Classification of nilpotent and semisimple fourvectors of a real eight-dimensional space

Abstract

In 1981 Antonyan classified the orbits of SL$(8,\mathbb{C})$ on ${\bigwedge }^4{\mathbb{C}}^8$. This is an example of a $\theta$-group action as introduced and studied by Vinberg. The orbits of a $\theta$-group are divided into three classes: nilpotent, semisimple and mixed. We consider the action of SL$(8,\mathbb{R})$ on ${\bigwedge }^4{\mathbb{R}}^8$ and classify the nilpotent and semisimple orbits as well as the Cartan subspaces. The semisimple orbits are divided into 1441 parametrized classes. Due to this high number a classification of the mixed orbits does not seem feasible. Our methods are based on Galois cohomology.