RUS, Ioan A. “Some Variants of Contraction Principle, Generalizations and Applications: Dedicated to Professor Gheorghe Coman on the Occasion of His 80th Anniversary”. Studia Universitatis Babeș-Bolyai Mathematica, vol. 61, no. 3, Sept. 2016, pp. 343-58, https://studia.reviste.ubbcluj.ro/index.php/subbmathematica/article/view/5585.