Abstract. Let H be a Hopf algebra with bijective antipode. In a previous paper, we introduced H-Azumaya Yetter-Drinfel d module algebras, and the Brauer group BQ(k, H) classifying them. We continue our study of BQ(k, H), and we generalize some properties that were previously known for the BrauerLong group. We also investigate separability properties for H-Azumaya algebras, and this leads to the notion of strongly separable H-Azumaya algebra, and to a new subgroup of the Brauer group BQ(k, H).
IntroductionLet k be a commutative ring. Wall [26] introduced a Brauer group of Z/2Z-graded algebras. During the early seventies, some generalizations to gradings by other groups have been proposed, cf. e.g. [9]. In [14], Long introduced a Brauer group of H-dimodule algebras (that is, algebras with an H-action and an H-coaction), where H is a finitely generated, projective, commutative and cocommutative Hopf algebra. Long's Brauer group, today usually referred to as the Brauer-Long group, turned out to be the appropriate generalization of the BrauerWall group, since it contains all previously proposed generalizations as subgroups.Several authors have investigated the properties of the Brauer-Long group, cf. e.g. [1,2,5,6,7,10,19,20] (this list is not exhaustive). In all computations, the fact that the basic Hopf algebra is finitely generated, projective, commutative and cocommutative seems to be essential. This is a severe restriction, since, apart from finite abelian group rings, examples of such Hopf algebras are scarce. In a previous paper [8], we have been able to generalize Long's construction to arbitrary Hopf algebras-the only remaining condition is that the Hopf algebra H needs to have a bijective antipode. The underlying philosophy is that we replace Long's dimodules and dimodule algebras by Yetter-Drinfel d modules (also called quantum YangBaxter modules or crossed modules) and Yetter-Drinfel d module algebras. The Brauer group BQ(k, H) then consists of Brauer equivalence classes of H-Azumaya algebras ; if H is finitely generated, projective, commutative and cocommutative, then BQ(k, H) is anti-isomorphic to the Brauer-Long group BD(k, H). The fact that we have an anti-isomorphism rather than an isomorphism comes from the