On cap sets and the group-theoretic approach to matrix multiplication

Abstract

In 2003, Cohn and Umans described a framework for proving upper bounds on the exponent $\ddot{I}‰$ of matrix multiplication by reducing matrix multiplication to group algebra multiplication, and in 2005 Cohn, Kleinberg, Szegedy, and Umans proposed specific conjectures for how to obtain $\ddot{I}‰=2$. In this paper we rule out obtaining $\ddot{I}‰=2$ in this framework from abelian groups of bounded exponent. To do this we bound the size of tricolored sum-free sets in such groups, extending the breakthrough results of Croot, Lev, Pach, Ellenberg, and Gijswijt on cap sets. As a byproduct of our proof, we show that a variant of tensor rank due to Tao gives a quantitative understanding of the notion of unstable tensor from geometric invariant theory.

27 pages