On commuting varieties of nilradicals of Borel subalgebras of reductive Lie algebras

Let G be a connected reductive algebraic group defined over an algebraically closed field k of characteristic zero. We consider the commuting variety C(u) of the nilradical u of the Lie algebra b of a Borel subgroup B of G. In case B acts on u with only a finite number of orbits, we verify that C(u) is equidimensional and that the irreducible components are in correspondence with the distinguished B-orbits in u. We observe that in general C(u) is not equidimensional, and determine the irreducible components of C(u) in the minimal cases where there are infinitely many B-orbits in u