📝 SummarXiv

Abstract

Hurwitz spaces are moduli of isotopy classes of covers. A specific space is formed from a finite group G and C, r of its conjugacy classes and an equivalence relation \dagger. Components, interpret as a braid orbits on Nielsen classes. Fried-V\"olklein 1991 related absolute (corresponding to a permutation representation, T, of G) and inner equivalence classes. It noted two situations producing multiple components: #1. The action of a normalizer subgroup from $T$ on components; and #2. distinct components from the Schur multiplier of $G$ (the Fried-Serre lift invariant). Here we consider components of type #1 and #2 under one umbrella using a definition in G.~Gonz\'alez-D\'iez and W.J. Harvey, 1992 to generalize this paper and A. Ghigi and C. Tamborini, 2025. Our applications use Modular Towers to generalize Serre's Open Image Theorem. That distinguishes two types of decomposition groups -- designated GL_2 and CM -- that occur on towers of modular curves. Our generalization -- with mild constraints -- for any pair (G,C), generalizes modular curve towers to Modular Towers uses arithmetic properties of Jacobian varieties, the lift invariant, the shift-incidence pairing on cusps of reduced Hurwitz spaces to connect Hilbert's Irreducibility theorem to the Coleman-Oort conjecture.