Beyond the Borsuk-Ulam theorem: The topological Tverberg story (1605.07321v3)

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.