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A fixed point approach to the Hyers-Ulam stability of an AQ functional equation on β-Banach modules
Journal of Inequalities and Applications volume 2012, Article number: 204 (2012)
Abstract
In this paper, we establish the general solution and investigate the generalized Hyers-Ulam stability of the following mixed additive and quadratic functional equation: () in β-Banach modules on a Banach algebra. In addition, we show that under some suitable conditions, an approximately mixed additive-quadratic function can be approximated by a mixed additive and quadratic mapping.
MSC:39B82, 39B52, 46H25.
1 Introduction and preliminaries
Starting with the article of Hyers [1] and continuing with those of Rassias [2], Găvruţa [3], Czervick [4] and so on, the authors used the ‘direct method’ to prove the stability properties for functional equations. Namely, the exact solution of the functional equation is explicitly constructed as a limit of a sequence starting from the given approximate solution.
On the other hand, Baker [5] used the Banach fixed point theorem to give a Hyers-Ulam stability result for a nonlinear functional equation in a single variable. In 2002, Cădariu and Radu delivered a lecture entitled ‘On the stability of the Cauchy functional equation: a fixed point approach’ in ‘The 14th European Conference on Iteration Theory - ECIT 2002, Evora, Portugal, 2002’. Their idea was to obtain, in β-normed spaces, the existence of the exact solution and the error estimations by using the fixed point alternative theorem [6]. This new method was used in two successive papers [7, 8] in 2003 to obtain the properties of generalized Hyers-Ulam stability for Jensen’s functional equation. Also, the lecture from ECIT 2002 was materialized in [9]. After that, a lot of papers used the ‘fixed point alternative’ to obtain generalized Hyers-Ulam stability results for different functional equations in various spaces. The reader is referred to the following books and research papers which provide an extensive account of the progress made on Ulam’s problem during the last seventy years (see, for instance, [10–25]).
The motivation of this paper is to present a new mixed additive and quadratic (‘AQ’ for short) functional equation. We obtain the general solution of the AQ-functional equation. Moreover, we prove the generalized Hyers-Ulam stability of the AQ-functional equation in Banach modules on a Banach algebra using the fixed point method.
The functional equation
is related to a symmetric bi-additive function [20]. It is natural that such an equation is called a quadratic functional equation. In particular, every solution of the quadratic equation (1.1) is said to be a quadratic function. It is well known that a function f between real vector spaces is quadratic if and only if there exists a unique symmetric bi-additive function B such that for all x (see [20]). The bi-additive function B is given by . In [4], Czerwik proved the Hyers-Ulam stability of the quadratic functional equation (1.1). A Hyers-Ulam stability problem for the quadratic functional equation (1.1) was proved by Skof for functions , where is a normed space and is a Banach space (see [26]). Cholewa [27] noticed that the theorem of Skof is still true if the relevant domain is replaced by an Abelian group. Grabiec in [28] has generalized the above mentioned results. The quadratic functional equation and several other functional equations are useful to characterize inner product spaces (see, for instance, [10, 17, 18, 21, 29]).
Now, we consider a mapping that satisfies the following general mixed additive and quadratic functional equation:
where . It is easy to see that the function is a solution of the functional equation (1.2).
Let β be a real number with , and let denote either or . Let X be a linear space over . A real-valued function is called a β-norm on X if and only if it satisfies
() if and only if ;
() for all and all ;
() for all .
The pair is called a β-normed space (see [30]). A β-Banach space is a complete β-normed space.
For explicit later use, we recall the following result by Diaz and Margolis [6].
Theorem 1.1 Let be a complete generalized metric space and be a strictly contractive mapping with the Lipschitz constant , that is,
Then for each given , either
or there exists a non-negative integer such that
-
(1)
for all ;
-
(2)
the sequence converges to a fixed point of J;
-
(3)
is the unique fixed point of J in the set ;
-
(4)
for all .
2 General solution
Throughout this section, X and Y will be real vector spaces. Before proceeding with the proof of Theorem 2.3, which is the main result in this section, we shall need the following two lemmas:
Lemma 2.1 If an odd mapping satisfies (1.2) for all , then f is additive.
Proof Since f satisfies the functional equation (1.2), putting in (1.2), we get , or if . Note that, in view of the oddness of f, we have for all . If in , one gets , or without assuming the condition . Yet, it is assumed that f is odd. Hence, (1.2) implies the following equation:
for all . Letting in (2.1), we get
for all . Replacing x and y by lx and kx, respectively, in (2.1) and using (2.2), we have
for all . Replacing y by in (2.3), we get
for all . Replacing y by in (2.1) and using (2.2), we obtain
for all . Replacing x and y by and , respectively, in (2.5) and using (2.4), we get
for all . Therefore, the mapping is additive. □
Lemma 2.2 If an even mapping satisfies (1.2) for all , then f is quadratic.
Proof Since f satisfies the functional equation (1.2), putting in (1.2), we get . Note that, in view of the evenness of f, we have for all . Hence, (1.2) implies the following equation:
for all . Letting in (2.7) and using , we get
for all . Replacing x and y by 0 and x, respectively, in (2.7) and using , we have
for all . Replacing x by lx in (2.7) and using (2.9), we get
for all . Replacing y by ky in (2.10) and using (2.10), we obtain
for all . Therefore, the function is quadratic. □
Now, we are ready to find the general solution of (1.2).
