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Green's open problem 29 · Conjectures.io
Group theory
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Date added
Green's open problem 29
Suppose that A is a K-approximate group (not necessarily abelian). Is there S⊂A,
∣S∣≫K−O(1)∣A∣, with S8⊂A4?
Withdrawn 6 Aug 2026
SOLVED (approved submission 82ab85ee refutes the published target with a 3-approximate group A = ({-1} × H) ∪ ({+1} × H) ∪ {(0,1)} in Multiplicative ℤ × H, whose only element at coordinate 0 is the identity, so every S ⊆ A with S^8 ⊆ A^4 is a singleton; the formalization is faithful and no defect was found)
True ↔
∃ C c,
0 < C ∧
0 < c ∧
∀ {G : Type u_1} [inst : Group G] [inst_1 : DecidableEq G] (K : ℝ) (A : Finset G),
1 ≤ K → IsApproximateSubgroup K ↑A → ∃ S ⊆ A, C * K ^ (-c) * ↑A.card ≤ ↑S.card ∧ S ^ 8 ⊆ A ^ 4
[Br13] Breuillard, Emmanuel, Ben Green, and Terence Tao. "Small doubling in groups." Erdős Centennial. Berlin, Heidelberg: Springer Berlin Heidelberg, 2013. 129-151.
[Sa10] Sanders, Tom. "On a nonabelian Balog–Szemerédi-type lemma." Journal of the Australian Mathematical Society 89.1 (2010): 127-132.
[CrSi10] Croot, Ernie, and Olof Sisask. "A probabilistic technique for finding almost-periods of convolutions." Geometric and functional analysis 20.6 (2010): 1367-1396.
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