Hopf Algebroids, Bimodule Connections and Noncommutative Geometry

Abstract: We construct new examples of left bialgebroids and Hopf algebroids, arising from noncommutative geometry. Given a first order differential calculus $\Omega$ on an algebra $A$, with the space of left vector fields $\mathfrak{X}$, we construct a left $A$-bialgeroid $B\mathfrak{X}$, whose category of left modules is isomorphic to the category of left bimodule connections over the calculus. When $\Omega$ is a pivotal bimodule, we construct a Hopf algebroid $H\mathfrak{X}$ over $A$, by restricting to a subcategory of bimodule connections which intertwine with both $\Omega$ and $\mathfrak{X}$ in a compatible manner. Assuming the space of 2-forms $\Omega^{2}$ is pivotal as well, we construct the corresponding Hopf algebroid $\mathcal{D}\mathfrak{X}$ for flat bimodule connections, and recover Lie-Rinehart Hopf algebroids as a quotient of our construction in the commutative case. We use these constructions to provide explicit examples of Hopf algebroids over noncommutative bases.