On the semi-infinite Deligne--Lusztig varieties for $\mathrm{GSp}$

Abstract: We prove that the structure of Lusztig's semi-infinite Deligne--Lusztig variety for $\mathrm{GSp}$ (and its inner form) is isomorphic to an affine Deligne--Lusztig variety at infinite level, as a generalization of Chan--Ivanov's result. Furthermore, we show that a component of some affine Deligne--Lusztig variety for $\mathrm{GSp}$ can be written as a direct product of a classical Deligne--Lusztig variety and an affine space, up to perfection. We also study varieties $X_{r}$ defined by Chan and Ivanov, and we show that $X_{r}$ at infinite level can be regarded as a component of semi-infinite Deligne--Lusztig varieties even in the $\mathrm{GSp}$ case. This result interprets previous studies on representations coming from $X_{r}$ as a realization of Lusztig's conjecture.