Team, S. “The Stability of the Pexiderized Cosine Functional Equation: C. Kusollerschariya, P. Nakmahachalasint”. Thai Journal of Mathematics, vol. 6, no. 3, Dec. 2008, pp. 39-44, https://thaijmath2.in.cmu.ac.th/index.php/thaijmath/article/view/132.