arXiv subjects
Publications and source records attributed to Sławomir Borzdyński.
Suppose that E is a Banach space, τ a topology under which the norm of E becomes τ-lower semicontinuous and S a commuting family of τ-continuous nonexpansive mappings defined on a τ-compact convex subset C of E: It is shown that the set of common fixed points of S is a nonempty nonexpansive retract of C. Along the way, a few other related fixed point theorems are derived.
We show that there are no nontrivial surjective uniformly asymptotically regular mappings acting on a metric space and derive some consequences of this fact. In particular, we prove that a jointly continuous left amenable or left reversible semigroup generated by firmly nonexpansive mappings on a bounded $τ$-compact subset of a Banach space has a common fixed point, and give a qualitative complement to the Markov-Kakutani theorem.
It is shown that if $S$ is a commuting family of weak$^{\ast }$ continuous nonexpansive mappings acting on a weak$^{\ast }$ compact convex subset $C$ of the dual Banach space $E$, then the set of common fixed points of $S$ is a nonempty nonexpansive retract of $C$. This partially solves a long-standing open problem in metric fixed point theory in the case of commutative semigroups.