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Published online by Cambridge University Press: 28 October 2022
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.