constrained environments. In certain
multi-threaded applications where high performance is a requirement and/or
memory constraints exist, we recommend therefore that the object pool pattern
be given consideration and tested for possible run-time as well as memory
footprint improvements. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science"
] |
Title: Quenching of supermassive black hole growth around the apparent maximum mass,
Abstract: Recent quasar surveys have revealed that supermassive black holes (SMBHs)
rarely exceed a mass of $M_{\rm BH} \sim {\rm a~few}\times10^{10}~M_{\odot}$
during the entire cosmic history. It has been argued that quenching of the BH
growth is caused by a transition of a nuclear accretion disk into an advection
dominated accretion flow, with which strong outflows and/or jets are likely to
be associated. We investigate a relation between the maximum mass of SMBHs and
the radio-loudness of quasars with a well-defined sample of $\sim 10^5$ quasars
at a redshift range of $0<z<2$, obtained from the Sloan Digital Sky Surveys DR7
catalog. We find that the number fraction of the radio-loud (RL) quasars
increases above a threshold of $M_{\rm BH} \simeq 10^{9.5}~M_{\odot}$,
independent of their redshifts. Moreover, the number fraction of RL quasars
with lower Eddington ratios (out of the whole RL quasars), indicating lower
accretion rates, increases above the critical BH mass. These observational
trends can be natural consequences of the proposed scenario of suppressing BH
growth around the apparent maximum mass of $\sim 10^{10}~M_{\odot}$. The
ongoing VLA Sky Survey in radio will allow us to estimate of the exact number
fraction of RL quasars more precisely, which gives further insights to
understand quenching processes for BH growth. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Phase induced transparency mediated structured beam generation in a closed-loop tripod configuration,
Abstract: We present a phase induced transparency based scheme to generate structured
beam patterns in a closed four level atomic system. We employ phase structured
probe beam and a transverse magnetic field (TMF) to create phase dependent
medium susceptibility. We show that such phase dependent modulation of
absorption holds the key to formation of a structured beam. We use a full
density matrix formalism to explain the experiments of Radwell et al. [Phys.
Rev. Lett. 114,123603 (2015)] at weak probe limits. Our numerical results on
beam propagation confirms that the phase information present in the absorption
profile gets encoded on the spatial probe envelope which creates petal-like
structures even in the strong field limit. The contrast of the formed
structured beam can be enhanced by changing the strength of TMF as well as of
the probe intensity. In weak field limits an absorption profile is solely
responsible for creating a structured beam, whereas in the strong probe regime,
both dispersion and absorption profiles facilitate the generation of high
contrast structured beam. Furthermore we find the rotation of structured beams
owing to strong field induced nonlinear magneto-optical rotation (NMOR). | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Phasebook and Friends: Leveraging Discrete Representations for Source Separation,
Abstract: Deep learning based speech enhancement and source separation systems have
recently reached unprecedented levels of quality, to the point that performance
is reaching a new ceiling. Most systems rely on estimating the magnitude of a
target source by estimating a real-valued mask to be applied to a
time-frequency representation of the mixture signal. A limiting factor in such
approaches is a lack of phase estimation: the phase of the mixture is most
often used when reconstructing the estimated time-domain signal. Here, we
propose `MagBook', `phasebook', and `Combook', three new types of layers based
on discrete representations that can be used to estimate complex time-frequency
masks. MagBook layers extend classical sigmoidal units and a recently
introduced convex softmax activation for mask-based magnitude estimation.
