“…In the last ten years, by classical variational methods, there are many interesting results about the existence and nonexistence of solutions, sign-changing solutions, ground state solutions, the existence of positive solutions and positive ground states, least energy nodal solutions, multiplicity of solutions, semiclassical limit and concentrations of solutions to Kirchhoff type problems, see e.g. [3,4,5,11,12,13,20,25,26] and the references therein.…”