A multiplicity result for the problem $\delta d \xi = f'(<\xi,\xi>)\xi$
In this paper we consider the nonlinear equation involving differential forms on a compact Riemannian manifold $\delta d \xi = f'(<\xi,\xi>)\xi$. This equation is a generalization of the semilinear Maxwell equations recently introduced in a paper by Benci and Fortunato. We obtain a multiplicity result both in the positive mass case (i.e. $f'(t)\geq\epsilon>0$ uniformly) and in the zero mass case ($f'(t)\geq 0$ and $f'(0)=0$) where a strong convexity hypothesis on the nonlinearity is assumed.