Phasebook layers use a similar structure to give an estimate of the phase mask
without suffering from phase wrapping issues. Combook layers are an alternative
to the MagBook-Phasebook combination that directly estimate complex masks. We
present various training and inference regimes involving these representations,
and explain in particular how to include them in an end-to-end learning
framework. We also present an oracle study to assess upper bounds on
performance for various types of masks using discrete phase representations. We
evaluate the proposed methods on the wsj0-2mix dataset, a well-studied corpus
for single-channel speaker-independent speaker separation, matching the
performance of state-of-the-art mask-based approaches without requiring
additional phase reconstruction steps. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science"
] |
Title: Adsorption and desorption of hydrogen at nonpolar GaN(1-100) surfaces: Kinetics and impact on surface vibrational and electronic properties,
Abstract: The adsorption of hydrogen at nonpolar GaN(1-100) surfaces and its impact on
the electronic and vibrational properties is investigated using surface
electron spectroscopy in combination with density functional theory (DFT)
calculations. For the surface mediated dissociation of H2 and the subsequent
adsorption of H, an energy barrier of 0.55 eV has to be overcome. The
calculated kinetic surface phase diagram indicates that the reaction is
kinetically hindered at low pressures and low temperatures. At higher
temperatures ab-initio thermodynamics show, that the H-free surface is
energetically favored. To validate these theoretical predictions experiments at
room temperature and under ultrahigh vacuum conditions were performed. They
reveal that molecular hydrogen does not dissociatively adsorb at the GaN(1-100)
surface. Only activated atomic hydrogen atoms attach to the surface. At
temperatures above 820 K, the attached hydrogen gets desorbed. The adsorbed
hydrogen atoms saturate the dangling bonds of the gallium and nitrogen surface
atoms and result in an inversion of the Ga-N surface dimer buckling. The
signatures of the Ga-H and N-H vibrational modes on the H-covered surface have
experimentally been identified and are in good agreement with the DFT
calculations of the surface phonon modes. Both theory and experiment show that
H adsorption results in a removal of occupied and unoccupied intragap electron
states of the clean GaN(1-100) surface and a reduction of the surface upward
band bending by 0.4 eV. The latter mechanism largely reduces surface electron
depletion. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Application of the Fast Multipole Fully Coupled Poroelastic Displacement Discontinuity Method to Hydraulic Fracturing Problems,
Abstract: In this study, a fast multipole method (FMM) is used to decrease the
computational time of a fully-coupled poroelastic hydraulic fracture model with
a controllable effect on its accuracy. The hydraulic fracture model is based on
the poroelastic formulation of the displacement discontinuity method (DDM)
which is a special formulation of the boundary element method (BEM). DDM is a
powerful and efficient method for problems involving fractures. However, this
method becomes slow as the number of temporal, or spatial elements increases,
or necessary details such as poroelasticity, that makes the solution
history-dependent, are added to the model. FMM is a technique to expedite
matrix-vector multiplications within a controllable error without forming the
matrix explicitly. Fully-coupled poroelastic formulation of DDM involves the
multiplication of a dense matrix with a vector in several places. A crucial
modification to DDM is suggested in two places in the algorithm to leverage the
speed efficiency of FMM for carrying out these multiplications. The first
modification is in the time-marching scheme, which accounts for the solution of
previous time steps to compute the current time step. The second modification
is in the generalized minimal residual method (GMRES) to iteratively solve for
the problem unknowns. Several examples are provided to show the efficiency of
the proposed approach in problems with large degrees of freedom (in time and
space). Examples include hydraulic fracturing of a horizontal well and randomly
distributed pressurized fractures at different orientations with respect to
horizontal stresses. The results are compared to the conventional DDM in terms
of computational processing time and accuracy. Accordingly, the proposed
algorithm may be used for fracture propagation studies while substantially
reducing the processing time with a controllable error. | [
1,
0,
0,
0,
0,
0
] | [
"Physics",
"Mathematics"
] |
Title: Two-level Chebyshev filter based complementary subspace method: pushing the envelope of large-scale electronic structure calculations,
Abstract: We describe a novel iterative strategy for Kohn-Sham density functional
theory calculations aimed at large systems (> 1000 electrons), applicable to
metals and insulators alike. In lieu of explicit diagonalization of the
Kohn-Sham Hamiltonian on every self-consistent field (SCF) iteration, we employ
a two-level Chebyshev polynomial filter based complementary subspace strategy
to: 1) compute a set of vectors that span the occupied subspace of the
Hamiltonian; 2) reduce subspace diagonalization to just partially occupied
states; and 3) obtain those states in an efficient, scalable manner via an
