English

Eventually Constant and stagnating functions in non-Lindel\"of spaces

General Topology 2024-09-26 v3

Abstract

Inspired by recent work of A. Mardani which elaborates on the elementary fact that for any continuous function f:ω1×RRf:\omega_1\times\mathbb{R}\to\mathbb{R}, there is an αω1\alpha\in\omega_1 such that f(β,x)=f(α,x)f(\langle\beta,x\rangle) = f(\langle\alpha,x\rangle) for all βα\beta\ge\alpha and xRx\in\mathbb{R}, we introduce four properties P(X,Y)\mathsf{P}(X,Y), P{EC,S,L,BR}\mathsf{P}\in\{\mathsf{EC},\mathsf{S},\mathsf{L},\mathsf{BR}\}, which are different formalizations of the idea vaguely stated as "given a continuous f:XYf:X\to Y, there is a small subspace of XX outside of which ff does not do anything much new". We say that the spaces X,YX,Y satisfy the property EC(X,Y)\mathsf{EC}(X,Y) (resp. S(X,Y)\mathsf{S}(X,Y)) [resp. L(X,Y)\mathsf{L}(X,Y)] iff given f:XYf:X\to Y, then there is a Lindel\"of ZXZ\subset X such that f(XZ)f(X-Z) is a singleton (resp. there is a retraction r:XZr:X\to Z such that fr=ff\circ r = f) [resp. f(Z)=f(X)f(Z) = f(X)]. (BR(X,Y)\mathsf{BR}(X,Y) is defined similarly.) We investigate the relations between these four and other classical topological properties. Two variants of each property are given depending on whether ZZ can be chosen to be closed. Here is a sample of our results. An uncountable subspace TT of a tree of height ω1\omega_1 is ω1\omega_1-compact iff S(T,Y)\mathsf{S}(T,Y) holds for any metrizable space YY of cardinality >1>1. If MM is a 1\aleph_1-strongly collectionwise Hausforff non-metrizable manifold satisfying either a weakening of S(M,R)\mathsf{S}(M,\mathbb{R}) or EC(M,R)\mathsf{EC}(M,\mathbb{R}), then MM is ω1\omega_1-compact. The property L(M,R)\mathsf{L}(M,\mathbb{R}) holds for any manifold while L(M,R2)\mathsf{L}(M,\mathbb{R}^2) does not. Under PFA, a locally compact countably tight space YY for which EC(ω1,Y)\mathsf{EC}(\omega_1,Y) holds is isocompact, while there are counterexamples under C\clubsuit_C. Some of our results are restatements of other researchers work put in our context.

Keywords

Cite

@article{arxiv.2308.12763,
  title  = {Eventually Constant and stagnating functions in non-Lindel\"of spaces},
  author = {Mathieu Baillif},
  journal= {arXiv preprint arXiv:2308.12763},
  year   = {2024}
}

Comments

35 pages, 4 figures. V3: Corrected inaccuracies in Section 4, added small new results in Sections 4 and 5, and some cosmetic correction