Groupes libres et algebses de groupes en geometrie algebrique

Let X=Sp A be an affine scheme over a perfect field k of positive caracteristic. The functor D(k)-mod (W(A), (w(?)) is the unipotent part of the affine commutative free group generated by X (W is the ring of Witt-vectors and D the ring of Dieudonné). If X is a monoïd-scheme, this free group has a structure of ring scheme. In particular, if X=0 _k ^x , we obtain a new proof of the theorem of decomposition of the universal ring.