inner Chebyshev-filter iteration. By reducing the necessary computation to just
partially occupied states, and obtaining these through an inner Chebyshev
iteration, our approach reduces the cost of large metallic calculations
significantly, while eliminating subspace diagonalization for insulating
systems altogether. We describe the implementation of the method within the
framework of the Discontinuous Galerkin (DG) electronic structure method and
show that this results in a computational scheme that can effectively tackle
bulk and nano systems containing tens of thousands of electrons, with chemical
accuracy, within a few minutes or less of wall clock time per SCF iteration on
large-scale computing platforms. We anticipate that our method will be
instrumental in pushing the envelope of large-scale ab initio molecular
dynamics. As a demonstration of this, we simulate a bulk silicon system
containing 8,000 atoms at finite temperature, and obtain an average SCF step
wall time of 51 seconds on 34,560 processors; thus allowing us to carry out 1.0
ps of ab initio molecular dynamics in approximately 28 hours (of wall time). | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Mathematics",
"Computer Science"
] |
Title: SRM: An Efficient Framework for Autonomous Robotic Exploration in Indoor Environments,
Abstract: In this paper, we propose an integrated framework for the autonomous robotic
exploration in indoor environments. Specially, we present a hybrid map, named
Semantic Road Map (SRM), to represent the topological structure of the explored
environment and facilitate decision-making in the exploration. The SRM is built
incrementally along with the exploration process. It is a graph structure with
collision-free nodes and edges that are generated within the sensor coverage.
Moreover, each node has a semantic label and the expected information gain at
that location. Based on the concise SRM, we present a novel and effective
decision-making model to determine the next-best-target (NBT) during the
exploration. The model concerns the semantic information, the information gain,
and the path cost to the target location. We use the nodes of SRM to represent
the candidate targets, which enables the target evaluation to be performed
directly on the SRM. With the SRM, both the information gain of a node and the
path cost to the node can be obtained efficiently. Besides, we adopt the
cross-entropy method to optimize the path to make it more informative. We
conduct experimental studies in both simulated and real-world environments,
which demonstrate the effectiveness of the proposed method. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Robotics"
] |
Title: Electrocaloric effects in the lead-free Ba(Zr,Ti)O$_{3}$ relaxor ferroelectric from atomistic simulations,
Abstract: Atomistic effective Hamiltonian simulations are used to investigate
electrocaloric (EC) effects in the lead-free Ba(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$
(BZT) relaxor ferroelectric. We find that the EC coefficient varies
non-monotonically with the field at any temperature, presenting a maximum that
can be traced back to the behavior of BZT's polar nanoregions. We also
introduce a simple Landau-based model that reproduces the EC behavior of BZT as
a function of field and temperature, and which is directly applicable to other
compounds. Finally, we confirm that, for low temperatures (i.e., in non-ergodic
conditions), the usual indirect approach to measure the EC response provides an
estimate that differs quantitatively from a direct evaluation of the
field-induced temperature change. | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Materials Science"
] |
Title: Who Said What: Modeling Individual Labelers Improves Classification,
Abstract: Data are often labeled by many different experts with each expert only
labeling a small fraction of the data and each data point being labeled by
several experts. This reduces the workload on individual experts and also gives
a better estimate of the unobserved ground truth. When experts disagree, the
standard approaches are to treat the majority opinion as the correct label or
to model the correct label as a distribution. These approaches, however, do not
make any use of potentially valuable information about which expert produced
which label. To make use of this extra information, we propose modeling the
experts individually and then learning averaging weights for combining them,
possibly in sample-specific ways. This allows us to give more weight to more
reliable experts and take advantage of the unique strengths of individual
experts at classifying certain types of data. Here we show that our approach
leads to improvements in computer-aided diagnosis of diabetic retinopathy. We
also show that our method performs better than competing algorithms by Welinder
and Perona (2010), and by Mnih and Hinton (2012). Our work offers an innovative
approach for dealing with the myriad real-world settings that use expert
opinions to define labels for training. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Statistics"
] |
Title: Operational Semantics of Process Monitors,
Abstract: CSPe is a specification language for runtime monitors that can directly
express concurrency in a bottom-up manner that composes the system from
simpler, interacting components. It includes constructs to explicitly flag
failures to the monitor, which unlike deadlocks and livelocks in conventional
process algebras, propagate globally and aborts the whole system's execution.
