English

Returning functions with closed graph are continuous

General Topology 2020-04-09 v1

Abstract

A function f:XRf:X\to \mathbb R defined on a topological space XX is called returning if for any point xXx\in X there exists a positive real number MxM_x such that for every path-connected subset CxXC_x\subset X containing the point xx and any yCx{x}y\in C_x\setminus\{x\} there exists a point zCx{x,y}z\in C_x\setminus\{x,y\} such that f(z)max{Mx,f(y)}|f(z)|\le \max\{M_x,|f(y)|\}. A topological space XX is called path-inductive if a subset UXU\subset X is open if and only if for any path γ:[0,1]X\gamma:[0,1]\to X the preimage γ1(U)\gamma^{-1}(U) is open in [0,1][0,1]. The class of path-inductive spaces includes all first-countable locally path-connected spaces and all sequential locally contractible space. We prove that a function f:XRf:X\to \mathbb R defined on a path-inductive space XX is continuous if and only of it is returning and has closed graph. This implies that a (weakly) \'Swi\c atkowski function f:RRf:\mathbb R\to\mathbb R is continuous if and only if it has closed graph, which answers a problem of Maliszewski, inscibed to Lviv Scottish Book.

Keywords

Cite

@article{arxiv.1903.01937,
  title  = {Returning functions with closed graph are continuous},
  author = {Taras Banakh and Małgorzata Filipczak and Julia Wódka},
  journal= {arXiv preprint arXiv:1903.01937},
  year   = {2020}
}

Comments

5 pages

R2 v1 2026-06-23T07:58:54.106Z