@book{16402, author = {Ekeland, Ivar and Szulkin, Andrzej}, title = {Minimax results of Lusternik-Schnirelman type and applications . Part 2 of the proceedings of the NATO ASI "Variational methods in nonlinear problems". Montreal , 1986.}, publisher = {Universite de Montreal}, year = {1989}, address = {Montreal} }