Nilpotent orbits and theta-stable parabolic subalgebras

In this work, we present a new classification of nilpotent orbits in a real reductive Lie algebra g under the action of its adjoint group. Our classification generalizes the Bala-Carter classification of the nilpotent orbits of complex semisimple Lie algebras. Our theory takes full advantage of the work of Kostant and Rallis on p_ℂ, the “complex symmetric space associated with g”. The Kostant-Sekiguchi correspondence, a bijection between nilpotent orbits in g and nilpotent orbits in p_ℂ, is also used. We identify a fundamental set of noticed nilpotents in p_ℂand show that they allow us to recover all other nilpotents. Finally, we study the behaviour of a principal orbit, that is an orbit of maximal dimension, under our classification. This is not done in the other classification schemes currently available in the literature.