We study the relationship between antipodes on a Hopf algebroid $\mathcal{ H}$ in the sense of B\"ohm-Szlachanyi and the group of twists that lies inside the associated convolution algebra. We specialize to the case of a faithfully flat $H$-Hopf-Galois extensions $B\subseteq A$ and related Ehresmann-Schauenburg bialgebroid. In particular, we find that the twists are in one-to-one correspondence with $H$-comodule algebra automorphism of $A$. We work out in detail the $U(1)$-extension ${ \mathcal O}(\mathbb{ C}P^{ n-1}_q)\subseteq { \mathcal O}(S^{ 2n-1}_q)$ on the quantum projective space and show how to get an antipode on the bialgebroid out of the $K$-theory of the base algebra ${ \mathcal O}(\mathbb{ C}P^{ n-1}_q)$.
Categories: math.QA