Ekeland, Takahashi and Caristi principles in quasi-pseudometric spaces

Abstract

We prove versions of Ekeland, Takahashi and Caristi principles in sequentially right $K$-complete quasi-pseudometric spaces (meaning asymmetric pseudometric spaces), the equivalence between these principles, as well as their equivalence to the completeness of the underlying quasi-pseudometric space. The key tools are Picard sequences for some special set-valued mappings corresponding to a function $\varphi$ on a quasi-pseudometric space, allowing a unitary treatment of all these principles.