Aissi, Youssef, et al. “Alienation of Drygas’ and Cauchy’s Functional Equations”. Annales Mathematicae Silesianae, vol. 35, no. 2, Apr. 2021, pp. 131-48, https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13440.