Oversampling on a class of symmetric regular de Branges spaces

Abstract

A de Branges space $\mathcal B$ is regular if the constants belong to its space of associated functions and is symmetric if it is isometrically invariant under the map $F(z) \mapsto F(-z)$. Let $K_\mathcal{B}(z,w)$ be the reproducing kernel in $\mathcal B$ and $S_{\mathcal{B}}$ be the operator of multiplication by the independent variable with maximal domain in $\mathcal B$. Loosely speaking, we say that $\mathcal B$ has the $\ell_p$-oversampling property relative to a proper subspace $\mathcal A$ of it, with $p\in(2,\infty]$, if there exists $J_{\mathcal A\mathcal B}:\mathbb{C}\times\mathbb{C}\to\mathbb{C}$ such that $J(\cdot,w)\in\mathcal B$ for all $w\in\mathbb{C}$, \begin{equation*} \sum_{\lambda\in\sigma(S_{\mathcal B}^{\gamma})} \left(\frac{\lvert J_{\mathcal{A}\mathcal{B}}(z,\lambda)\rvert}{K_\mathcal{B}(\lambda,\lambda)^{1/2}}\right)^{p/(p-1)} <\infty, \quad\text{and}\quad F(z) = \sum_{\lambda\in\sigma(S_{\mathcal B}^{\gamma})} \frac{J_{\mathcal{A}\mathcal{B}}(z,\lambda)}{K_\mathcal{B}(\lambda,\lambda)}F(\lambda), \end{equation*} for all $F\in\mathcal A$ and almost every self-adjoint extension $S_{\mathcal B}^{\gamma}$ of $S_{\mathcal{B}}$. This definition is motivated by the well-known oversampling property of Paley-Wiener spaces. In this paper we provide sufficient conditions for a symmetric, regular de Branges space to have the $\ell_p$-oversampling property relative to a chain of de Branges subspaces of it.