Cocycle deformations for liftings of quantum linear spaces

TL;DR

This paper investigates cocycle deformations of liftings of quantum linear spaces, showing when a natural cocycle candidate suffices and extending computations to quantum linear spaces of arbitrary dimension.

Contribution

It demonstrates that the cocycle $oldsymbol{ ext{lambda} ext{circ} ext{xi}}$ often suffices for twisting, extends cocycle calculations to quantum linear spaces of any dimension, and provides new examples.

Findings

The cocycle $ ext{lambda} ext{circ} ext{xi}$ often yields the desired twisting.

Explicit cocycle computations are extended to quantum linear spaces of arbitrary dimension.

Counterexamples show cases where the cocycle differs from $ ext{lambda} ext{circ} ext{xi}$.

Abstract

Let $A$ be a Hopf algebra over a field $K$ of characteristic 0 and suppose there is a coalgebra projection $\pi$ from $A$ to a sub-Hopf algebra $H$ that splits the inclusion. If the projection is $H$-bilinear, then $A$ is isomorphic to a biproduct $R #_{\xi}H$ where $(R,\xi)$ is called a pre-bialgebra with cocycle in the category $_{H}^{H}\mathcal{YD}$. The cocycle $\xi$ maps $R \otimes R$ to $H$. Examples of this situation include the liftings of pointed Hopf algebras with abelian group of points $\Gamma$ as classified by Andruskiewitsch and Schneider [AS1]. One asks when such an $A$ can be twisted by a cocycle $\gamma:A\otimes A\rightarrow K$ to obtain a Radford biproduct. By results of Masuoka [Ma1, Ma2], and Gr\"{u}nenfelder and Mastnak [GM], this can always be done for the pointed liftings mentioned above. In a previous paper [ABM1], we showed that a natural candidate for a twisting cocycle is {$\lambda \circ \xi$} where $\lambda\in H^{\ast}$ is a total integral for $H$ and $\xi$ is as above. We also computed the twisting cocycle explicitly for liftings of a quantum linear plane and found some examples where the twisting cocycle we computed was different from {$\lambda \circ \xi$}. In this note we show that in many cases this cocycle is exactly $\lambda\circ\xi$ and give some further examples where this is not the case. As well we extend the cocycle computation to quantum linear spaces; there is no restriction on the dimension.