Asymptotic representation-growth conjecture for Fuchsian groups

Let Γ\Gamma be the Fuchsian group from the setup, with orbifold Euler characteristic

χ(Γ)=22gi=1r(11ai)<0.\chi(\Gamma)=2-2g-\sum_{i=1}^r\left(1-\frac{1}{a_i}\right)<0.

Let AA be the least common multiple of a1,,ara_1,\ldots,a_r, with A=1A=1 if r=0r=0, and let qq be a prime power relatively prime to AA. Write GLn(q)\operatorname{GL}_n(q) for the general linear group over Fq\mathbb{F}_q.

Representation-growth conjecture. (a) There exists a 2A2A-periodic sequence cq,1,cq,2,c_{q,1},c_{q,2},\ldots of positive numbers such that

Hom(Γ,GLn(q))cq,nq(1χ(Γ))n2.|\operatorname{Hom}(\Gamma,\operatorname{GL}_n(q))|\sim c_{q,n}q^{(1-\chi(\Gamma))n^2}.

(b) There exist a 22-dimensional array eΓ,q,ne_{\Gamma,q,n} of rational numbers and a 22-dimensional array fΓ,q,nf_{\Gamma,q,n} of half-integral weight meromorphic modular forms, periodic in both qq and nn, such that

cq,n=(q1)qeΓ,q,nfΓ,q,n(ilogq2π).c_{q,n}=(q-1)q^{e_{\Gamma,q,n}}f_{\Gamma,q,n}\left(\frac{i\log q}{2\pi}\right).

Moreover, fΓ,q,nf_{\Gamma,q,n} is holomorphic on the upper half plane and has integer Fourier coefficients at ii\infty.

The conjecture predicts both the leading asymptotic growth of homomorphisms from Γ\Gamma to finite general linear groups and a modular-form description of the periodic leading constants. The supplied text gives no resolution evidence, so the conjecture is recorded as open.

Sources & referencesView supporting material

Primary source

Michael Larsen, Jay Taylor and Pham Huu Tiep, “Representation growth of Fuchsian groups and modular forms”, arXiv:2407.07193 (2024).

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