Bisections and cocycles on Hopf algebroids

Abstract

We introduce and study the group $\mathcal{B}(\mathcal{L})$ of bisections of a Hopf algebroid $\mathcal{L}$ and show that they form a group crossed module or 2-group with the group $\Aut(\mathcal{L})$ of automorphisms. Moreover, the group of vertical bisections turns out to be part of a certain non-Abelian cohomology of which $\mathcal{H}^2(\mathcal{L},B)$ governs cotwisting of a Hopf algebroid with base $B$. For the Ehresmann-Schauenburg Hopf algebroid $\mathcal{L}(P,H)$ of a quantum principal bundle or Hopf-Galois extension, $\mathcal{B}(\mathcal{L}(P,H))$ reduces to the group $\Aut_H(P)$ of bundle automorphisms and vertical bisections to the group of `gauge transformations' of the bundle. The general $\mathcal{H}^2(\mathcal{L}(P,H),B)$ reduces to a known non-Abelian cohomology in the case where $P$ is a trivial principal bundle or cleft extension. Parallel characterisations are obtained for the bisections and non-Abelian cohomology of the action Hopf algebroid $B\# H^{op}$ associated to a braided-commutative algebra $B$ in the category of Drinfeld-Yetter modules over a Hopf algebra $H$. Examples include the Heisenberg double or Weyl Hopf algebroid of a Hopf algebra and a canonical action Hopf algebroid $\underline H\# H^{op}$ when $H$ is coquasitriangular and $\underline{H}$ is its transmutation.