Topology of scrambled simplices

TL;DR

This paper introduces a combinatorial family of topological spaces generalizing Dunce hats, determines their homotopy types as either contractible or odd-dimensional spheres, and links their structure to indexing words.

Contribution

It provides a complete classification of the homotopy types of these spaces based on combinatorial data, expanding understanding of contractible but non-collapsible complexes.

Findings

Spaces are either contractible or homotopy equivalent to odd-dimensional spheres.

Homotopy type can be determined directly from the combinatorics of the indexing word.

Constructs a large family of contractible, non-collapsible complexes.

Abstract

In this paper we define a family of topological spaces, which contains and vastly generalizes the higher-dimensional Dunce hats. Our definition is purely combinatorial, and is phrased in terms of identifications of boundary simplices of a~standard d-simplex. By virtue of the construction, the obtained spaces may be indexed by words, and they automatically carry the structure of a $\Delta$-complex. As our main result, we completely determine the homotopy type of these spaces. In fact, somewhat surprisingly, we are able to prove that each of them is either contractible or homotopy equivalent to an odd-dimensional sphere. We develop the language to determine the homotopy type directly from the combinatorics of the indexing word. As added benefit of our investigation, we are able to emulate the Dunce hat phenomenon, and to obtain a large family of both $\Delta$-complexes, as well as simplicial complexes, which are contractible, but not collapsible.