Elqorachi, Elhoucien, and Ahmed Redouani. “Solutions and Stability of Generalized Kannappan’s and Van Vleck’s Functional Equations”. Annales Mathematicae Silesianae 32 (August 5, 2017): 169–200. Accessed July 3, 2025. https://trrest.vot.pl/ojsus/index.php/AMSIL/article/view/13919.