Kottwitz-style component conjecture for parahoric bundle moduli

Assume that char(k)2\operatorname{char}(k)\neq 2, that π:X~X\pi:\widetilde X\rightarrow X is a possibly ramified double cover with involution σ\sigma, and set G=SUn(X~/X)\mathcal G={\rm SU}_n(\widetilde X/X). Let MG/X\mathcal M_{\mathcal G/X} be the moduli stack of G\mathcal G-torsors, and let ηˉ\bar\eta and η\eta denote geometric and generic points of XX. Denote by π1(Gηˉ)\pi_1(\mathcal G_{\bar\eta}) the algebraic fundamental group in the sense of Borovoi, and write Γ=Gal(ηˉ/η)\Gamma=\operatorname{Gal}(\bar\eta/\eta). Kottwitz-style component conjecture. The connected components satisfy

π0(MG/X)=π1(Gηˉ)Γ.\pi_0(\mathcal M_{\mathcal G/X})=\pi_1(\mathcal G_{\bar\eta})_{\Gamma}.

Here the right-hand side denotes the coinvariants under Γ\Gamma. In particular, if Gηˉ\mathcal G_{\bar\eta} is semisimple and simply connected, the moduli stack should be connected. The statement is known in the constant case over C\mathbf C by topological uniformization, but is open in the stated generality.

Sources & referencesView supporting material

Primary source

G. Pappas and M. Rapoport, “Some questions about G-bundles on curves”, arXiv:0808.3743 (2008).

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