Stronger counterexamples to the topological Tverberg conjecture

TL;DR

This paper advances the construction of counterexamples to the topological Tverberg conjecture, especially for non-prime power r, by developing stronger counterexamples using generalized topological and combinatorial methods.

Contribution

It provides improved bounds and stronger counterexamples for the topological Tverberg conjecture and related van Kampen-Flores conjecture, extending previous constructions with new topological tools.

Findings

Constructed larger counterexamples for non-prime power r

Developed stronger counterexamples for the van Kampen-Flores conjecture

Extended the use of equivariant topology methods in combinatorial topology

Abstract

Denote by $\Delta_M$ the $M$-dimensional simplex. A map $f\colon \Delta_M\to\mathbb R^d$ is an almost $r$-embedding if $f\sigma_1\cap\ldots\cap f\sigma_r=\emptyset$ whenever $\sigma_1,\ldots,\sigma_r$ are pairwise disjoint faces. A counterexample to the topological Tverberg conjecture asserts that if $r$ is not a prime power and $d\ge2r+1$, then there is an almost $r$-embedding $\Delta_{(d+1)(r-1)}\to\mathbb R^d$. This was improved by Blagojevi\'c-Frick-Ziegler using a simple construction of higher-dimensional counterexamples by taking $k$-fold join power of lower-dimensional ones. We improve this further (for $d$ large compared to $r$): If $r$ is not a prime power and $N:=(d+1)r-r\Big\lceil\dfrac{d+2}{r+1}\Big\rceil-2$, then there is an almost $r$-embedding $\Delta_N\to\mathbb R^d$. For the $r$-fold van Kampen-Flores conjecture we also produce counterexamples which are stronger than previously known. Our proof is based on generalizations of the Mabillard-Wagner theorem on construction of almost $r$-embeddings from equivariant maps, and of the \"Ozaydin theorem on existence of equivariant maps.