Infinitesimal commutative unipotent group schemes with one-dimensional Lie algebra

Abstract

We prove that over an algebraically closed field of characteristic $p>0$ there are exactly, up to isomorphism, $n$ infinitesimal commutative unipotent $k$-group schemes of order $p^n$ with one-dimensional Lie algebra, and we explicitly describe them. We consequently recover an explicit description of the $p^n$-torsion of any supersingular elliptic curve over an algebraically closed field. Finally, we use these results to answer a question of Brion on rational actions of infinitesimal commutative unipotent group schemes on curves.