Theorem 2.3 A mapping satisfies (1.2) for all if and only if there exist a symmetric bi-additive mapping and an additive mapping such that for all .
Proof If there exist a symmetric bi-additive function and an additive function such that for all , it is easy to show that
for all . Therefore, the function satisfies (1.2).
Conversely, we decompose f into the odd part and the even part by putting
for all . It is clear that for all . It is easy to show that the functions and satisfy (1.2). Hence, by Lemmas 2.1 and 2.2, we achieve that the functions and are additive and quadratic, respectively. Therefore, there exists a symmetric bi-additive function such that for all (see [10]). So,
for all , where for all . □
3 Approximate mixed additive and quadratic mappings
In this section, we prove the generalized Hyers-Ulam stability of the mixed additive and quadratic functional equation (1.2) using the fixed point method introduced by Radu in [7] (see also [8, 9, 31–35]).
Throughout this section, let B be a unital Banach algebra with norm , , X be a β-normed left B-module and Y be a β-normed left Banach B-module, and let be fixed integers. For a given mapping , we define the difference operators
and
for all and .
Theorem 3.1 Let be a function such that
for all . Let be an odd mapping such that
for all and all . If there exists a Lipschitz constant such that
for all , then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear, i.e., for all and all .
Proof Letting and in (3.2), we get
for all . Consider the set and introduce the generalized metric on Ω:
It is easy to show that is a complete generalized metric space (see the Theorem 2.1 of [13]). We now define a function by
Let and be an arbitrary constant with ; by the definition of d, it follows
By the given hypothesis and the last inequality, one has
Hence, it holds that . It follows from (3.5) that . Therefore, by Theorem 1.1, J has a unique fixed point in the set such that
and for all . Also,
This means that (3.4) holds for all .
Now, we show that A is additive. By (3.1), (3.2) and (3.10), we have
that is,
for all . Therefore, by Lemma 2.1, we get that the mapping A is additive. To prove the uniqueness assertion, let us assume that there exists an additive function which satisfies (3.4). Since and T is additive, we get and for all , i.e., T is a fixed point of J. Since A is the unique fixed point of J in , then .
Moreover, if is continuous in for each fixed , then, by the same reasoning as in the proof of [2], A is -linear. Since A is additive, for any rational number r. Fix and (the dual space of Y). Consider the mapping , , . Then , , i.e. φ is a group homomorphism. Moreover, φ is a Borel function because of the following reasoning. Let and put . Then are continuous functions. But is the pointwise limit of continuous functions, thus is a Borel function. It is a known fact that if is a function such that φ is a group homomorphism, i.e. and φ is a measurable function, then φ is continuous. Therefore, is a continuous function. Let . Then , where is a sequence of rational numbers. Thus
Therefore, for any . And then, for any . Hence, the additive mapping A is -linear.
Letting in (3.2), we get
for all and all . By definition of A, (3.1) and (3.12), we obtain
for all and all . So, for all and all . Since A is additive, we get for all and all . Now, let . Since A is -linear,
for all and all . This proves that A is B-linear. □
Corollary 3.2 Let , , and let be an odd mapping for which
for all and . Then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear.
Proof The proof follows from Theorem 3.1 by taking for all . We can choose to get the desired result. □
The generalized Hyers-Ulam stability problem for the case of was excluded in Corollary 3.2. In fact, the functional equation (1.2) is not stable for in (3.13) as we shall see in the following example, which is based on the example given in [36] (see also [34]).
Example 3.3 Let be defined by
Consider the function be defined by
for all , where . Let
for all and . Then f satisfies the functional inequality
for all , but there do not exist an additive mapping and a constant such that for all .
It is clear that f is bounded by on . If or , then
Now, suppose that . Then there exists an integer such that
Hence,
for all . From the definition of f and (3.15), we obtain that
Therefore, f satisfies (3.14). Now, we claim that the functional equation (1.2) is not stable for in Corollary 3.2. Suppose, on the contrary, that there exist an additive mapping and a constant such that for all . Then there exists a constant such that for all rational numbers x. So, we obtain that
for all rational numbers x. Let with . If x is a rational number in , then for all , and for this x, we get
which contradicts (3.16).
Corollary 3.4 Let such that and δ, θ be non-negative real numbers, and let be an odd mapping for which
for all and . Then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear.
Proof The proof follows from Theorem 3.1 by taking for all . We can choose to get the desired result. □
The generalized Hyers-Ulam stability problem for the case of was excluded in Corollary 3.4. Similar to Theorem 3.1, one can obtain the following theorem.
Theorem 3.5 Let be a function such that
for all . Let be an odd mapping such that
for all and all . If there exists a Lipschitz constant such that for all , then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear.
As applications of Theorem 3.5, one can get the following Corollaries 3.6 and 3.7.
Corollary 3.6 Let and θ be a non-negative real number, and let be an odd mapping such that
for all and . Then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear.
