On the group of separable quadratic algebras and stacks
  The aim of this paper is to study the group of isomorphism classes of torsors of finite flat group schemes of rank 2 over a commutative ring $R$. This, in particular, generalises the group of quadratic algebras (free or projective), which is especially well studied. Our approach however, yields new results even in this case. 
Ilia Pirashvili
math.AG math.AC