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Fuzzy Stability of an AdditiveQuadraticQuartic Functional Equation
Journal of Inequalities and Applications volume 2010, Article number: 253040 (2010)
Abstract
Using the fixed point method, we prove the generalized HyersUlam stability of the following additivequadraticquartic functional equation: in fuzzy Banach spaces.
1. Introduction and Preliminaries
Katsaras [1] 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 [2–4]. In particular, Bag and Samanta [5], following Cheng and Mordeson [6], gave an idea of fuzzy norm in such a manner that the corresponding fuzzy metric is of Kramosil and Michálek type [7]. They established a decomposition theorem of a fuzzy norm into a family of crisp norms and investigated some properties of fuzzy normed spaces [8].
We use the definition of fuzzy normed spaces given in [5, 9, 10] to investigate a fuzzy version of the generalized HyersUlam stability forthe following functional equation
in the fuzzy normed vector space setting.
Definition 1.1 (see [5, 9–11]).
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 nondecreasing 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 [9, 12].
Definition 1.2 (see [5, 9–11]).
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 thelimit of the sequence and we denote it by .
Definition 1.3 (see [5, 9, 10]).
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 wellknown 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 , the sequence converges to . If is continuous at each , then is said to be continuous on (see [8]).
The stability problem of functional equations originated from a question of Ulam [13] concerning the stability of group homomorphisms. Hyers [14] gave a first affirmative partial answer to the question of Ulam for Banach spaces. Hyers' theorem was generalized by Aoki [15] for additive mappings and by Th. M. Rassias [16] for linear mappings by considering an unbounded Cauchy difference. The paper of Th. M. Rassias [16] has provided a lot of influence in the development of what we call generalized HyersUlam stability or as HyersUlamRassias stability of functional equations. A generalization of the Th. M. Rassias theorem was obtained by Gvruta [17] by replacing the unbounded Cauchy difference by a general control function in the spirit of Th.M. Rassias' approach.
The functional equation
is called a quadratic functional equation. In particular, every solution of the quadratic functional equation is said to be aquadratic mapping. A generalized HyersUlam stability problem for the quadratic functional equation was proved by Skof [18] for mappings , where is a normed space and is a Banach space. Cholewa [19] noticed that the theorem of Skof is still true if the relevant domain is replaced by an Abelian group. Czerwik [20] proved the generalized HyersUlam stability of the quadratic functional equation. The stability problems of several functional equations have been extensively investigated by a number of authors and there are many interesting results concerning this problem (see [16, 21–39]).
In [40], Lee et al. considered the following quartic functional equation:
It is easy to show that the function satisfies the functional equation (1.3), which is called a quartic functional equation and every solution of the quartic functional equation is said to be a quartic mapping.
Let be a set. A function is called a generalized metric on if satisfies
(1) if and only if ;
(2) for all ;
(3) for all .
We recall a fundamental result in fixed point theory.
Let be a complete generalized metric space and let be a strictly contractive mapping with Lipschitz constant . Then for each given element , either
for all nonnegative integers or there exists a positive integer such that
(1);
(2)the sequence converges to a fixed point of ;
(3) is the unique fixed point of in the set ;
(4) for all .
In 1996, G. Isac and Th. M. Rassias [43] were the first to provide applications of stability theory of functional equations for the proof of new fixed point theorems with applications. By using fixed point methods, the stability problems of several functional equations have been extensively investigated by a number of authors (see [12, 44–48]).
This paper is organized as follows. In Section 2, we prove the generalized HyersUlam stability of the additivequadraticquartic functional equation (1.1) in fuzzy Banach spaces for an odd case. In Section 3, we prove the generalized HyersUlam stability of the additivequadraticquartic functional equation (1.1) in fuzzy Banach spaces for an even case.
Throughout this paper, assume that is a vector space and that is a fuzzy Banach space.
2. Generalized HyersUlam Stability of the Functional Equation (1.1): An Odd Case
One can easily show that an odd mapping satisfies (1.1) if and only if the odd mapping mapping is an additive mapping, that is,
One can easily show that an even mapping satisfies (1.1) if and only if the even mapping is a quadraticquartic mapping, that is,
It was shown in [49, Lemma 2.1] that and are quartic and quadratic, respectively, and that .
For a given mapping , we define
for all .
Using the fixed point method, we prove the generalized HyersUlam stability of the functional equation in fuzzy Banach spaces: an odd case.
Theorem 2.1.
Let be a function such that there exists an with
for all . Let be an odd mapping satisfying
for all and all . Then
exists for each and defines an additive mapping such that
for all and all .
Proof.
Letting in (2.5), we get
for all and all .
Consider the set
and introduce the generalized metric on
where, as usual, . It is easy to show that is complete. (see the proof of Lemma 2.1 of [50].)
Now we consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (2.8) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is odd, is an odd mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (2.16) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all ;

