Real graded Lie algebras, Galois cohomology, and classification of trivectors in R⁹

In this paper we classify real trivectors in dimension 9. The corresponding classification over the field C of complex numbers was done by Vinberg and Elashvili in 1978. One of the main tools used for their classification was the construction of the representation of SL(9,C) on the space of complex trivectors of C^9 as a theta-representation corresponding to a Z/3Z-grading of the simple complex Lie algebra of type E8. This divides the trivectors into three groups: nilpotent, semisimple, and mixed trivectors. Our classification follows the same pattern. We use Galois cohomology to obtain the classification over R. For the nilpotent orbits this is in principle rather straightforward and the main problem is to determine the first Galois cohomology sets of a long list of centralizers (we compute the centralizers using computer). For the semisimple and mixed orbits we develop new methods based on Galois cohomology, first and second. We also consider real theta-representations in general, and derive a number of results that are useful for the classification of their orbits.