Beyond the Borsuk-Ulam theorem: The topological Tverberg story

TL;DR

This paper advances the topological Tverberg theorem by applying equivariant topology methods to prove it for prime powers and introduces the constraint method to derive various corollaries, impacting high-dimensional counterexamples.

Contribution

It extends the topological Tverberg conjecture proof to prime power cases and develops the constraint method for broader applications and corollaries.

Findings

Proved the conjecture for prime power r using equivariant cohomology.

Determined the Fadell-Husseini index of chessboard complexes in the prime case.

Introduced the constraint method for deriving new Tverberg-type results.

Abstract

B\'ar\'any's "topological Tverberg conjecture" from 1976 states that any continuous map of an $N$-simplex $\Delta_N$ to $\mathbb{R}^d$, for $N\ge(d+1)(r-1)$, maps points from $r$ disjoint faces in $\Delta_N$ to the same point in $\mathbb{R}^d$. The proof of this result for the case when $r$ is a prime, as well as some colored version of the same result, using the results of Borsuk-Ulam and Dold on the non-existence of equivariant maps between spaces with a free group action, were main topics of Matou\v{s}ek's 2003 book "Using the Borsuk-Ulam theorem." In this paper we show how advanced equivariant topology methods allow one to go beyond the prime case of the topological Tverberg conjecture. First we explain in detail how equivariant cohomology tools (employing the Borel construction, comparison of Serre spectral sequences, Fadell-Husseini index, etc.) can be used to prove the topological Tverberg conjecture whenever $r$ is a prime power. Our presentation includes a number of improved proofs as well as new results, such as a complete determination of the Fadell-Husseini index of chessboard complexes in the prime case. Then we introduce the "constraint method," which applied to suitable "unavoidable complexes" yields a great variety of variations and corollaries to the topological Tverberg theorem, such as the "colored" and the "dimension-restricted" (Van Kampen-Flores type) versions. Both parts have provided crucial components to the recent spectacular counter-examples in high dimensions for the case when $r$ is not a prime power.