Extending Doi--Takeuchi correspondence to bimodules and noncommutative covers

Abstract

Doi and Takeuchi, and independently Blattner, Cohen and Montgomery, have exhibited a correspondence between cleft extensions of an algebra $U$ by a Hopf algebra $H$ and $H$-measurings on $U$ equipped with a compatible normalized 2-cocycle. Each cleft extension is isomorphic as an algebra, a left $U$-module and a right $H$-comodule to the crossed product algebra constructed from the measuring and the cocycle. Cleft $H$-extensions are a special case of $H$-Hopf-Galois extensions, describing noncommutative principal $H$-bundles with affine total and affine base space. Corings are used to describe noncommutative space covers, allowing a description of possibly nonaffine noncommutative spaces. We propose to describe the total space of a noncommutative $H$-principal bundle by an internal $E$-coring in $H$-comodules where $E$ is an $H$-comodule algebra which is itself $H$-Hopf-Galois. This article focuses on the cleft case. As corings involve bimodules and morphisms of bimodules we extend the Doi--Takeuchi correspondence to bimodule $H$-comodules between cleft extensions on one side and bimodules of $H$-coinvariants equipped with bimodule $H$-measurings compatible with 2-cocycles on the other side. In fact, the correspondences for algebras and for bimodules are parts of a biequivalence of suitable bicategories defined in this article. As a corollary one obtains a correspondence between cleft $H$-extensions of corings and corings equipped with a nonabelian Čech 1-cocycle covalued in $H$.