Although CSPe has a trace semantics along with an implementation demonstrating
acceptable performance, it lacks an operational semantics. An operational
semantics is not only more accessible than trace semantics but also
indispensable for ensuring the correctness of the implementation. Furthermore,
a process algebra like CSPe admits multiple denotational semantics appropriate
for different purposes, and an operational semantics is the basis for
justifying such semantics' integrity and relevance. In this paper, we develop
an SOS-style operational semantics for CSPe, which properly accounts for
explicit failures and will serve as a basis for further study of its
properties, its optimization, and its use in runtime verification. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics"
] |
Title: Anyonic Entanglement and Topological Entanglement Entropy,
Abstract: We study the properties of entanglement in two-dimensional topologically
ordered phases of matter. Such phases support anyons, quasiparticles with
exotic exchange statistics. The emergent nonlocal state spaces of anyonic
systems admit a particular form of entanglement that does not exist in
conventional quantum mechanical systems. We study this entanglement by adapting
standard notions of entropy to anyonic systems. We use the algebraic theory of
anyon models (modular tensor categories) to illustrate the nonlocal
entanglement structure of anyonic systems. Using this formalism, we present a
general method of deriving the universal topological contributions to the
entanglement entropy for general system configurations of a topological phase,
including surfaces of arbitrary genus, punctures, and quasiparticle content. We
analyze a number of examples in detail. Our results recover and extend prior
results for anyonic entanglement and the topological entanglement entropy. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Thermal diffusivity and chaos in metals without quasiparticles,
Abstract: We study the thermal diffusivity $D_T$ in models of metals without
quasiparticle excitations (`strange metals'). The many-body quantum chaos and
transport properties of such metals can be efficiently described by a
holographic representation in a gravitational theory in an emergent curved
spacetime with an additional spatial dimension. We find that at generic
infra-red fixed points $D_T$ is always related to parameters characterizing
many-body quantum chaos: the butterfly velocity $v_B$, and Lyapunov time
$\tau_L$ through $D_T \sim v_B^2 \tau_L$. The relationship holds independently
of the charge density, periodic potential strength or magnetic field at the
fixed point. The generality of this result follows from the observation that
the thermal conductivity of strange metals depends only on the metric near the
horizon of a black hole in the emergent spacetime, and is otherwise insensitive
to the profile of any matter fields. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Electrical control of metallic heavy-metal/ferromagnet interfacial states,
Abstract: Voltage control effects provide an energy-efficient means of tailoring
material properties, especially in highly integrated nanoscale devices.
However, only insulating and semiconducting systems can be controlled so far.
In metallic systems, there is no electric field due to electron screening
effects and thus no such control effect exists. Here we demonstrate that
metallic systems can also be controlled electrically through ionic not
electronic effects. In a Pt/Co structure, the control of the metallic Pt/Co
interface can lead to unprecedented control effects on the magnetic properties
of the entire structure. Consequently, the magnetization and perpendicular
magnetic anisotropy of the Co layer can be independently manipulated to any
desired state, the efficient spin toques can be enhanced about 3.5 times, and
the switching current can be reduced about one order of magnitude. This ability
to control a metallic system may be extended to control other physical
phenomena. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: An Improved SCFlip Decoder for Polar Codes,
Abstract: This paper focuses on the recently introduced Successive Cancellation Flip
(SCFlip) decoder of polar codes. Our contribution is twofold. First, we propose
the use of an optimized metric to determine the flipping positions within the
SCFlip decoder, which improves its ability to find the first error that
occurred during the initial SC decoding attempt. We also show that the proposed
metric allows closely approaching the performance of an ideal SCFlip decoder.
