Some generalizations of Ekeland-type variational principle with applications to equilibrium problems and fixed point theory

In this paper, we introduce the concept of a Q-function defined on a quasi-metric space which generalizes the notion of a τ-function and a w-distance. We establish Ekeland-type variational principles in the setting of quasi-metric spaces with a Q-function. We also present an equilibrium version of the Ekeland-type variational principle in the setting of quasi-metric spaces with a Q-function. We prove some equivalences of our variational principles with Caristi-Kirk type fixed point theorems for multivalued maps, the Takahashi minimization theorem and some other related results. As applications of our results, we derive existence results for solutions of equilibrium problems and fixed point theorems for multivalued maps. We also extend the Nadler's fixed point theorem for multivalued maps to a Q-function and in the setting of complete quasi-metric spaces. As a consequence, we prove the Banach contraction theorem for a Q-function and in the setting of complete quasi-metric spaces. The results of this paper extend and generalize many results appearing recently in the literature.