Point evaluation in Paley--Wiener spaces

TL;DR

This paper investigates the norm of point evaluation at zero in Paley--Wiener spaces, proposing a monotonicity conjecture for a related constant and analyzing extremal functions and their zeros.

Contribution

It introduces a new extremal problem for point evaluation in Paley--Wiener spaces, providing evidence for a monotonicity conjecture and characterizing extremal functions.

Findings

Proves $ extscr{C}_p < p/2$ for $2<p< finite$

Numerically verifies $ extscr{C}_p > p/2$ for $1 leq p < 2$

Estimates asymptotic behavior of $ extscr{C}_p$ as $p o 0^+$ and $p o finite$

Abstract

We study the norm of point evaluation at the origin in the Paley--Wiener space $PW^p$ for $0 < p < \infty$, i. e., we search for the smallest positive constant $C$, called $\mathscr{C}_p$, such that the inequality $|f(0)|^p \leq C \|f\|_p^p$ holds for every $f$ in $PW^p$. We present evidence and prove several results supporting the following monotonicity conjecture: The function $p\mapsto \mathscr{C}_p/p$ is strictly decreasing on the half-line $(0,\infty)$. Our main result implies that $\mathscr{C}_p <p/2$ for $2<p<\infty$, and we verify numerically that $\mathscr{C}_p > p/2$ for $1 \leq p < 2$. We also estimate the asymptotic behavior of $\mathscr{C}_p$ as $p \to \infty$ and as $p \to 0^+$. Our approach is based on expressing $\mathscr{C}_p$ as the solution of an extremal problem. Extremal functions exist for all $0<p<\infty$; they are real entire functions with only real zeros, and the extremal functions are known to be unique for $1\leq p < \infty$. Following work of H\"{o}rmander and Bernhardsson, we rely on certain orthogonality relations associated with the zeros of extremal functions, along with certain integral formulas representing respectively extremal functions and general functions at the origin. We also use precise numerical estimates for the largest eigenvalue of the Landau--Pollak--Slepian operator of time--frequency concentration. A number of qualitative and quantitative results on the distribution of the zeros of extremal functions are established. In the range $1<p<\infty$, the orthogonality relations associated with the zeros of the extremal function are linked to a de Branges space. We state a number of conjectures and further open problems pertaining to $\mathscr{C}_p$ and the extremal functions.