Prehomogeneous vector spaces and field extensions II
  The purpose of the paper is to prove an analogous correspondence for the above prehomogeneous vector space (1)-(3). For case (1) the correspondence is bijective. However, it turns out that the correspondence is not bijective for cases (2), (3). We describe the fiber structures of this correspondence in subsection 2-4. In section 1 we briefly review basic properties of the non-abelian Galois cohomology set and its relation to the orbit space G_k \ V_k^ss. In subsection 2-4 we consider the prehomogeneous vector spaces (1)-(3) respectively. 
Anthony C. Kable, Akihiko Yukie