# Variants of a selection principle for sequences of regulated and non-regulated functions

Caterina Maniscalco, Yuliya V. Tretyachenko, Vyacheslav V. Chistyakov

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7 Citazioni (Scopus)

## Abstract

Let $T$ be a nonempty subset of $\RB$, $X$ a metric space withmetric $d$ and $X^T$ the set of all functions mapping $T$ into$X$. Given $\vep>0$ and $f\in X^T$, we denote by $N(\vep,f,T)$ theleast upper bound of those $n\in\NB$, for which there existnumbers $s_1,\dots,s_n,t_1,\dots,t_n$ from $T$ such that$s_1<t_1\le s_2<t_2\le\dots\le s_n<t_n$ and$d(f(s_i),f(t_i))>\vep$ for all $i=1,\dots,n$ ($N(\vep,f,T)=0$ ifthere are no such $n$'s). The following pointwise selectionprinciple is proved: {\em If a sequence of functions\/$\{f_j\}_{j=1}^\infty\subset X^T$ is such that the closure in $X$of the sequence\/ $\{f_j(t)\}_{j=1}^\infty$ is compact for each$t\in T$ and\/ $\limsup_{j\to\infty}N(\vep,f_j,T)<\infty$ for all$\vep>0$, then\/ $\{f_j\}_{j=1}^\infty$ contains a subsequence,converging pointwise on $T$ to a function $f\in X^T$, such that$N(\vep,f,T)<\infty$ for all $\vep>0$}. We establish severalvariants of this result for functions with values in a metricsemigroup and reflexive separable Banach space as well as for theweak pointwise and almost everywhere convergence of extractedsubsequences, and comment on the necessity of conditions in theselection principles. We show that many Helly-type pointwiseselection principles are consequences of our results, which can beapplied to sequences of non-regulated functions, and compare themwith recent results by Chistyakov [J. Math. Anal. Appl. 310(2005) 609--625] and Chistyakov and Maniscalco [J. Math. Anal.Appl. 341 (2008) 613--625].
Lingua originale English Topics in Classical Analysis and Applications in Honor of Professor Dan Waterman on the occasion of his 80th birthday 45-72 28 Published - 2008

## All Science Journal Classification (ASJC) codes

• Mathematics(all)