“…There have been many works about the existence of nontrivial solutions to (1.1) by using variational methods, see for example, [1,5,6,7,8,9,11,12,13,18,20,21,23,24,25,27,28,29,32,34,35] and the references therein. A typical way to deal with (1.1) is to use the mountain-pass theorem.…”