... critical point theory and applications, ” Journal of Functional Analysis, vol. 14, pp. 349–381, 1973.24 M. Willem, Minimax Theorems,vol.24ofProgress in Nonlinear Differential Equations and Their Applications, Birkh¨auser, ... Ekeland, “On the variational principle,” Journal of Mathematical Analysis and Applications, vol. 47, pp.324–353, 1974.22 P. H. Rabinow it z, Minimax Methods in Critical Point Theory with Applications ... 11, and their references. However, little has been done for this type of problem in unboundeddomains. For ΩN, we are only aware of the works 12–15 which studied the existence of solutions for...