Sarnak's bounded-clustering conjecture for cofinite Fuchsian groups

Let Γ\Gamma be a cofinite Fuchsian group. Its trace set, denoted Tr(Γ)\mathrm{Tr}(\Gamma), is the set of traces of elements of Γ\Gamma, defined up to sign. A set of complex numbers has the bounded clustering property (or B-C property) if there is a constant KAK_A such that, for every m,nZm,n\in\mathbb Z, its intersection with each unit square S(m,n)S(m,n) contains fewer than KAK_A elements, where

S(m,n)={zCmRe(z)m+1, nIm(z)n+1}.S(m,n)=\left\{z\in\mathbb C\mid m\leq\operatorname{Re}(z)\leq m+1,\ n\leq\operatorname{Im}(z)\leq n+1\right\}.

Also define

Gap(A)=inf{aba,bA, ab}.\operatorname{Gap}(A)=\inf\{|a-b|\mid a,b\in A,\ a\neq b\}.

Sarnak's conjecture. If Tr(Γ)\mathrm{Tr}(\Gamma) satisfies the B-C property, then Γ\Gamma is arithmetic. If Gap(Tr(Γ))>0\operatorname{Gap}(\mathrm{Tr}(\Gamma))>0, then Γ\Gamma is derived from a quaternion algebra.

Luo and Sarnak had shown that the trace set of an arithmetic Fuchsian group satisfies the B-C property, and the conjecture proposes converses of this phenomenon. The source does not state whether these assertions have been resolved.

Sources & referencesView supporting material

Primary source

Yanlong Hao, “Bounded clustering property characterizes arithmetic nonuniform Kleinian groups”, arXiv:2303.01395 (2023).

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