Nonabelian Cohen–Lenstra distribution conjecture for pro-prime-to-roots-of-unity class groups

Let Q=QQ={\mathbb Q} or Fq(t)\mathbb F_q(t), and let μQ\mu_Q be the group of roots of unity contained in QQ. Let Γ\Gamma be the finite group and Γ\Gamma_\infty the cyclic subgroup governing the extensions under consideration. As KK varies over (Γ,Γ)(\Gamma,\Gamma_\infty)-extensions of QQ, consider the pro-prime-to-μQ|\mu_Q| completion of GO#(K)G_{\mathcal O}^{\#}(K). For an admissible Γ\Gamma-group HH with gcd(H,μQ)=1\gcd(|H|,|\mu_Q|)=1 and char(Q)H\operatorname{char}(Q)\nmid |H|, let its HH-moment be the expected number of Γ\Gamma-equivariant surjections onto HH. Nonabelian Cohen–Lenstra conjecture. The distribution of the pro-prime-to-μQ|\mu_Q| completion of GO#(K)G_{\mathcal O}^{\#}(K) is predicted by the probability measure μΓ,Γ\mu_{\Gamma,\Gamma_\infty} defined by the random group model. Moreover, the HH-moment of the distribution of GO#(K)G_{\mathcal O}^{\#}(K) is

1[HΓ:HΓ].\frac{1}{[H^{\Gamma_\infty}:H^{\Gamma}]}.

This is the main conjectural extension of the Cohen–Lenstra heuristics to nonabelian admissible Γ\Gamma-groups. The source gives no evidence that the distribution statement or the stated moments are resolved in the generality claimed.

Sources & referencesView supporting material

Primary source

Yuan Liu and Ken Willyard, “The imaginary case of the nonabelian Cohen–Lenstra heuristics”, arXiv:2507.21558 (2025).

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