1. P. Drábek, J. Milota, Methods of Nonlinear Analysis. Applications to Differential Equations, 2nd edn. Birkhäuser Advanced Texts. Basler Lehrbücher (Springer, Basel, 2013)
2. I. Ekeland and R. Temam, Convex Analysis and Variational Problems (North-Holland, Amsterdam, 1976)
3. A.D. Ioffe, V.M. Tikhomirov, Theory of Extremal Problems (in Russian). Series in Nonlinear Analysis and its Applications. Izdat., Nauka, Moscow (1974)
4. see also: Theory of extremal problems. Translated from the Russian by Karol Makowski. Studies in Mathematics and its Applications, vol. 6 (North-Holland Publishing Co., Amsterdam, 1979), xii+460 pp
5. N. Iusem, D. Reem, S. Reich, Fixed points of Legendre-Fenchel type transforms. J. Convex Anal. 26, 275–298 (2019)