Different versions of the nerve theorem and rainbow simplices

TL;DR

This paper extends the nerve theorem by replacing intersection connectivity conditions with union connectivity conditions and generalizes the Meshulam lemma, broadening applications in topological combinatorics.

Contribution

It introduces a novel extension of the nerve theorem and generalizes the Meshulam lemma using different approaches, enhancing tools in topological combinatorics.

Findings

Extended nerve theorem with union connectivity conditions

Generalized Meshulam lemma for broader applications

Connected to polytopal generalizations of Sperner's lemma

Abstract

Given a simplicial complex and a collection of subcomplexes covering it, the nerve theorem, a fundamental tool in topological combinatorics, guarantees a certain connectivity of the simplicial complex when connectivity conditions on the intersection of the subcomplexes are satisfied. We show that it is possible to extend this theorem by replacing some of these connectivity conditions on the intersection of the subcomplexes by connectivity conditions on their union. While this is interesting for its own sake, we use this extension to generalize in various ways the Meshulam lemma, a powerful homological version of the Sperner lemma. We also prove a generalization of the Meshulam lemma that is somehow reminiscent of the polytopal generalization of the Sperner lemma by De Loera, Peterson, and Su. For this latter result, we use a different approach and we do not know whether there is a way to get it via a nerve theorem of some kind.