Product mixing in the alternating group
Discrete analysis (2016)
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We prove the following one-sided product-mixing theorem for the alternating group: Given subsets $X,Y,Z \subset A_n$ of densities $α,β,γ$ satisfying $\min(αβ,αγ,βγ)\gg n^{-1}(\log n)^7$, there are at least $ (1+o(1))αβγ|A_n|^2$ solutions to $xy=z$ with $x\in X, y\in Y, z\in Z$. One consequence is that the largest product-free subset of $A_n$ has density at most $n^{-1/2}(\log n)^{7/2}$, which is best possible up to logarithms and improves the best previous bound of $n^{-1/3}$ due to Gowers. The main tools are a Fourier-analytic reduction noted by Ellis and Green to a problem just about the standard representation, a Brascamp--Lieb-type inequality for the symmetric group due to Carlen, Lieb, and Loss, and a concentration of measure result for rearrangements of inner products.
@article{DAS_2016_a17,
author = {Sean Eberhard},
title = {Product mixing in the alternating group},
journal = {Discrete analysis},
year = {2016},
language = {en},
url = {http://geodesic.mathdoc.fr/item/DAS_2016_a17/}
}
Sean Eberhard. Product mixing in the alternating group. Discrete analysis (2016). http://geodesic.mathdoc.fr/item/DAS_2016_a17/