#### Abstract

We investigate the stability problems for the -dimensional mixed-type additive and quadratic functional equation in random normed spaces by applying the fixed point method.

#### 1. Introduction

In 1940, Ulam [1] gave a wide-ranging talk before a mathematical colloquium at the University of Wisconsin, in which he discussed a number of important unsolved problems. Among those was the following question concerning the stability of homomorphisms.

*Let ** be a group, and let ** be a metric group with a metric **. Given **, does there exist a ** such that if a function ** satisfies the inequality ** for all **, then there is a homomorphism ** with ** for all **?*

If the answer is affirmative, we say that the functional equation for homomorphisms is stable. Hyers [2] was the first mathematician to present the result concerning the stability of functional equations. He answered the question of Ulam for the case where and are assumed to be Banach spaces. This result of Hyers is stated as follows.

*Let ** be a function between Banach spaces such that** for some ** and for all **. Then the limit ** exists for each **, and ** is the unique additive function such that ** for every **. Moreover, if ** is continuous in ** for each fixed **, then function ** is linear.*

We remark that the additive function is directly constructed from the given function , and this method is called the *direct method*. The direct method is a very powerful method for studying the stability problems of various functional equations. Taking this famous result into consideration, the additive Cauchy equation is said to have the *Hyers-Ulam stability* on if for every function satisfying the inequality (1.1) for some and for all , there exists an additive function such that is bounded on .

In 1950, Aoki [3] generalized the theorem of Hyers for additive functions, and in the following year, Bourgin [4] extended the theorem without proof. Unfortunately, it seems that their results failed to receive attention from mathematicians at that time. No one has made use of these results for a long time.

In 1978, Rassias [5] addressed the Hyers’s stability theorem and attempted to weaken the condition for the bound of the norm of Cauchy difference and generalized the theorem of Hyers for linear functions.

*Let ** be a function between Banach spaces. If ** satisfies the functional inequality**for some **, ** with ** and for all **, then there exists a unique additive function ** such that ** for each **. If, in addition, ** is continuous in ** for each fixed **, then the function ** is linear.*

This result of Rassias attracted a number of mathematicians who began to be stimulated to investigate the stability problems of functional equations. By regarding a large influence of Ulam, Hyers, and Rassias on the study of stability problems of functional equations, the stability phenomenon proved by Rassias is called the *Hyers-Ulam-Rassias stability*. For the last thirty years, many results concerning the Hyers-Ulam-Rassias stability of various functional equations have been obtained (see [6–17]).

In this paper, applying the fixed point method, we prove the Hyers-Ulam-Rassias stability of the *-dimensional mixed-type additive and quadratic functional equation *
in random normed spaces. Every solution of (1.3) is called a *quadratic-additive function*.

Throughout this paper, let be an integer larger than 1.

#### 2. Preliminaries

We introduce some terminologies, notations, and conventions usually used in the theory of random normed spaces (see [18, 19]). The set of all probability distribution functions is denoted by Let us define . The set is partially ordered by the usual pointwise ordering of functions, that is, if and only if for all . The maximal element for in this order is the distribution function given by

*Definition 2.1 (See [18]). *A function is called a *continuous triangular norm* (briefly, *continuous **-norm*) if satisfies the following conditions: is commutative and associative;*τ* is continuous; for all ; for all with and .

Typical examples of continuous -norms are , , and .

*Definition 2.2 (See [19]). *Let be a vector space, a continuous -norm, and let be a function satisfying the following conditions: for all if and only if ; for all , , and for all ; for all and all . A triple is called a *random normed space* (briefly, *RN-space*).

If is a normed space, we can define a function by
for all and . Then is a random normed space, which is called the *induced random normed space*.

*Definition 2.3. *Let be an -space.A sequence in is said to be *convergent* to a point if, for every and , there exists a positive integer such that whenever .A sequence in is called a *Cauchy sequence* if, for every and , there exists a positive integer such that whenever .An RN-space is called *complete* if and only if every Cauchy sequence in converges to a point in .

*Definition 2.4. *Let be a nonempty set. A function is called a *generalized metric* on if and only if satisfies if and only if ; for all ; for all .

We now introduce one of the fundamental results of the fixed point theory. For the proof, we refer to [20] or [21].

Theorem 2.5 (See [20, 21]). *Let be a complete generalized metric space. Assume that is a strict contraction with the Lipschitz constant . If there exists a nonnegative integer such that for some , then the following statements are true:**the sequence converges to a fixed point of ;** is the unique fixed point of in ;**if , then*

In 2003, Radu [22] noticed that many theorems concerning the Hyers-Ulam stability of various functional equations follow from the fixed point alternative (Theorem 2.5). Indeed, he applied the fixed point method to prove the existence of a solution of the inequality (1.1) and investigated the Hyers-Ulam stability of the additive Cauchy equation (see also [23–26]). Furthermore, Miheţ and Radu [27] applied the fixed point method to prove the stability theorems of the additive Cauchy equation in random normed spaces.

