Azumaya Algebras and Obstructions to Quadratic Pairs over a Scheme

Abstract: We investigate quadratic pairs for Azumaya algebras with involutions over a base scheme S as defined by Calm{`e}s and Fasel, generalizing the case of quadratic pairs on central simple algebras over a field (Knus, Merkurjev, Rost, Tignol). We describe a cohomological obstruction for an Azumaya algebra over S with orthogonal involution to admit a quadratic pair. When S is affine this obstruction vanishes, however it is non-trivial in general. In particular, we construct explicit examples with non-trivial obstructions.