Mixing, Communication Complexity and Conjectures of Gowers and Viola

We study the distribution of products of conjugacy classes in finite simple groups, obtaining effective two-step mixing results, which give rise to an approximation to a conjecture of Thompson. Our results, combined with work of Gowers and Viola, also lead to the solution of recent conjectures they posed on interleaved products and related complexity lower bounds, extending their work on the groups SL(2, q) to all (non-abelian) finite simple groups. In particular it follows that, if G is a finite simple group, and A, B ⊂ G^t for t ≥ 2 are subsets of fixed positive densities, then, as a = (a₁,..., at ) ∈ A and b = (b 1,..., bt ) ∈ B are chosen uniformly, the interleaved product a • b:=a₁b₁.. a_tb_t is almost uniform on G (with quantitative estimates) with respect to the ℓ_∈-norm. It also follows that the communication complexity of an old decision problem related to interleaved products of a, b ∈ G^t is at least Ω(t log |G|) when G is a finite simple group of Lie type of bounded rank, and at least Ω(t log log |G|) when G is any finite simple group. Both these bounds are best possible.