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Null, recursively starlike-equivalent decompositions shrink

Published online by Cambridge University Press:  28 October 2022

Jeffrey Meier
Affiliation:
Department of Mathematics, Western Washington University, Bellingham, WA 98225, USA
Patrick Orson*
Affiliation:
Department of Mathematics, California Polytechnic State University, San Luis Obispo, CA 93407, USA
Arunima Ray
Affiliation:
Max-Planck-Institut für Mathematik, Bonn, Germany
*
*Corresponding author. E-mail: patrickorson@gmail.com

Abstract

A subset E of a metric space X is said to be starlike-equivalent if it has a neighbourhood which is mapped homeomorphically into $\mathbb{R}^n$ for some n, sending E to a starlike set. A subset $E\subset X$ is said to be recursively starlike-equivalent if it can be expressed as a finite nested union of closed subsets $\{E_i\}_{i=0}^{N+1}$ such that $E_{i}/E_{i+1}\subset X/E_{i+1}$ is starlike-equivalent for each i and $E_{N+1}$ is a point. A decomposition $\mathcal{D}$ of a metric space X is said to be recursively starlike-equivalent, if there exists $N\geq 0$ such that each element of $\mathcal{D}$ is recursively starlike-equivalent of filtration length N. We prove that any null, recursively starlike-equivalent decomposition $\mathcal{D}$ of a compact metric space X shrinks, that is, the quotient map $X\to X/\mathcal{D}$ is the limit of a sequence of homeomorphisms. This is a strong generalisation of results of Denman–Starbird and Freedman and is applicable to the proof of Freedman’s celebrated disc embedding theorem. The latter leads to a multitude of foundational results for topological 4-manifolds, including the four-dimensional Poincaré conjecture.

MSC classification

Type
Research Article
Copyright
© The Author(s), 2022. Published by Cambridge University Press on behalf of Glasgow Mathematical Journal Trust

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