Second, we introduce a generalisation of the SCFlip decoder to a number of
$\omega$ nested flips, denoted by SCFlip-$\omega$, using a similar optimized
metric to determine the positions of the nested flips. We show that the
SCFlip-2 decoder yields significant gains in terms of decoding performance and
competes with the performance of the CRC-aided SC-List decoder with list size
L=4, while having an average decoding complexity similar to that of the
standard SC decoding, at medium to high signal to noise ratio. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics"
] |
Title: Static Dalvik VM bytecode instrumentation,
Abstract: This work proposes a novel approach to restricting the access for blacklisted
Android system API calls. Main feature of the suggested method introduced in
this paper is that it requires only rootless or (user-mode) access to the
system unlike previous works. For that reason this method is valuable for
end-users due to the possibility of project distribution via Play Market and it
does not require any phone system modifications and/or updates. This paper
explains the required background of Android OS necessary for understanding and
describes the method for modification Android application. In this paper the
proof-of-concept implementation. That is able to block the application's IMEI
requests is introduced. Also this paper lists unsuccessful methods that tried
to provide the user security. Obviously with those restrictions application may
lack some of features that can only be granted in unsecured environment. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science"
] |
Title: Nonlinear Unknown Input and State Estimation Algorithm in Mobile Robots,
Abstract: This technical report provides the description and the derivation of a novel
nonlinear unknown input and state estimation algorithm (NUISE) for mobile
robots. The algorithm is designed for real-world robots with nonlinear dynamic
models and subject to stochastic noises on sensing and actuation. Leveraging
sensor readings and planned control commands, the algorithm detects and
quantifies anomalies on both sensors and actuators. Later, we elaborate the
dynamic models of two distinctive mobile robots for the purpose of
demonstrating the application of NUISE. This report serves as a supplementary
document for [1]. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics"
] |
Title: Personalized Driver Stress Detection with Multi-task Neural Networks using Physiological Signals,
Abstract: Stress can be seen as a physiological response to everyday emotional, mental
and physical challenges. A long-term exposure to stressful situations can have
negative health consequences, such as increased risk of cardiovascular diseases
and immune system disorder. Therefore, a timely stress detection can lead to
systems for better management and prevention in future circumstances. In this
paper, we suggest a multi-task learning based neural network approach (with
hard parameter sharing of mutual representation and task-specific layers) for
personalized stress recognition using skin conductance and heart rate from
wearable devices. The proposed method is tested on multi-modal physiological
responses collected during real-world and simulator driving tasks. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Quantitative Biology"
] |
Title: Evaluating the Robustness of Rogue Waves Under Perturbations,
Abstract: Rogue waves, and their periodic counterparts, have been shown to exist in a
number of integrable models. However, relatively little is known about the
existence of these objects in models where an exact formula is unattainable. In
this work, we develop a novel numerical perspective towards identifying such
states as localized solutions in space-time. Importantly, we illustrate that
this methodology in addition to benchmarking known solutions (and confirming
their numerical propagation under controllable error) enables the continuation
of such solutions over parametric variations to non-integrable models. As a
result, we can answer in the positive the question about the parametric
robustness of Peregrine-like waveforms and even of generalizations thereof on a
cnoidal wave background. | [
0,
1,
0,
0,
0,
0
] | [
"Physics",
"Mathematics"
] |
Title: Experimental study of electron and phonon dynamics in nanoscale materials by ultrafast laser time-domain spectroscopy,
Abstract: With the rapid advances in the development of nanotechnology, nowadays, the
sizes of elementary unit, i.e. transistor, of micro- and nanoelectronic devices
are well deep into nanoscale. For the pursuit of cheaper and faster nanoscale
electronic devices, the size of transistors keeps scaling down. As the
miniaturization of the nanoelectronic devices, the electrical resistivity
increases dramatically, resulting rapid growth in the heat generation. The heat
generation and limited thermal dissipation in nanoscale materials have become a
critical problem in the development of the next generation nanoelectronic
devices. Copper (Cu) is widely used conducting material in nanoelectronic
devices, and the electron-phonon scattering is the dominant contributor to the
resistivity in Cu nanowires at room temperature. Meanwhile, phonons are the
main carriers of heat in insulators, intrinsic and lightly doped
semiconductors. The thermal transport is an ensemble of phonon transport, which
strongly depends on the phonon frequency. In addition, the phonon transport in
nanoscale materials can behave fundamentally different than in bulk materials,
because of the spatial confinement. However, the size effect on electron-phonon
scattering and frequency dependent phonon transport in nanoscale materials
remain largely unexplored, due to the lack of suitable experimental techniques.
