Affine smooth group schemes over the dual numbers

Matthieu Romagny (1) , Dajano Tossici (2)

1 IRMAR - Institut de Recherche Mathématique de Rennes
2 IMB - Institut de Mathématiques de Bordeaux

We provide an equivalence between the category of affine, smooth group schemes over the ring of generalized dual numbers $k[I]$, and the category of extensions of the form $1 → Lie(G, I) → E → G → 1$ where G is an affine, smooth group scheme over k. Here k is an arbitrary commutative ring and $k[I] = k ⊕ I$ with $I^2 = 0$. The equivalence is given by Weil restriction, and we provide a quasi-inverse which we call Weil extension. It is compatible with the exact structures and the $O_k$-module stack structures on both categories. Our constructions rely on the use of the group algebra scheme of an affine group scheme; we introduce this object and establish its main properties. As an application, we establish a Dieudonné classification for smooth, commutative, unipotent group schemes over $k[I]$.

Domaines: Géométrie algébrique [math.AG]; Théorie des nombres [math.NT]; Théorie des représentations [math.RT]

Matthieu Romagny, Dajano Tossici. Affine smooth group schemes over the dual numbers. 2018. ⟨hal-01712886v2⟩