Existence and multiplicity of periodic solutions for some second order Hamiltonian systems

Abstract

In this paper, we study the existence of nontrivial periodic solutions for the second order Hamiltonian systems $ \ddot u(t)+\nabla F(t,u(t))=0$, where $F(t,x)$ is either nonquadratic or superquadratic as $|u|\mathbb{R}ightarrow \infty$. Furthermore, if $F(t,x)$ is even in $x$, we prove the existence of infinitely many periodic solutions for the general Hamiltonian systems $ \ddot u(t)+A(t)u(t)+\nabla F(t,u(t))=0$, where $A(\cdot)$ is a continuous $T$-periodic symmetric matrix. Our theorems mainly improve the recent result of Tang and Jiang [X.H. Tang, J. Jiang, Existence and multiplicity of periodic solutions for a class of second-order Hamiltonian systems, Comput. Math. Appl. 59 (2010) 3646-3655].