Take the 2-minute tour ×

Consider an enumeration $\{q_1,q_2,\ldots\}$ of $\mathbb{Q}\cap [1,\infty)$ and a orthogonal Schauder basis $\{e_1,e_2,\ldots\}$ of $\ell^2(\mathbb{N})$. Define $Ae_{2k-1}=e_{2k-1}$ and $Ae_{2k}=q_ke_{2k}$ for all $k\geq 1$.
Question 1: Is it possible to extend $A$ to a linear self-adjoint operator defined in some infinite dimensional subspace of $\ell^2(\mathbb{N})$ ?

If I am not wrong, this possible linear self-adjoint extension of $A$ can not be defined everywhere in $\ell^2(\mathbb{N})$ and I would like to know if the following set $$ \left\{ v\in \ell^2(\mathbb{N}); v=\lim_{n\to\infty} \sum_{i=1}^n\alpha_ie_i \ \text{and}\ \ \lim_{n\to\infty}\sum_{i=1}^n q_i\alpha_ie_i \in \ell^2(\mathbb{N}) \right\} $$ is a good candidate to be the domain of $A$ ?
Question 2: Is the point spectrum $\sigma_p(A)\supset \{q_1,\ldots,q_n,\ldots\}$ ?

Motivavation: I would like to know if there is an example of an unbounded self-adjoint operator such that the point spectrum is not composed only by isolated points in $\mathbb{R}$ and there is at least one eigenvalue with infinite dimensional eigenspace.

share|improve this question
Can't you just use Friedrichs extension? define $A$ on the linear subspace of finite sequences, which is dense. $A$ is clearly a symmetric operator, bounded from below, thus it admits a selfadjoint extension, and the $e_k$ are eigenvectors as you require –  Piero D'Ancona May 4 '11 at 22:35
Just to clarify: are you asking if there is a choice of Schauder basis for which things work, or if things work for every choice of Schauder basis? –  Yemon Choi May 5 '11 at 1:00
Hi Yemon, If this works for one basis it is good for me. –  Leandro May 5 '11 at 1:22
Thanks for the remark Yemon, I added inthe question that I was thinking about orthogonal basis. –  Leandro May 5 '11 at 1:32

1 Answer 1

up vote 4 down vote accepted

The spectral theorem for unbounded self-adjoint operators says the following:

Up to isomorphism, any unbounded self-adjoint operator $A$ on a Hilbert space $H$ can be written in the following form:



for some measure space $(X,\mu)$ and some $\mu$-measurable real valued function $a:X\to \mathbb R$.

The domain of the operator is $H$ iff the function $a$ is bounded. If $a$ is unbounded, then the domain is $\{ f\in L^2(X,\mu)\\,|\\,af\in L^2(X,\mu)\}$.

You operator is given in that form. So, yes, it is (i.e. extends to) a self adjoint operator.

The answer to your second question is also yes, and your construction indeed provides an example of what you're looking for.

share|improve this answer
doesn't this assume that the Schauder basis mentioned in the question consists of orthogonal vectors? or have I misunderstood? –  Yemon Choi May 5 '11 at 1:01

Your Answer


