Sarnak's bounded-clustering conjecture for cofinite Fuchsian groups

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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,n∈Zm,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)={z∈C∣m≤Re⁡(z)≤m+1, n≤Im⁡(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⁡{∣a−b∣∣a,b∈A, a≠b}.\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.

References

Primary source

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

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