Small doubling implies small tripling for locally compact groups

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Let GG be a locally compact group with Haar measure μ\mu, and let S⊆GS\subseteq G be a precompact symmetric open set containing the identity. Assume that, for some integer nn sufficiently large depending on KK and ε\varepsilon, one has

μ(S⌈(1+ε)n⌉)≤Kμ(Sn).\mu(S^{\lceil(1+\varepsilon)n\rceil})\le K\mu(S^n).

Small tripling conjecture. For every K≥1K\ge 1 and ε>0\varepsilon>0, this implies

μ(S3n)≪K,εμ(Sn).\mu(S^{3n})\ll_{K,\varepsilon}\mu(S^n).

This asks whether a weaker growth hypothesis than the usual tripling assumption already forces small tripling for sufficiently large radii. The supplied text presents it as a natural question and gives no resolution, so its status remains open.

References

Primary source

Romain Tessera and Matthew Tointon, “Small doubling implies small tripling for balls of large radius”, arXiv:2310.20500 (2025).

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