1. J. P. Aubin and I. Ekeland, Applied Nonlinear Analysis, John Wiley & Sons, United States, 1984.
2. A. Ambrosetti and P. H. Rabinowitz, Dual variational methods in critical point theory and applications, J. Funct. Anal. 14 (1973), no. 4, 349–381.
3. G. Bonanno, A critical point theorem via the Ekeland variational principle, Nonlinear Anal. Theory Meth. Appl. Ser A Theory Meth. 75 (2012), no. 5, 2992–3007.
4. F. H. Clarke, Optimization and nonsmooth analysis, Can. Math. Soc. Ser. Monogr. Adv. Texts, John Wiley, New York, NY, 1983.
5. D. G. De Figueiredo, Lectures on the Ekeland Variational Principle with Applications and Detours, vol. 81, Springer, Berlin, 1989.