The oldest quartic functional equation was introduced by J. M. Rassias in (1999), and then was employed by other authors. The functional equation is called a quartic functional equation, all of its solution is said to be a quartic function. In the current paper, the Hyers-Ulam stability and the superstability for quartic functional equations are established by using the fixed-point alternative theorem.
"Ulam Stability of a Quartic Functional Equation." Abstr. Appl. Anal. 2012 (SI14) 1 - 9, 2012. https://doi.org/10.1155/2012/232630