Another multiplicity result for the periodic solutions of certain systems

In this paper, we deal with a problem of the type \cases{(\phi(u'))'=\nabla_xF(t,u) & in $[0,T]$\cr & \cr u(0)=u(T)\ , \hskip 3pt u'(0)=u'(T)\ ,\cr} where, in particular, $\phi$ is a homeomorphism from an open ball of ${\bf R}^n$ onto ${\bf R}^n$. Using the theory developed by Brezis and Mawhin in [1] jointly with our minimax theorem proved in [3], we obtain a general multiplicity result, under assumptions of qualitative nature only. Three remarkable corollaries are also presented.