Nonlinear operators and the calculus of variations : summer school held in Bruxelles, 8-19 September, 1975 - Gossez, J. P., 1943-; Gossez, J. P.; Lami Dozo, E. J.; Mawhin, J.; Waelbroeck, L. (Lecture Notes in Mathematics / Springer-Verlag ;, 1976)