Fixed Point Properties and reflexivity in Variable Lebesgue Spaces

TL;DR

This paper characterizes fixed point properties in Variable Lebesgue Spaces, revealing that some nonreflexive spaces still have the fixed point property, contrasting with classical Lebesgue spaces, and extends known results to these variable exponent spaces.

Contribution

It provides a complete characterization of the weak fixed point property in Variable Lebesgue Spaces and identifies nonreflexive spaces with the fixed point property, a novel finding.

Findings

Reflexive $L^{p(ullet)}$ spaces have the FPP.

Some nonreflexive $L^{p(ullet)}$ spaces satisfy the $w$-FPP.

Certain nonreflexive Nakano sequence spaces have the FPP without renorming.

Abstract

In this paper the weak fixed point property ($w$-FPP) and the fixed point property (FPP) in Variable Lebesgue Spaces are studied. Given $(\Omega,\Sigma,\mu)$ a $\sigma$-finite measure and $p(\cdot)$ a variable exponent function, the $w$-FPP is completely characterized for the variable Lebesgue space $L^{p(\cdot)}(\Omega)$ in terms of the exponent function $p(\cdot)$ and the absence of an isometric copy of $L_1[0,1]$. In particular, every reflexive $L^{p(\cdot)}(\Omega)$ has the FPP and our results bring to light the existence of some nonreflexive variable Lebesgue spaces satisfying the $w$-FPP, in sharp contrast with the classic Lebesgue $L^p$-spaces. In connection with the FPP, we prove that Maurey's result for $L^1$-spaces can be extended to the larger class of variable $L^{p(\cdot)}(\Omega)$ spaces with order continuous norm, that is, every reflexive subspace of $L^{p(\cdot)}(\Omega)$ has the FPP. Never\-theless, Maurey's converse does not longer hold in the variable setting, since some nonreflexive subspaces of $L^{p(\cdot)}(\Omega)$ satisfying the FPP can be found. As a consequence, we discover that several nonreflexive Nakano sequence spaces $\ell^{p_n}$ do have the FPP endowed with the Luxemburg norm. As far as the authors are concerned, this family of sequence spaces gives rise to the first known nonreflexive classic Banach spaces enjoying the FPP without requiring of any renorming procedure. The failure of asympto\-tically isometric copies of $\ell_1$ in $L^{p(\cdot)}(\Omega)$ is also analyzed.