Corollary 3.7 Let such that and θ be a non-negative real number, and let be an odd mapping such that
for all and . Then there exists a unique additive mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear.
Theorem 3.8 Let be a function such that
for all . Let be an even mapping with such that
for all and all . If there exists a Lipschitz constant such that
for all , then there exists a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then Q is B-quadratic, i.e., for all and all .
Proof Letting and in (3.18), we get
for all . Consider the set and introduce the generalized metric on Ω:
It is easy to show that is a complete generalized metric space (see Theorem 2.1 of [13]). We now define a function by
Let and be an arbitrary constant with ; by the definition of d, it follows
By the given hypothesis and the last inequality, one has
Hence, it holds that . It follows from (3.21) that . Therefore, by Theorem 1.1, J has a unique fixed point in the set such that
and for all . Also,
This means that (3.20) holds for all . The mapping Q is quadratic because it satisfies equation (1.2) as follows:
for all ; therefore, by Lemma 2.2, it is quadratic. To prove the uniqueness assertion, let us assume that there exists a quadratic mapping which satisfies (3.20). Since and S is quadratic, we get and for all , i.e., S is a fixed point of J. Since Q is the unique fixed point of J in , then .
Moreover, if is continuous in for each fixed , then by the same reasoning as in the proof of [2], Q is -quadratic. Letting in (3.18), we get
for all and all . By definition of Q, (3.17) and (3.23), we obtain
for all and all . So, for all and all . Since , we get for all and all . Now, let . Since Q is -quadratic,
for all and all . This proves that Q is B-quadratic. □
Corollary 3.9 Let , , and let be an even mapping with such that
for all and . Then there exists a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then Q is B-quadratic.
Proof The proof follows from Theorem 3.8 by taking for all . We can choose to get the desired result. □
The following example shows that the generalized Hyers-Ulam stability problem for the case of was excluded in Corollary 3.9. This example is a modified version of Czerwik [4].
Example 3.10 Let be defined by
Consider the function be defined by
for all , where . Let
for all and . Then f satisfies the functional inequality
for all , but there do not exist a quadratic mapping and a constant such that for all .
It is clear that f is bounded by on . If or , then
Now, suppose that . Then there exists an integer such that
Hence,
for all . From the definition of f and the inequality (3.25), we obtain that
Now, we claim that the functional equation (1.2) is not stable for in Corollary 3.9. Suppose, on the contrary, that there exist a quadratic mapping and a constant such that for all . Then there exists a constant such that for all rational numbers x. So, we obtain that
for all rational numbers x. Let with . If x is a rational number in , then for all , and for this x, we get
which contradicts (3.26).
Similar to Corollary 3.9, one can obtain the following corollary.
Corollary 3.11 Let such that and δ, θ be non-negative real numbers, and let be an even mapping with such that
for all and . Then there exists a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then Q is B-quadratic.
Similar to Theorem 3.8, one can obtain the following theorem.
Theorem 3.12 Let be a function such that
for all . Let be an even mapping such that
for all and all . If there exists a Lipschitz constant such that for all , then there exists a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then Q is B-quadratic.
Remark 3.13 Let be a mapping for which there exists a function satisfying (3.28). Let be a constant such that for all . , since .
We now prove our main theorem in this section.
Theorem 3.14 Let be a function such that
for all . Let be a mapping with such that
for all and all . If there exists a Lipschitz constant such that
for all , then there exist a unique additive mapping and a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear and Q is B-quadratic.
Proof If we decompose f into the even and the odd parts by putting
for all , then . Let , then by (3.30)-(3.32) and (3.34), we have
Hence, by Theorems 3.1 and 3.8, there exist a unique additive mapping and a unique quadratic mapping such that
for all . Therefore,
for all . □
Corollary 3.15 Let and δ, θ be non-negative real numbers, and let be a mapping with such that
for all and . Then there exist a unique additive mapping and a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear and Q is B-quadratic.
Thanks to Remark 3.13, by a similar method to the proof of Theorem 3.14, one can obtain the following theorem.
Theorem 3.16 Let be a function such that
for all . Let be a mapping such that
for all and all . If there exists a Lipschitz constant such that
for all , then there exist a unique additive mapping and a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear and Q is B-quadratic.
Corollary 3.17 Let and θ be a non-negative real number, and let be a mapping for which
for all and . Then there exist a unique additive mapping and a unique quadratic mapping such that
for all . Moreover, if is continuous in for each fixed , then A is B-linear and Q is B-quadratic.
Remark 3.18 The generalized Hyers-Ulam stability problem for the cases of and were excluded in Corollaries 3.15 and 3.17, respectively.
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Acknowledgements
The authors would like to thank the referees and the editors for their help and suggestions in improving this paper. The first author TZX was supported by the National Natural Science Foundation of China (NNSFC) (Grant No. 11171022).
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Xu, T.Z., Rassias, J.M. A fixed point approach to the Hyers-Ulam stability of an AQ functional equation on β-Banach modules. J Inequal Appl 2012, 204 (2012). https://doi.org/10.1186/1029-242X-2012-204
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DOI: https://doi.org/10.1186/1029-242X-2012-204