The approximate variation to pointwise selection principles

TL;DR

This paper investigates the properties of approximate variation in functions mapping into metric spaces, establishing a pointwise selection principle that guarantees convergence of subsequences under certain boundedness and compactness conditions.

Contribution

It provides a direct proof of a basic pointwise selection principle for functions with bounded approximate variation, extending to various function classes and convergence types.

Findings

Established a pointwise selection principle for functions with bounded approximate variation.

Extended the principle to regulated, nonregulated, and Banach space-valued functions.

Illustrated the sharpness of results with examples.

Abstract

Let $T\subset\mathbb{R}$, $M$ be a metric space with metric $d$, and $M^T$ be the set of all functions mapping $T$ into $M$. Given $f\in M^T$, we study the properties of the approximate variation $\{V_\varepsilon(f)\}_{\varepsilon>0}$, where $V_\varepsilon(f)$ is the greatest lower bound of Jordan variations $V(g)$ of functions $g\in M^T$ such that $d(f(t),g(t))\le\varepsilon$ for all $t\in T$. The notion of $\varepsilon$-variation $V_\varepsilon(f)$ was introduced by Fra\'nkov\'a [Math. Bohem. 116 (1991), 20-59] for intervals $T=[a,b]$ in $\mathbb{R}$ and $M=\mathbb{R}^N$ and extended to the general case by Chistyakov and Chistyakova [Studia Math. 238 (2017), 37-57]. We prove directly the following basic pointwise selection principle: If a sequence of functions $\{f_j\}_{j=1}^\infty$ from $M^T$ is such that the closure in $M$ of the set $\{f_j(t):j\in\mathbb{N}\}$ is compact for all $t\in T$ and $\limsup_{j\to\infty}V_\varepsilon(f_j)$ is finite for all $\varepsilon>0$, then it contains a subsequence, which converges pointwise on $T$ to a bounded regulated function $f\in M^T$. We establish several variants of this result for sequences of regulated and nonregulated functions, for functions with values in reflexive separable Banach spaces, for the almost everywhere convergence and weak pointwise convergence of extracted subsequences, and comment on the necessity of assumptions in the selection principles. The sharpness of all assertions is illustrated by examples.