Abstract and Applied Analysis

VolumeΒ 2011Β (2011), Article IDΒ 456182, 19 pages

http://dx.doi.org/10.1155/2011/456182

## Intuitionistic Fuzzy Stability of Functional Equations Associated with Inner Product Spaces

^{1}School of Science, Hubei University of Technology, Wuhan, Hubei 430068, China^{2}Department of Mathematics, National Technical University of Athens, Zografou Campus, 15780 Athens, Greece

Received 2 September 2011; Accepted 29 September 2011

Academic Editor: GabrielΒ Turinici

Copyright Β© 2011 Zhihua Wang and Themistocles M. Rassias. 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

In intuitionistic fuzzy normed spaces, we investigate some stability results for the functional equation which is said to be a functional equation associated with inner products space.

#### 1. Introduction and Preliminaries

The aim of this article is to prove an intuitionistic fuzzy version of the Hyers-Ulam-Rassias stability for the functional equation: which is said to be a functional equation associated with inner product spaces. It was shown by Rassias [1] that the norm defined over a real vector space is induced by an inner product if and only if for a fixed integer it follows for all . Interesting new results concerning functional equations associated with inner product spaces have recently been obtained by Park et al. [2, 3] and Najati and Rassias [4] as well as for the fuzzy stability of a functional equation associated with inner product spaces [5].

Stability problem of a functional equation was first posed by Ulam [6] which was answered by Hyers [7] on approximately additive mappings and then generalized by Aoki [8] and Rassias [9] for additive mappings and linear mappings, respectively. Later there have been proved several new results on stability of various classes of functional equations in the Hyers-Ulam sense (cf. the following books and papers [10β18] and the references cited therein), as well as various fuzzy stability results concerning Cauchy, Jensen, quadratic and cubic functional equations (cf. [19β22]). Furthermore some stability results concerning Jensen, cubic, mixed-type additive and cubic functional equations were investigated (cf. [23β26]) in the spirit of intuitionistic fuzzy normed spaces, while the idea of intuitionistic fuzzy normed space was introduced in [27] and further studied in [28β35].

In this section, we recall some notations and basic definitions used in this paper as follows.

*Definition 1.1 (cf. [36]). *A binary operation is said to be a continuous -norm if it satisfies the following conditions:(a) is commutative and associative, (b) is continuous,(c) for all , (d) whenever and for each .

*Definition 1.2 (cf. [36]). * A binary operation is said to be a continuous -conorm if it satisfies the following conditions:(a^{β}) is commutative and associative, (b^{β}) is continuous,(c^{β}) for all , (d^{β}) whenever and for each .

Using the notions of continuous -norm and continuous -conorm, Saadati and Park [27] have recently introduced the concept of intuitionistic fuzzy normed spaces as follows.

*Definition 1.3. * The five-tuple is said to be an intuitionistic fuzzy normed space (for short, IFNS) if is a vector space, is a continuous -norm, is a continuous -conorm, and are fuzzy sets on satisfying the following conditions. For every and ,(i), (ii), (iii) if and only if ,(iv) for each , (v), (vi) is continuous, (vii) and ,(viii), (ix) if and only if , (x) for each , (xi) , (xii) is continuous, (xiii) and . In this case is called an intuitionistic fuzzy norm.

*Example 1.4 (cf. [37]). * Letbe a normed space, andfor all *. *For all and everyand*, *consider
Then is an IFNS.

The concepts of convergence and Cauchy sequences in an intuitionistic fuzzy normed space are studied in [27].

Let be an IFNS. Then, a sequence is said to be intuitionistic fuzzy convergent to if, for every and , there exists such that and for all . In this case we write . The sequence is said to be intuitionistic fuzzy Cauchy sequence if, for every and , there exists such that and for all . is said to be complete if every intuitionistic fuzzy Cauchy sequence in is intuitionistic fuzzy convergent in .

#### 2. Intuitionistic Fuzzy Stability

Throughout this section, assume that , , and are linear space, IFNS, and intuitionistic fuzzy Banach space, respectively. For convenience, we use the following abbreviation for a given function :

We begin with the Hyers-Ulam-Rassias type theorem in IFNS for the functional (1.1) which is said to be a functional equation associated with inner product spaces.

Theorem 2.1. *Let be a function such that for some real number with . Suppose that an even function with satisfies the inequality
**
for all and all . Then there exists a unique quadratic function such that
**
for all and , where
*

*Proof. *Put in (2.2), and, using the evenness of , we obtain
for all and . Interchanging with in (2.5) and using the evenness of , we get
for all and . It follows from (2.5) and (2.6) that
for all and . Putting in (2.2) and using the evenness of , we obtain
for all and . Hence, we obtain from (2.7) and (2.8) that
for all and . So
for all and . Putting in (2.2), we obtain
for all and . It follows from (2.10) and (2.11) that
for all and . Letting in (2.8) and replacing by in the obtained inequality, we get
for all and . It follows from (2.10), (2.11), (2.12), and (2.13) that
for all and . Applying (2.12) and (2.14), we obtain
for all and . Setting in (2.2), we obtain
for all and . It follows from (2.15) and (2.16) that
It follows that
Define
Then, by our assumption,
Replacing by in (2.18) and applying (2.20), we get
Thus for each , we have
where . Let and be given. Since and , there exists some such that and . Since , there is some such that for each . It follows that
for all . This shows that the sequence is Cauchy in . Since is intuitionistic fuzzy Banach space, converges to some point . Thus, we can define a mapping such that . Moreover, if we put in (2.22), we get
Thus,

Now, we will show that is quadratic. Setting and in (2.2), we obtain
for all and . Letting in (2.26), we obtain
for all and all . This means that satisfies the functional (1.1) and so it is quadratic (see Lemma 2.2 of [4]).

Next, we approximate the difference between and in intuitionistic fuzzy sense. By (2.25), we have
for every and large enough . To prove the uniqueness of , assume that is another quadratic mapping from to , which satisfies the required inequality. Then, for each and ,
Since and are quadratic, we have
for all and . Since and , we
Therefore and for all and . Hence, for all . This completes the proof of the theorem.

Theorem 2.2. *Let be a function such that for some real number with . Suppose that an odd function satisfies the inequality
**
for all and all . Then there exists a unique additive function such that
**
for all and , where
*

*Proof. * Put in (2.32) and using the oddness of , we obtain
for all and . Interchanging with in (2.35) and using the oddness of , we get
for all and . It follows from (2.35) and (2.36) that
for all and . Setting in (2.32) and using the oddness of , we get
for all and . It follows from (2.37) and (2.38) that
for all and . Putting in (2.32), we obtain
for all and . It follows from (2.39) and (2.40) that
for all and . Replacing and by and in (2.41); respectively, we have
It follows that
Define
Then by the assumption
Replacing by in (2.43) and using (2.45), we obtain
Thus, for each , we have
where , . Let and be given. Since and , there exists some such that and . Since , there is some such that for each . It follows that
for all . This shows that the sequence is Cauchy in . Since is intuitionistic fuzzy Banach space, converges to some point . Thus, we can define a mapping such that . Moreover, if we put in (2.47), we get
Thus,
Next we will show that is additive. Putting and