A note on the order of the antipode of a pointed Hopf algebra

Abstract

Let $k$ be a field and let $H$ denote a pointed Hopf $k$-algebra with antipode $S$. We are interested in determining the order of $S$. Building on the work done by Taft and Wilson $[7]$, we define an invariant for $H$, denoted $m_H$, and prove that the value of this invariant is connected to the order of $S$. In the case where $\mathrm{char}k=0$, it is shown that if $S$ has finite order then it is either the identity or has order $2m_H$. If in addition $H$ is assumed to be coradically graded, it is shown that the order of $S$ is finite if and only if $m_H$ is finite. We also consider the case where $\mathrm{char}k=p>0$, generalising the results of $[7]$ to the infinite-dimensional setting.