In 2009, Towanlong and Nakmahachalasint [28] established the general solution and the stability of the -dimensional mixed-type additive and quadratic functional equation (1.3) by using the direct method. According to [28], a function is a quadratic-additive function, where and are vector spaces, if and only if there exist an additive function and a quadratic function such that for all .

#### 3. Hyers-Ulam-Rassias Stability

Throughout this paper, let be a real vector space and let be a complete RN-space. For a given function , we use the following abbreviation: for all .

We will now prove the stability of the functional equation (1.3) in random normed spaces by using fixed point method.

Theorem 3.1. *Let be a real vector space, an RN-space, a complete RN-space, and let be a function. Assume that satisfies one of the following conditions:** for some ;** for some ** for all and . If a function satisfies and
**
for all and , then there exists a unique function such that
**
for all and
**
for all and , where , and .*

*Proof. *We will first treat the case where satisfies the condition . Let be the set of all functions with , and let us define a generalized metric on by
It is not difficult to show that is a complete generalized metric space (see [29] or [30, 31]).

Consider the operator defined by
Then we can apply induction on to prove
for all and .

Let and let be an arbitrary constant with . For some satisfying the condition , it follows from the definition of , , , and that
for all and , which implies that
That is, is a strict contraction with the Lipschitz constant .

Moreover, by , , and (3.2), we see that
for all and . Hence, it follows from the definition of that
Now, in view of Theorem 2.5, the sequence converges to the unique “fixed point” of in the set and is represented by
for all .

By Theorem 2.5, (3.11), and the definition of , we have
that is, the first inequality in (3.4) holds true.

We will now show that is a quadratic-additive function. It follows from and the definition of that
for all , , and . Due to the definition of , the first four terms on the right-hand side of the above inequality tend to 1 as .

By a somewhat tedious manipulation, we have
Hence, it follows from , , definition of , (3.2), and that
which tends to 1 as for all and . Therefore, (3.14) implies that
for any and . By , this implies that for all , which ends the proof of the first part.

Now, assume that satisfies the condition . Let be the same as given in the first part. We now consider the operator defined by
for all and . Notice that
for all and .

Let and let be an arbitrary constant with . From , , the definition of , and , we have
for all , , and for some satisfying , which implies that
That is, is a strict contraction with the Lipschitz constant .

Moreover, by , (3.2), and , we see that
for all and . This implies that by the definition of . Therefore, according to Theorem 2.5, the sequence converges to the unique “fixed point” of in the set and is represented by
for all . Since
the second inequality in (3.4) holds true.

Next, we will show that is a quadratic-additive function. As we did in the first part, we obtain the inequality (3.14). In view of the definition of , the first four terms on the right-hand side of the inequality (3.14) tend to 1 as . Furthermore, a long manipulation yields
Thus, it follows from , , definition of , (3.2), and that
which tends to 1 as for all and . Therefore, it follows from (3.14) that
for any and . By , this implies that
for all , which ends the proof.

By a similar way presented in the proof of Theorem 3.1, we can also prove the preceding theorem if the domains of relevant functions include 0.

Theorem 3.2. *Let be a real vector space, an RN-space, a complete RN-space, and let be a function. Assume that satisfies one of the conditions and in Theorem 3.1 for all and . If a function satisfies and (3.2) for all and , then there exists a unique quadratic-additive function satisfying (3.4) for all and .*

Now, we obtain general Hyers-Ulam stability results of (1.3) in normed spaces. If is a normed space, then is an induced random normed space. We get the following result.

Corollary 3.3. *Let be a real vector space, a complete normed space, and let be a function. Assume that satisfies one of the following conditions:** for some ;** for some ** for all . If a function satisfies and
**
for all , then there exists a unique function such that
**
for all and
**
for all .*

*Proof. *Let us put
for all , , and . If satisfies the condition for all and for some , then
for all and , that is, satisfies the condition . In a similar way, we can show that if satisfies , then it satisfies the condition .

Moreover, we get
for all and , that is, satisfies the inequality (3.2) for all .

According to Theorem 3.1, there exists a unique function such that
for all and
for all and , which ends the proof.

We now prove the Hyers-Ulam-Rassias stability of (1.3) in the framework of normed spaces.

Corollary 3.4. *Let be a real normed space, , and let be a complete normed space. If a function satisfies and
**
for all and for some , then there exists a unique quadratic-additive function such that
**
for all .*

*Proof. *If we put
then the induced random normed space satisfies the conditions stated in Theorem 3.2 with .

Corollary 3.5. *Let be a real normed space, , and let be a complete normed space. If a function satisfies and
**
for all and for some , then there exists a unique quadratic-additive function satisfying
*

*Proof. *If we put , , and define
for all and , then we have
that is, satisfies condition given in Theorem 3.1 for all and . We moreover get
that is, satisfies the inequality (3.2) for all and .

According to Theorem 3.2, there exists a unique quadratic-additive function satisfying
for all , or equivalently
for all , which ends the proof.

#### Acknowledgment

The second author was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (no. 2011-0004919).