# Hopf algebroids and twists for quantum projective spaces

## TL;DR

This paper explores the structure of Hopf algebroids and twists in quantum projective spaces, establishing a correspondence between twists and automorphisms, and constructing antipodes using K-theory.

## Contribution

It provides a detailed analysis of antipodes and twists in Hopf algebroids related to quantum projective spaces, including explicit constructions and classifications.

## Key findings

- Twists correspond to $H$-comodule algebra automorphisms.
- Antipodes can be constructed from the K-theory of the base algebra.
- Explicit example on quantum projective space demonstrates the theory.

## Abstract

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 ${\mathcal O}(\mathbb{C}P^{n-1}_q)$.

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Source: https://tomesphere.com/paper/2302.12073