Cohomological concentration conjecture for 0-toral characters

Let Tr\mathbb{T}_r and Ur\mathbb{U}_r be the groups appearing in the cohomology varieties YTr,UrY_{\mathbb{T}_r,\mathbb{U}_r}, and let θ ⁣:Tr(Fq)F×\theta\colon \mathbb{T}_r(\mathbb{F}_q)\to\overline{\mathbb{F}}_\ell^{\times} be the reduction of a character θ~ ⁣:Tr(Fq)Z×\widetilde{\theta}\colon \mathbb{T}_r(\mathbb{F}_q)\to\overline{\mathbb{Z}}_\ell^{\times}. The notation Hc(YTr,Ur;F)θH_c^*(Y_{\mathbb{T}_r,\mathbb{U}_r};\overline{\mathbb{F}}_\ell)_\theta denotes the corresponding character component of compactly supported étale cohomology, and a 0-toral character is understood in the sense of Cunningham–Osserman. Cohomological concentration conjecture. If θ\theta is a 0-toral character, then

Hc(YTr,Ur;F)θH_c^*(Y_{\mathbb{T}_r,\mathbb{U}_r};\overline{\mathbb{F}}_\ell)_\theta

is non-zero in only one degree. The conjecture extends known concentration results for ordinary Deligne–Lusztig varieties and is proposed in the context of torsion in integral cohomology; its resolution is not stated in the source.

Sources & referencesView supporting material

Primary source

Gebhard Böckle, Tony Feng, Michael Harris, Chandrashekhar Khare and Jack A. Thorne, “Cyclic base change of cuspidal automorphic representations over function fields”, arXiv:2205.04499 (2023).

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