Approximate orbit structure conjecture for group actions

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 K1K\geq 1 satisfy A(Y)KY|A(Y)|\leq K|Y|. Approximate orbit structure conjecture. There is a constant C>0C>0, a subset BGB\subseteq G, a subgroup HGH\leq G, and a finite subset ZXZ\subseteq X such that

BKC,|B|\ll K^C,

ABHA\subseteq BH, and

H(Z)YKCY.|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|.

Sources & referencesView supporting material

Primary source

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

Progress summary

Never refreshed

Nothing recorded yet. Refresh searches the literature and the public web for attempts on this problem, and writes the first summary here.

Solutions 0

No solutions have been posted yet.