Abstract and Applied Analysis

Volume 2009, Article ID 535678, 17 pages

http://dx.doi.org/10.1155/2009/535678

## Fuzzy Stability of Jensen-Type Quadratic Functional Equations

^{1}Department of Mathematics, University of Ulsan, Ulsan 680-749, South Korea^{2}Department of Mathematics, Daejin University, Kyeonggi 487-711, South Korea^{3}Department of Mathematics, Hanyang University, Seoul 133-791, South Korea^{4}Department of Mathematics, University of Seoul, Seoul 130-743, South Korea

Received 29 December 2008; Revised 26 March 2009; Accepted 10 April 2009

Academic Editor: John Rassias

Copyright © 2009 Sun-Young Jang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We prove the generalized Hyers-Ulam stability of the following quadratic functional equations and in fuzzy Banach spaces for a nonzero real number with .

#### 1. Introduction and Preliminaries

The stability problem of functional equations originated from a question of Ulam [1] concerning the stability of group homomorphisms. Hyers [2] gave a first affirmative partial answer to the question of Ulam for Banach spaces. Hyers' theorem was generalized by Aoki [3] for additive mappings and by Th. M. Rassias [4] for linear mappings by considering an unbounded Cauchy difference. The work of Th. M. Rassias [4] has provided a lot of influence in the development of what we call * generalized Hyers-Ulam stability* of functional equations. A generalization of the Th. M. Rassias theorem was obtained by Găvruţa [5] by replacing the unbounded Cauchy difference by a general control function in the spirit of Th. M. Rassias' approach.

J. M. Rassias [6] proved a similar stability theorem in which he replaced the factor by for with (see also [7, 8] for a number of other new results). The papers of J. M. Rassias [6–8] introduced the Ulam- Găvruţa-Rassias stability of functional equations. See also [9–11].

The functional equation
is called a *quadratic functional equation*. In particular, every solution of the quadratic functional equation is said to be a* quadratic mapping*. A generalized Hyers-Ulam stability problem for the quadratic functional equation was proved by Skof [12] for mappings , where is a normed space and is a Banach space. Cholewa [13] noticed that the theorem of Skof is still true if the relevant domain is replaced by an Abelian group. In [14], Czerwik proved the generalized Hyers-Ulam stability of the quadratic functional equation.

J. M. Rassias [15] introduced and solved the stability problem of Ulam for the Euler-Lagrange-type quadratic functional equation

motivated from the following pertinent algebraic equation

The solution of the functional equation (1.2) is called a * Euler-Lagrange-type quadratic mapping*. J. M. Rassias [16, 17] introduced and investigated the relative functional equations. In addition, J. M. Rassias [18] generalized the algebraic equation (1.3) to the following equation
and introduced and investigated the general pertinent Euler-Lagrange quadratic mappings. Analogous quadratic mappings were introduced and investigated in [19, 20].

These Euler-Lagrange mappings are named * Euler-Lagrange-Rassias mappings* and the corresponding Euler-Lagrange equations are called * Euler-Lagrange-Rassias equations*. Before 1992, these mappings and equations were not known at all in functional equations and inequalities. However, a completely different kind of Euler-Lagrange partial differential equations are known in calculus of variations. Therefore, we think that J. M. Rassias' introduction of Euler-Lagrange mappings and equations in functional equations and inequalities provides an interesting cornerstone in analysis. Already some mathematicians have employed these Euler-Lagrange mappings.

Recently, Jun and Kim [21] solved the stability problem of Ulam for another Euler-Lagrange-Rassias-type quadratic functional equation. Jun and Kim [22] introduced and investigated the following quadratic functional equation of Euler-Lagrange-Rassias type:

whose solution is said to be a generalized quadratic mapping of Euler-Lagrange-Rassias type.

During the last two decades a number of papers and research monographs have been published on various generalizations and applications of the generalized Hyers-Ulam stability to a number of functional equations and mappings (see [9, 23–26]).

Katsaras [27] defined a fuzzy norm on a vector space to construct a fuzzy vector topological structure on the space. Some mathematicians have defined fuzzy norms on a vector space from various points of view [28–30]. In particular, Bag and Samanta [31], following Cheng and Mordeson [32], gave an idea of fuzzy norm in such a manner that the corresponding fuzzy metric is of Kramosil and Michálek type [33]. They established a decomposition theorem of a fuzzy norm into a family of crisp norms and investigated some properties of fuzzy normed spaces [34].

We use the definition of fuzzy normed spaces given in [31] and [35–38] to investigate a fuzzy version of the generalized Hyers-Ulam stability for the quadratic functional equations

in the fuzzy normed vector space setting.

*Definition 1.1 (see [31, 35–38]). *Let be a real vector space. A function is called a *fuzzy norm* on if for all and all ,

() for ;

() if and only if for all ;

() if ;

() ;

() is a non-decreasing function of and ;

() for , is continuous on .

The pair is called a * fuzzy normed vector space*.

The properties of fuzzy normed vector spaces and examples of fuzzy norms are given in [35–38].

