“…For homoclinic solutions (HS), in the last thirty years the existence and multiplicity of homoclinic solutions have been extensively investigated via critical point theory and variational methods. Under the assumption that L(t) and W (t, u) are either independent of t or periodic in t, many authors have investigated the existence of (HS), see, for instance, [6,9,12,36,51] and the references therein, and some more general Hamiltonian systems are considered in the recent papers [19,27,45]. For the case that L(t) and W (t, u) are neither autonomous nor periodic in t, the existence of homoclinic solutions of (HS) is in sharp contrast to the periodic systems, because of the lack of compactness of Sobolev embedding [2,24,33,37].…”