Solution and Stability of a General Mixed Type Cubic and Quartic Functional Equation

We consider the following mixed type cubic and quartic functional equation = where is a fixed integer. We establish the general solution of the functional equation when the integer , and then, by using the fixed point alternative, we investigate the generalized Hyers-Ulam-Rassias stability for this functional equation when the integer . 1. Introduction In 1940, Ulam [1] asked the fundamental question for the stability for the group homomorphisms. Let be a group, and let be a metric group with the metric . Given , does there exist a such that if a mapping satisfies the inequality for all , then there is a homomorphism with for all ? In other words, under what conditions, does there exist a homomorphism near an approximately homomorphism? In the next year, Hyers [2] gave the first affirmative answer to the question of Ulam for Cauchy equation in the Banach spaces. Then, Rassias [3] generalized Hyers’ result by considering an unbounded Cauchy difference, and this stability phenomenon is known as generalized Hyers-Ulam-Rassias stability or Hyers-Ulam-Rassias stability. During the last three decades, the stability problem for several functional equations has been extensively investigated by many mathematicians; see, for example, [4–9] and the references therein. We also refer the readers to the books [10–13]. In [14], Jun and Kim introduced the following functional equation It is easy to see that the function satisfies the functional equation (3). Thus, it is natural that (3) is called a cubic functional equation and every solution of the cubic functional equation is said to be a cubic function. In [14], Jun and Kim established the general solution and the generalized Hyers-Ulam-Rassias stability for (3). They proved that a function between real vector spaces is a solution of the functional equation (3) if and only if there exists a function such that for all and is symmetric for each fixed one variable and is additive for fixed two variables. In [15], Lee et al. considered the following quartic functional equation Since the function satisfies the functional equation (4), the functional equation (4) is called a quartic functional equation and every solution of the quartic functional equation is said to be a quartic function. In [15], the authors solved the functional equation (4) and proved the stability for it. Actually, they obtained that a function between real vector spaces satisfies the functional equation (4) if and only if there exists a symmetric biquadratic function such that for all . A function between real vector spaces is said to be quadratic if