Akkouchi, Mohamed, et al. “Fixed Point Approach to the Stability of an Integral Equation in the Sense of Ulam-Hyers-Rassias”. Annales Mathematicae Silesianae, vol. 25, Sept. 2011, pp. 27-44, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/14019.