We analyze the geometry of some $p$-adic Deligne--Lusztig spaces $X_w(b)$ introduced in [Iva21] attached to an unramified reductive group ${\bf G}$ over a non-archimedean local field. We prove that when ${\bf G}$ 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.

Comment: \'Epijournal de Géométrie Algébrique, Volume 7 (2023), Article no. 19

• 14F20 - Étale and other Grothendieck topologies and (co)homologies
• 14M15 - Grassmannians, Schubert varieties, flag manifolds
• 20G25 - Linear algebraic groups over local fields and their integers