On the approximate fixed point property in abstract spaces

TL;DR

This paper investigates the approximate fixed point property in various abstract topological vector spaces, extending previous Banach space results through new topological and duality-based approaches.

Contribution

It introduces novel results on the $\sigma(X,Z)$-approximate fixed point property in general topological vector spaces, broadening the scope beyond Banach spaces.

Findings

Established the $\sigma(X,Z)$-approximate fixed point property for specific space classes.

Proved the Fréchet-Urysohn property for relevant sets under the $\sigma(X,Z)$-topology.

Extended fixed point theorems using tools like Brouwer's theorem and Rosenthal's $ extit{ ext{l}}_1$-theorem.

Abstract

Let $X$ be a Hausdorff topological vector space, $X^*$ its topological dual and $Z$ a subset of $X^*$. In this paper, we establish some results concerning the $\sigma(X,Z)$-approximate fixed point property for bounded, closed convex subsets $C$ of $X$. Three major situations are studied. First when $Z$ is separable in the strong topology. Second when $X$ is a metrizable locally convex space and $Z=X^*$, and third when $X$ is not necessarily metrizable but admits a metrizable locally convex topology compatible with the duality. Our approach focuses on establishing the Fr\'echet-Urysohn property for certain sets with regarding the $\sigma(X,Z)$-topology. The support tools include the Brouwer's fixed point theorem and an analogous version of the classical Rosenthal's $\ell_1$-theorem for $\ell_1$-sequences in metrizable case. The results are novel and generalize previous work obtained by the authors in Banach spaces.