Mawhin, J., & Willem, M. (1986). Critical-points of Convex Perturbations of Some Indefinite Quadratic-forms and Semilinear Boundary-value-problems At Resonance. Annales de l’Institut Henri Poincaré - C - Non Linear Analysis, 3(6), 431-453. https://hdl.handle.net/2078.5/80365 (Original work published 1986)