(3)
, which implies the inequality
This implies that inequality (2.7) holds.
By (2.5),
for all , all and all . So
for all , all and all . Since for all and all ,
for all and all . Thus the mapping is additive, as desired.
Corollary 2.2.
Let and let be a real number with . Let be a normed vector space with norm . Let be an odd mapping satisfying
for all and all . Then
exists for each and defines an additive mapping such that
for all and all .
Proof.
The proof follows from Theorem 2.1 by taking
for all . Then we can choose and we get the desired result.
Theorem 2.3.
Let be a function such that there exists an with
for all . Let be an odd mapping satisfying (2.5). Then
exists for each and defines an additive mapping such that
for all and all .
Proof.
Let be the generalized metric space defined in the proof of Theorem 2.1.
Consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (2.8) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is odd, is an odd mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (2.36) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all ;

(3)
, which implies the inequality
This implies that the inequality (2.30) holds.
The rest of the proof is similar to the proof of Theorem 2.1.
Corollary 2.4.
Let and let be a real number with . Let be a normed vector space with norm . Let be an odd mapping satisfying (2.24). Then
exists for each and defines an additive mapping such that
for all and all .
Proof.
The proof follows from Theorem 2.3 by taking
for all . Then we can choose and we get the desired result.
3. Generalized HyersUlam Stability of the Functional Equation (1.1):An Even Case
Using the fixed point method, we prove the generalized HyersUlam stability of the functional equation in fuzzy Banach spaces: an even case.
Theorem 3.1.
Let be a function such that there exists an with
for all . Let be an even mapping satisfying and (2.5). Then
exists for each and defines a quartic mapping such that
for all and all .
Proof.
Letting in (2.5), we get
for all and all .
Replacing by in (2.5), we get
for all and all .
By (3.4) and (3.5),
for all and all . Letting for all , we get
for all and all .
Consider the set
and introduce the generalized metric on
where, as usual, . It is easy to show that is complete. (see the proof of Lemma 2.1 of [50]).
Now we consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (3.7) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is even, is an even mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (3.15) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all .

(3)
, which implies the inequality
This implies that inequality (3.3) holds.
The rest of the proof is similar to that of the proof of Theorem 2.1.
Corollary 3.2.
Let and let be a real number with . Let be a normed vector space with norm . Let be an even mapping satisfying and (2.24). Then
exists for each and defines a quartic mapping such that
for all and all .
Proof.
The proof follows from Theorem 3.1 by taking
for all . Then we can choose and we get the desired result.
Theorem 3.3.
Let be a function such that there exists an with
for all . Let be an even mapping satisfying and (2.5). Then
exists for each and defines a quartic mapping such that
for all and all .
Proof.
Let be the generalized metric space defined in the proof of Theorem 3.1.
Consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (3.7) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is even, is an even mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (3.31) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all ;

(3)
, which implies the inequality
This implies that inequality (3.25) holds.
The rest of the proof is similar to that of the proof of Theorem 2.1.
Corollary 3.4.
Let and let be a real number with . Let be a normed vector space with norm . Let be an even mapping satisfying and (2.24). Then
exists for each and defines a quartic mapping such that
for all and all .
Proof.
The proof follows from Theorem 3.3 by taking
for all . Then we can choose and we get the desired result.
Theorem 3.5.
Let be a function such that there exists an with
for all . Let be an even mapping satisfying and (2.5). Then
exists for each and defines a quadratic mapping such that
for all and all .
Proof.
Let be the generalized metric space defined in the proof of Theorem 3.1.
Letting for all in (3.6), we get
for all and all .
Now we consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (3.42) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is even, is an even mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (3.48) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all .

(3)
, which implies the inequality
This implies that inequality (3.41) holds.
The rest of the proof is similar to that of the proof of Theorem 2.1.
Corollary 3.6.
Let and let be a real number with . Let be a normed vector space with norm . Let be an even mapping satisfying and (2.24). Then
exists for each and defines a quadratic mapping such that
for all and all .
Proof.
The proof follows from Theorem 3.5 by taking
for all . Then we can choose and we get the desired result.
Theorem 3.7.
Let be a function such that there exists an with
for all . Let be an even mapping satisfying and (2.5). Then
exists for each and defines a quadratic mapping such that
for all and all .
Proof.
Let be the generalized metric space defined in the proof of Theorem 3.1.
Consider the linear mapping such that
for all .
Let be given such that . Then
for all and all . Hence
for all and all . So implies that . This means that
for all .
It follows from (3.42) that
for all and all . So .
By Theorem 1.4, there exists a mapping satisfying the following.

(1)
is a fixed point of , that is,
for all . Since is even, is an even mapping. The mapping is a unique fixed point of in the set
This implies that is a unique mapping satisfying (3.64) such that there exists a satisfying
for all and all .

(2)
as . This implies the equality
for all .

(3)
, which implies the inequality
This implies that inequality (3.58) holds.
The rest of the proof is similar to that of the proof of Theorem 2.1.
Corollary 3.8.
Let and let be a real number with . Let be a normed vector space with norm . Let be an even mapping satisfying and (2.24). Then
exists for each and defines a quadratic mapping such that
for all and all .
Proof.
The proof follows from Theorem 3.7 by taking
for all . Then we can choose and we get the desired result.
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Acknowledgment
This work was supported by the Hanyang University in 2009.
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Park, C. Fuzzy Stability of an AdditiveQuadraticQuartic Functional Equation. J Inequal Appl 2010, 253040 (2010) doi:10.1155/2010/253040
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Keywords
 Functional Equation
 Unique Mapping
 Additive Mapping
 Unique Fixed Point
 Normed Vector Space