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We give a detailed and easily accessible proof of Gromov's Topological Overlap Theorem. Let X be a finite simplicial complex or, more generally, a finite polyhedral cell complex of dimension d. Informally, the theorem states that if X has sufficiently strong higher-dimensional expansion properties (which generalize edge expansion of graphs and are defined in terms of cellular cochains of X) then X has the following topological overlap property: for every continuous map X -> R^d there exists a point p in R^d whose preimage intersects a positive fraction mu > 0 of the d-cells of X. More generally, the conclusion holds if R^d is replaced by any d-dimensional piecewise-linear (PL) manifold M, with a constant \mu that depends only on d and on the expansion properties of X, but not on M.
@InProceedings{dotterrer_et_al:LIPIcs.SoCG.2016.35,
author = {Dotterrer, Dominic and Kaufman, Tali and Wagner, Uli},
title = {{On Expansion and Topological Overlap}},
booktitle = {32nd International Symposium on Computational Geometry (SoCG 2016)},
pages = {35:1--35:10},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-009-5},
ISSN = {1868-8969},
year = {2016},
volume = {51},
editor = {Fekete, S\'{a}ndor and Lubiw, Anna},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2016.35},
URN = {urn:nbn:de:0030-drops-59270},
doi = {10.4230/LIPIcs.SoCG.2016.35},
annote = {Keywords: Combinatorial Topology, Selection Lemmas, Higher-Dimensional Expanders}
}