Approximate orbit structure conjecture for group actions

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Let GG be a group acting on a set XX, let AA be a subset of GG, and let YY be a finite subset of XX. Write A(Y)A(Y) for the image of YY under AA, and let K≥1K\geq 1 satisfy ∣A(Y)∣≤K∣Y∣|A(Y)|\leq K|Y|. Approximate orbit structure conjecture. There is a constant C>0C>0, a subset B⊆GB\subseteq G, a subgroup H≤GH\leq G, and a finite subset Z⊆XZ\subseteq X such that

∣B∣≪KC,|B|\ll K^C,

A⊆BHA\subseteq BH, and

∣H(Z)∩Y∣≫K−C∣Y∣.|H(Z)\cap Y|\gg K^{-C}|Y|.

This conjecture asks whether small image growth forces YY to contain a substantial portion of an orbit-like set for a subgroup generated, up to a bounded set, by AA. It is a group-action analogue of approximate structure results in multiplicative combinatorics; the statement is intended to relax the exact orbit decomposition obtained when ∣A(Y)∣=∣Y∣|A(Y)|=|Y|.

References

Primary source

Brendan Murphy, “Group Action Combinatorics”, arXiv:1907.13569 (2019).

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