POPA, Valeriu. A general fixed point theorem for implicit cyclic multi-valued contraction mappings. Annales Mathematicae Silesianae, [S. l.], v. 29, p. 119–129, 2015. Disponível em: https://journals.us.edu.pl/index.php/AMSIL/article/view/13981. Acesso em: 5 dec. 2025.