Compactifications of the Drinfeld half space over a Finite Field

Abstract

When considered as a Deligne-Lusztig variety, the Drinfeld half space ${\Omega }_V$ over a finite field $k$ has a compactification whose boundary divisor is normal crossing and which can be obtained by successively blowing-up projective space along linear subspaces. Pink and Schieder (2014) have introduced a new compactification of ${\Omega }_V$ whose strata of the boundary are glued together in a way dual to the way they are for the tautological compactification by projective space. We show that by applying an analogous sequence of blow-ups to this new compactification we arrive at the compactification by Deligne and Lusztig as well. Moreover, we compute for each of these three compactifications the stabilizers of $\bar{k}$-valued points under the canonical $\mathrm{PGL}(V)$-action. We find that in each case the stratification can be recovered from the unipotent radicals of these stabilizers.

28 pages