We study the subspace of the exterior algebra of a simple complex Lie algebra
linearly spanned by the copies of the little adjoint representation or, in the
case of the Lie algebra of traceless matrices, by the copies of the n-th
symmetric power of the defining representation. As main result we prove that
this subspace is a free module over the subalgebra of the exterior algebra
generated by all primitive invariants except the one of highest degree.Comment: Latex file, 11 pages, Final version, appeared in "Rendiconti Lincei -
Matematica e Applicazioni
