Conjectural splitting of the virtual class for twisted maps to the universal root stack

Let XX be a smooth pair with boundary divisor DD, let rr be a positive integer, and let Λ\Lambda denote the discrete data of the twisted maps. Write OrbΛ(Ar)\mathrm{Orb}_{\Lambda}(\mathcal{A}_r) for the corresponding moduli space of maps to the universal rr-th root stack, let vd\mathrm{vd} be its virtual dimension, and let Z[τ],ΓZ_{[\tau],\Gamma'} be strata indexed by types [τ][\tau] and graphs Γ\Gamma'. For each stratum, let a[τ],Γa_{[\tau],\Gamma'} be a coefficient and let C[τ],ΓC_{[\tau],\Gamma'} be a Chow class depending on rr. Conjectural splitting. There exists an expression

[OrbΛ(Ar)]vir=([τ],Γ)a[τ],Γ[Z[τ],Γ]C[τ],Γ[\mathrm{Orb}_{\Lambda}(\mathcal{A}_r)]^{\mathrm{vir}} = \sum_{([\tau], \Gamma')} a_{[\tau], \Gamma'} [Z_{[\tau], \Gamma'}] \cap C_{[\tau], \Gamma'}

where the sum is over strata of dimension at least vd\mathrm{vd} and C[τ],ΓC_{[\tau],\Gamma'} is a Chow class which is a Laurent polynomial in rr such that

degr([Z[τ],Γ])+degr(C[τ],Γ)0.\operatorname{deg}_r([Z_{[\tau], \Gamma'}]) + \operatorname{deg}_r(C_{[\tau], \Gamma'}) \geq 0.

This proposed decomposition is intended to provide the general structure of the virtual class needed to deduce the polynomiality results for twisted maps to smooth pairs. The source gives no resolution of the conjecture, so its status remains open.

Sources & referencesView supporting material

Primary source

Robert Crumplin, “Moduli spaces of twisted maps to smooth pairs”, arXiv:2501.15171 (2025).

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