Griesmer's inverse conjecture for Kneser's inequality in compact groups

Let GG be a compact group (not necessarily abelian) with probability Haar measure μG\mu_G, let ε>0\varepsilon > 0, and let δ>0\delta>0 be sufficiently small depending on ε\varepsilon. For any compact subsets A,BGA,B \subset G satisfying

μG(AB)μG(A)+μG(B)+δ\mu_G(AB) \leq \mu_G(A)+\mu_G(B)+\delta

and

μG(A)+μG(B)1ε,\mu_G(A) + \mu_G(B) \leq 1-\varepsilon,

there exist compact subsets A,BGA',B' \subset G such that

μG(AΔA),,μG(BΔB)ε\mu_G(A \mathbin{\Delta} A'),\\,\mu_G(B \mathbin{\Delta} B') \leq \varepsilon

and

μG(AB)μG(A)+μG(B).\mu_G(A'B') \leq \mu_G(A') + \mu_G(B').

Griesmer's conjecture. Under these hypotheses, such compact subsets AA' and BB' exist. This conjecture proposes an inverse theorem for near equality in Kneser's inequality that extends the preceding abelian-group results to arbitrary compact, possibly nonabelian, groups. Its resolution is not given in the source.

Sources & referencesView supporting material

Primary source

Terence Tao, “An inverse theorem for an inequality of Kneser”, arXiv:1711.04337 (2018).

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