Failure of large intersections for quadruple recurrence

Let GG be a countable discrete abelian group, and let φ1,,φk:GG\varphi_1,\dots,\varphi_k:G\to G be distinct, nonzero homomorphisms. Assume, as in the source's nontriviality qualification, that

{gG:φi(g)=φj(g)}\{g\in G:\varphi_i(g)=\varphi_j(g)\}

has infinite index in GG for every iji\ne j. A family has the large intersections property when its associated multiple intersections are uniformly large on a syndetic subset of GG.

Quadruple-recurrence conjecture. If k4k\ge4, then {φ1,,φk}\{\varphi_1,\dots,\varphi_k\} does not have the large intersections property.

The paper suspects that Khintchine-type large-intersection results fail for patterns of length five and longer, with the stated threshold k4k\ge4 covering the corresponding quadruple-recurrence formulation.

Sources & referencesView supporting material

Primary source

Ethan Ackelsberg, Vitaly Bergelson and Andrew Best, “Multiple recurrence and large intersections for abelian group actions”, arXiv:2101.02811 (2021).

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