Stable homology conjecture for Hurwitz spaces

Let GG) be a finite group, let c=c1ckGc=c_1\cup\cdots\cup c_k\subset G be a union of conjugacy classes, and let CHurn1,,nkG,c1,,ck\operatorname{CHur}^{G,c_1,\ldots,c_k}_{n_1,\ldots,n_k} be the Hurwitz space of connected GG-covers of AC1\mathbb A^1_{\mathbb C} with a trivialization at infinity and nin_i branch points having inertia in cic_i. Let \confn1,,nk\conf_{n_1,\ldots,n_k} be the corresponding multicolored configuration space. Fix an integer ii. For all n1,,nkn_1,\ldots,n_k sufficiently large, with the required bound depending on ii, and for every connected component ZCHurn1,,nkG,c1,,ckZ\subset\operatorname{CHur}^{G,c_1,\ldots,c_k}_{n_1,\ldots,n_k}, the branch-locus map induces an isomorphism

Hi(Z;Q)Hi(\confn1,,nk;Q).H_i(Z;\mathbb Q)\xrightarrow{\sim}H_i(\conf_{n_1,\ldots,n_k};\mathbb Q).

Stable homology conjecture. The only stable homology of these Hurwitz spaces should be the obvious stable homology coming from the configuration space. The conjecture is known for abelian GG and for the nonabelian cases proved in the paper, but in general it is not even known whether the homology stabilizes, except when k=1k=1 and c1c_1 is non-splitting. It is related to conjectures on counting GG-extensions of number fields.

Sources & referencesView supporting material

Primary source

Aaron Landesman and Ishan Levy, “The Cohen–Lenstra moments over function fields via the stable homology of non-splitting Hurwitz spaces”, arXiv:2410.22210 (2025).

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