Periodic solutions of Euler-Lagrange equations with sublinear   pontentials in an Orlicz-Sobolev space setting
  In this paper, we obtain existence results of periodic solutions of hamiltonian systems in the Orlicz-Sobolev space $W^1L^{\Phi}([0,T])$. We employ the direct method of calculus of variations and we consider a potential function $F$ satisfying the inequality $|\nabla F(t,x)|\leq b_1(t) \Phi_0'(|x|)+b_2(t)$, with $b_1, b_2\in L^1$ and certain $N$-functions $\Phi_0$. 
Sonia Acinas and Fernando Mazzone
math.CA