Outer Type Severi-Brauer Schemes

Abstract

We introduce the notion of a lowered flag of $\mathcal{O}$--modules in order to define a sheaf of flags of ideals isomorphic to the sheaf of parabolic subgroups for the general linear group $\mathbf{GL}_{1,\mathcal{A}}$ of an Azumaya algebra over a general scheme $S$. This notion is extended to the outer type $A_n$ case and we define a suitable sheaf of flags of ideals isomorphic to the sheaf of parabolic subgroups for a unitary group over $S$. When the group is suitably split these are related to flags of submodules in a vector bundle or in a vector bundle with hermitian form, respectively. We also define a sheaf of tuples of idempotents in the associated algebra which is isomorphic to the sheaf of parabolic and Levi subgroup pairs. We show how the type morphism from parabolic subgroups to the Dynkin scheme can be defined in terms of these sheaves of flags. We review how the Severi-Brauer scheme associated to an Azumaya algebra $\mathcal{A}$ is isomorphic to a particular fiber of this type morphism and we generalize this idea to the outer case in order to define outer Severi-Brauer schemes. We provide a new approach to Quillen's construction which produces an Azumaya algebra from a Severi-Brauer scheme and we show that an outer version of Quillen's construction also exists for outer Severi-Brauer schemes which produces an algebra with unitary involution.