Structure conjecture for small quotient sets in groups

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Let AA be a finite subset of a group GG, and let nn be a positive integer. Write N(H)N(H) for the normalizer of a subgroup HH in GG. A subset A0A_0 is contained in a single left N(H)N(H)-coset if there is some ginGgin G such that A0⊆gN(H)A_0\subseteq gN(H).

Structure conjecture. If

∣A−1A∣<(2−1n+1)∣A∣,|A^{-1}A|<\left(2-\frac{1}{n+1}\right)|A|,

then there are a finite subgroup H≤GH\le G and a subset A0⊆AA_0\subseteq A of size ∣A0∣≤n|A_0|\le n contained in a single left N(H)N(H)-coset such that

A⊆A0H,∣A0H∣=∣A0∣∣H∣,A\subseteq A_0H,\qquad |A_0H|=|A_0||H|,

and

∣A∣>(2−1n+1)−1(2∣A0∣−1)∣H∣.|A|>\left(2-\frac{1}{n+1}\right)^{-1}(2|A_0|-1)|H|.

Moreover,

A−1A=A0−1A0HA^{-1}A=A_0^{-1}A_0H

and

∣A−1A∣=(2∣A0∣−1)∣H∣.|A^{-1}A|=(2|A_0|-1)|H|.

This conjecture proposes a structural description of finite subsets with quotient set smaller than the stated threshold. The supplied text gives no resolution status or further evidence beyond presenting it as a conjecture, so its status remains open.

References

Primary source

Vsevolod F. Lev, “Quotient sets in nonabelian groups”, arXiv:2404.06887 (2024).

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