1. Abkar, A., Gabeleh, M.: Proximal quasi-normal structure and a best proximity point theorem. J. Nonlin. Convex Anal. 14, 653–659 (2013)
2. Ahuja, G.C., Narang, T.D., Trehan, S.: Best approximation on convex sets in a metric space. J. Approx. Theory 12, 94–97 (1974)
3. Alspach, D.E.: A fixed point free nonexpansive map. Proc. Amer. Math. Soc. 82, 423–424 (1981)
4. Banach, S.: Sur les opérations dans les ensembles abstraits et leur application aux équations intégrales. Fundamenta Mathematicae 3, 133–181 (1922)
5. Brodskii, M.S., Milman, D.P.: On the center of a convex set. Dokl. Akad. Nauk. USSR 59, 837–840 (1948). (in Russian)