This thesis is mainly focusing on the study of carrier dynamics and acoustic
phonon transport in nanoscale materials. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: Comparing the dark matter models, modified Newtonian dynamics and modified gravity in accounting for the galaxy rotation curves,
Abstract: We compare six models (including the baryonic model, two dark matter models,
two modified Newtonian dynamics models and one modified gravity model) in
accounting for the galaxy rotation curves. For the dark matter models, we
assume NFW profile and core-modified profile for the dark halo, respectively.
For the modified Newtonian dynamics models, we discuss Milgrom's MOND theory
with two different interpolation functions, i.e. the standard and the simple
interpolation functions. As for the modified gravity, we focus on Moffat's MSTG
theory. We fit these models to the observed rotation curves of 9 high-surface
brightness and 9 low-surface brightness galaxies. We apply the Bayesian
Information Criterion and the Akaike Information Criterion to test the
goodness-of-fit of each model. It is found that non of the six models can well
fit all the galaxy rotation curves. Two galaxies can be best fitted by the
baryonic model without involving the nonluminous dark matter. MOND can fit the
largest number of galaxies, and only one galaxy can be best fitted by MSTG
model. Core-modified model can well fit about one half LSB galaxies but no HSB
galaxy, while NFW model can fit only a small fraction of HSB galaxies but no
LSB galaxy. This may imply that the oversimplified NFW and Core-modified
profiles couldn't well mimic the postulated dark matter halo. | [
0,
1,
0,
0,
0,
0
] | [
"Physics"
] |
Title: A study on text-score disagreement in online reviews,
Abstract: In this paper, we focus on online reviews and employ artificial intelligence
tools, taken from the cognitive computing field, to help understanding the
relationships between the textual part of the review and the assigned numerical
score. We move from the intuitions that 1) a set of textual reviews expressing
different sentiments may feature the same score (and vice-versa); and 2)
detecting and analyzing the mismatches between the review content and the
actual score may benefit both service providers and consumers, by highlighting
specific factors of satisfaction (and dissatisfaction) in texts.
To prove the intuitions, we adopt sentiment analysis techniques and we
concentrate on hotel reviews, to find polarity mismatches therein. In
particular, we first train a text classifier with a set of annotated hotel
reviews, taken from the Booking website. Then, we analyze a large dataset, with
around 160k hotel reviews collected from Tripadvisor, with the aim of detecting
a polarity mismatch, indicating if the textual content of the review is in
line, or not, with the associated score.
Using well established artificial intelligence techniques and analyzing in
depth the reviews featuring a mismatch between the text polarity and the score,
we find that -on a scale of five stars- those reviews ranked with middle scores
include a mixture of positive and negative aspects.
