Popa, Valeriu. “A General Fixed Point Theorem for Two Pairs of Absorbing Mappings in Gp-Metric Spaces”. Annales Mathematicae Silesianae, vol. 34, no. 2, May 2020, pp. 268-85, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13617.