On a decomposition of $p$-adic Coxeter orbits

Abstract

We analyze the geometry of some $p$-adic Deligne--Lusztig spaces $X_w(b)$ introduced in [Iva21] attached to an unramified reductive group $\mathbf{G}$ over a non-archimedean local field. We prove that when $\mathbf{G}$ is classical, $b$ basic and $w$ Coxeter, $X_w(b)$ decomposes as a disjoint union of translates of a certain integral $p$-adic Deligne--Lusztig space. Along the way we extend some observations of DeBacker and Reeder on rational conjugacy classes of unramified tori to the case of extended pure inner forms, and prove a loop version of Frobenius-twisted Steinberg's cross section.

arXiv commentÉpijournal de Géométrie Algébrique, Volume 7 (2023), Article no. 19

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