Popa, Valeriu. “A General Fixed Point Theorem for Implicit Cyclic Multi-Valued Contraction Mappings”. Annales Mathematicae Silesianae 29 (September 30, 2015): 119–129. Accessed May 8, 2025. https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13981.