Team, S. “A Quartic Functional Equation and Its Generalized Hyers-Ulam-Rassias Stability: Montakarn Petapirak, Paisan Nakmahachalasint”. Thai Journal of Mathematics, vol. 6, no. 3, Dec. 2008, pp. 77-84, https://thaijmath2.in.cmu.ac.th/index.php/thaijmath/article/view/136.