*Definition 1.2 (see [31, 35–38]). *Let be a fuzzy normed vector space. A sequence in is said to be *convergent* or *converge* if there exists an such that for all In this case, is called the *limit* of the sequence and we denote it by * N-*

*Definition 1.3 (see [31, 35–38]). *Let be a fuzzy normed vector space. A sequence in is called *Cauchy* if for each and each there exists an such that for all and all we have

It is well known that every convergent sequence in a fuzzy normed vector space is Cauchy. If each Cauchy sequence is convergent, then the fuzzy norm is said to be * complete* and the fuzzy normed vector space is called a * fuzzy Banach space*.

We say that a mapping between fuzzy normed vector spaces and is continuous at a point if for each sequence converging to in , then the sequence converges to . If is continuous at each , then is said to be * continuous* on (see [34]).

In this paper, we prove the generalized Hyers-Ulam stability of the quadratic functional equations (1.6) and (1.7) in fuzzy Banach spaces.

Throughout this paper, assume that is a vector space and that is a fuzzy Banach space. Let be a nonzero real number with ).

#### 2. Fuzzy Stability of Quadratic Functional Equations

We prove the fuzzy stability of the quadratic functional equation (1.6).

Theorem 2.1. *Let**be an even mapping with*. *Suppose that** is a mapping from **to a fuzzy normed space**such that**for all**and all positive real numbers*. *If**for some positive real number**with*, *then there is a unique quadratic mapping**such that*- and
*
where
*

*Proof. *Putting and in (2.1), we get
for all and all Replacing by by , and by in (2.1), we obtain
Thus
and so
Then by the assumption,
Replacing by in (2.7) and applying (2.8), we get
Thus for each we have
Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by . Moreover, if we put in (2.10), then we observe that
Thus
Next we show that is quadratic. Let . Then we have
The first four terms on the right-hand side of the above inequality tend to 1 as and the fifth term, by (2.1), is greater than or equal to
which tends to 1 as . Hence
for all and all . This means that satisfies the Jensen quadratic functional equation and so it is quadratic.

Next, we approximate the difference between and in a fuzzy sense. For every and , by (2.13), for large enough , we have
The uniqueness assertion can be proved by a standard fashion; cf. [36]: Let be another quadratic mapping from into , which satisfies the required inequality. Then for each and ,
Since and are quadratic,
for all , all and all .

Since , . Hence the right-hand side of the above inequality tends to 1 as . It follows that for all .

Theorem 2.2. *Let**be an even mapping with*. *Suppose that**is a mapping from**to a fuzzy normed space **satisfying* (2.1). *If**for some real number**with *, *then there is a unique quadratic mapping**such that*-* and**
where
*

*Proof. *It follows from (2.7) that
Then by the assumption,
Replacing by in (2.22) and applying (2.23), we get
Thus for each we have

Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by -. Moreover, if we put in , then we observe that
Thus
The rest of the proof is similar to the proof of Theorem 2.1.

Theorem 2.3. *Let**be a mapping with*. *Suppose that**is a mapping from**to a fuzzy normed space **satisfying* (2.1). *If**for some positive real number ** with*, *then there is a unique quadratic mapping**such that*-*and**
where .*

*Proof. *Letting and replacing by and by in (2.1), we obtain
Thus
Then by the assumption,
Replacing by in (2.31) and applying (2.32), we get
Thus for each we have

Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by -. Moreover, if we put in (2.34), then we observe that
Thus
The rest of the proof is similar to the proof of Theorem 2.1.

Theorem 2.4. *Let be a mapping with. Suppose thatis a mapping fromto a fuzzy normed space satisfying (2.1). Iffor some real numberwith, then there is a unique quadratic mappingsuch that-and*

*where*

*Proof. *It follows from (2.31) that
Then by the assumption,
Replacing by in (2.39) and applying (2.40), we get
Thus for each we have

Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by -. Moreover, if we put in (2.42), then we observe that
Thus
The rest of the proof is similar to the proof of Theorem 2.1.

Now we prove the fuzzy stability of the quadratic functional equation (1.7) for the case .

Theorem 2.5. *Let**and**a mapping with*. *Suppose that**is a mapping from**to a fuzzy normed space**such that**for all**and all positive real numbers*. *If**for some positive real number**with*, *then there is a unique quadratic mapping**such that*-*and** for all **and all*.

*Proof. *Putting and in (2.46), we get
for all and all Thus
and so
Replacing by in (2.50), we get
Thus for each we have

Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by -. Moreover, if we put in (2.52), then we observe that
Thus
The rest of the proof is similar to the proof of Theorem 2.1.

Theorem 2.6. *Let**and**a mapping with*. *Suppose that**is a mapping from**to a fuzzy normed space**satisfying * (2.46). *If**for some real number**with*, *then there is a unique quadratic mapping**such that*-*and**for all**and all*.

*Proof. *It follows from (2.50) that
for all and all Thus
Replacing by in (2.58), we get
Thus for each we have
Let and be given. Since , there is some such that . Since , there is some such that for . It follows that
for all . This shows that the sequence is Cauchy in . Since is complete, converges to some . Thus we can define a mapping by -. Moreover, if we put in (2.60), then we observe that
Thus
The rest of the proof is similar to the proof of Theorem 2.1.

#### Acknowledgment

Dr. Sun-Young Jang was supported by the Research Fund of University ofUlsan in 2008, and Dr. Choonkil Park was supported by National ResearchFoundation of Korea (NRF-2009-0070788).

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