Rassias, J. M., T. Z. Xu, and W. X. Xu. “A Generalized Mixed Type of Quartic, Cubic, Quadratic and Additive Functional Equation”. Ukrains’kyi Matematychnyi Zhurnal 63, no. 3 (March 25, 2011): 399-415. Accessed April 19, 2024. https://umj.imath.kiev.ua/index.php/umj/article/view/2725.