The approach proposed here, beside acting as a polarity detector, provides an
effective selection of reviews -on an initial very large dataset- that may
allow both consumers and providers to focus directly on the review subset
featuring a text/score disagreement, which conveniently convey to the user a
summary of positive and negative features of the review target. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Statistics"
] |
Title: Determining r-Robustness of Arbitrary Digraphs Using Zero-One Linear Integer Programming,
Abstract: There has been an increase in the use of resilient control algorithms based
on the graph theoretic properties of $r$- and $(r,s)$-robustness. These
algorithms guarantee consensus of normally behaving agents in the presence of a
bounded number of arbitrarily misbehaving agents if the values of the integers
$r$ and $s$ are sufficiently high. However, determining the largest integer $r$
for which an arbitrary digraph is $r$-robust is highly nontrivial. This paper
introduces a novel method for calculating this value using zero-one integer
programming. The method only requires knowledge of the graph Laplacian matrix,
and can be formulated with affine objective and constraints, except for the
integer constraint. Integer programming methods such as branch-and-bound can
allow both lower and upper bounds on $r$ to be iteratively tightened.
Simulations suggest the proposed method demonstrates greater efficiency than
prior algorithms. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Mathematics"
] |
Title: Multilayer Network Model of Movie Script,
Abstract: Network models have been increasingly used in the past years to support
summarization and analysis of narratives, such as famous TV series, books and
news. Inspired by social network analysis, most of these models focus on the
characters at play. The network model well captures all characters
interactions, giving a broad picture of the narration's content. A few works
went beyond by introducing additional semantic elements, always captured in a
single layer network. In contrast, we introduce in this work a multilayer
network model to capture more elements of the narration of a movie from its
script: people, locations, and other semantic elements. This model enables new
measures and insights on movies. We demonstrate this model on two very popular
movies. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science",
"Quantitative Biology"
] |
Title: Object Detection Using Deep CNNs Trained on Synthetic Images,
Abstract: The need for large annotated image datasets for training Convolutional Neural
Networks (CNNs) has been a significant impediment for their adoption in
computer vision applications. We show that with transfer learning an effective
object detector can be trained almost entirely on synthetically rendered
datasets. We apply this strategy for detecting pack- aged food products
clustered in refrigerator scenes. Our CNN trained only with 4000 synthetic
images achieves mean average precision (mAP) of 24 on a test set with 55
distinct products as objects of interest and 17 distractor objects. A further
increase of 12% in the mAP is obtained by adding only 400 real images to these
4000 synthetic images in the training set. A high degree of photorealism in the
synthetic images was not essential in achieving this performance. We analyze
factors like training data set size and 3D model dictionary size for their
influence on detection performance. Additionally, training strategies like
fine-tuning with selected layers and early stopping which affect transfer
learning from synthetic scenes to real scenes are explored. Training CNNs with
synthetic datasets is a novel application of high-performance computing and a
promising approach for object detection applications in domains where there is
a dearth of large annotated image data. | [
1,
0,
0,
0,
0,
0
] | [
"Computer Science"
] |
Title: The Network of U.S. Mutual Fund Investments: Diversification, Similarity and Fragility throughout the Global Financial Crisis,
Abstract: Network theory proved recently to be useful in the quantification of many
properties of financial systems. The analysis of the structure of investment
portfolios is a major application since their eventual correlation and overlap
impact the actual risk diversification by individual investors. We investigate
the bipartite network of US mutual fund portfolios and their assets. We follow
its evolution during the Global Financial Crisis and analyse the interplay
between diversification, as understood in classical portfolio theory, and
similarity of the investments of different funds. We show that, on average,
portfolios have become more diversified and less similar during the crisis.
However, we also find that large overlap is far more likely than expected from
models of random allocation of investments. This indicates the existence of
strong correlations between fund portfolio strategies. We introduce a
simplified model of propagation of financial shocks, that we exploit to show
that a systemic risk component origins from the similarity of portfolios. The
network is still vulnerable after crisis because of this effect, despite the
increase in the diversification of portfolios. Our results indicate that
diversification may even increase systemic risk when funds diversify in the
same way. Diversification and similarity can play antagonistic roles and the
trade-off between the two should be taken into account to properly assess
systemic risk. | [
0,
0,
0,
1,
0,
1
] | [
"Quantitative Finance",
"Statistics"
] |
Subsets and Splits