Proximinality and uniformly approximable sets in $L^p$

TL;DR

This paper investigates proximinal sets in $L^p$, introduces the larger class of uniformly approximable sets, and characterizes their properties and conditions for uniform approximability in different $L^p$ spaces.

Contribution

It proves proximinality of simple functions with limited values in $L^p$ and introduces the class of uniformly approximable sets, expanding understanding of approximation properties in $L^p$ spaces.

Findings

Simple functions with at most $k$ values are proximinal in $L^p$.

The class of uniformly approximable sets is characterized by $p$-variation and covering numbers.

The unit ball in $L^p$ is uniformly approximable iff $L^p$ is finite-dimensional for $p<\infty$, and always for $p=\infty$.

Abstract

For any $p\in[1,\infty]$, we prove that the set of simple functions taking at most $k$ different values is proximinal in $L^p$ for all $k\geq 1$. We introduce the class of uniformly approximable subsets of $L^p$, which is larger than the class of uniformly integrable sets. This new class is characterized in terms of the $p$-variation if $p\in[1,\infty)$ and in terms of covering numbers if $p=\infty$. We study properties of uniformly approximable sets. In particular, we prove that the convex hull of a uniformly approximable bounded set is also uniformly approximable and that this class is stable under H\"older transformations. We also prove that, for $p\in [1,\infty)$, the unit ball of $L^p$ is uniformly approximable if and only if $L^p$ is finite-dimensional, while for $p=\infty$ the unit ball is always uniformly approximable.