Hopf algebroids and twists for quantum projective spaces

We study the relationship between antipodes on a Hopf algebroid \sr{$\mathcal{H}$} in the sense of B\"ohm--Szlachanyi and the group of twists that lies inside the associated convolution algebra \sr{$\mathcal{H}^*$}. 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 ${\mathcal O}(\mathbb{C}P^{n-1}_q)$.