“…More precisely, there is no natural space in which we can define the variational functional of the problem such that it possesses both smoothness and compactness. Those specific mathematical difficulties attract many researchers, and several new ideas are used, such as minimization method [20,18], change of variables [21] and Nehari method [17]. However, the changing variable argument proves to be quite effective for dealing with the special problem as −Δu − Δ(u 2 )u = g (x, u) where the quasilinear term is exactly −Δ(u 2 )u, so they can be transformed to a semilinear problem and various techniques for semilinear problems can be adapted in this situation.…”