Partial generalized crossed products, Brauer groups and a comparison of seven-term exact sequences
Mikhailo Dokuchaev, Hector Pinedo, Itailma Rocha
Abstract
Given a unital partial action $α$ of a group $G$ on a commutative ring $R$ we denote by $ {\bf PicS} _{R^α}(R) $ the Picard monoid of the isomorphism classes of partially invertible $R$-bimodules, which are central over the subring $R^α \subseteq R$ of $α$-invariant elements, and consider a specific unital partial representation $Θ: G \to {\bf PicS} _{R^α}(R), $ along with the abelian group $\mathcal {C}(Θ/R)$ of the isomorphism classes of partial generalized crossed products related to $Θ,$ which already showed their importance in obtaining a partial action analogue of the Chase-Harrison-Rosenberg seven-term exact sequence. We give a description of $\mathcal {C}(Θ/R)$ in terms partial generalized products of the form $\mathcal D(f Θ)$ where $f$ is partial $1$-cocycle of $G$ with values in a submonoid of $ {\bf PicS}_{R^α}(R).$ Assuming that $G$ is finite and that $R^α \subseteq R$ is a partial Galois extension, we prove that any Azumaya $R^α$-algebra, containing $R$ as a maximal commutative subalgebra, is isomorphic to a partial generalized crossed product. Furthermore, we show that the relative Brauer group $\mathcal B(R/R^α)$ can be seen as a quotient of $\mathcal {C}(Θ/R)$ by a subgroup isomorphic to the Picard group of $R.$ Finally, we prove that the analogue of the Chase-Harrison-Rosenberg sequence, obtained earlier for partial Galois extensions of commutative rings, can be derived from a recent seven-term exact sequence established in a non-commutative setting.