Homological interpretation of extensions and biextensions of 1-motives

Let k be a separably closed field. Let (for i=1,2,3) be three 1-motives defined over k. We define the geometrical notions of extension of K1 by K3 and of biextension of (K1,K2) by K3. We then compute the homological interpretation of these new geometrical notions: namely, the group Biext0(K1,K2;K3) of automorphisms of any biextension of (K1,K2) by K3 is canonically isomorphic to the group , and the group Biext1(K1,K2;K3) of isomorphism classes of biextensions of (K1,K2) by K3 is canonically isomorphic to the group .