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Solution and Stability of a Mixed Type Additive, Quadratic, and Cubic Functional Equation
Advances in Difference Equations volume 2009, Article number: 826130 (2009)
Abstract
We obtain the general solution and the generalized HyersUlamRassias stability of the mixed type additive, quadratic, and cubic functional equation .
1. Introduction
The stability problem of functional equations originated from a question of Ulam [1] in 1940, concerning the stability of group homomorphisms. Let be a group, and let be a metric group with the metric Given , does there exist a , such that if a mapping satisfies the inequality for all , then there exists a homomorphism with for all ? In other words, under what condition does there exist a homomorphism near an approximate homomorphism?
In 1941, Hyers [2] gave a first affirmative answer to the question of Ulam for Banach spaces. Let be a mapping between Banach spaces such that
for all and for some Then there exists a unique additive mapping such that
for all Moreover if is continuous in for each fixed then is linear (see also [3]). In 1950, Aoki [4] generalized Hyers' theorem for approximately additive mappings. In 1978, Th. M. Rassias [5] provided a generalization of Hyers' theorem which allows the Cauchy difference to be unbounded. This new concept is known as HyersUlamRassias stability of functional equations (see [2–24]).
The functional equation
is related to symmetric biadditive function. In the real case it has among its solutions. Thus, it has been called quadratic functional equation, and each of its solutions is said to be a quadratic function. HyersUlamRassias stability for the quadratic functional equation (1.3) was proved by Skof for functions , where is normed space and Banach space (see [25–28]).
The following cubic functional equation was introduced by the third author of this paper, J. M. Rassias [29, 30] (in 20002001):
Jun and Kim [13] introduced the following cubic functional equation:
and they established the general solution and the generalized HyersUlamRassias stability for the functional equation (1.5).
The function satisfies the functional equation (1.5), which explains why it is called cubic functional equation.
Jun and Kim proved that a function between real vector spaces and is a solution of (1.5) if and only if there exists a unique function such that for all and is symmetric for each fixed one variable and is additive for fixed two variables (see also [31–33]).
We deal with the following functional equation deriving from additive, cubic and quadratic functions:
It is easy to see that the function is a solution of the functional equation (1.6). In the present paper we investigate the general solution and the generalized HyersUlamRassias stability of the functional equation (1.6).
2. General Solution
In this section we establish the general solution of functional equation (1.6).
Theorem 2.1.
Let , be vector spaces, and let be a function. Then satisfies (1.6) if and only if there exists a unique additive function , a unique symmetric and biadditive function and a unique symmetric and 3additive function such that for all .
Proof.
Suppose that for all , where is additive, is symmetric and biadditive, and is symmetric and 3additive. Then it is easy to see that satisfies (1.6). For the converse let satisfy (1.6). We decompose into the even part and odd part by setting
for all By (1.6), we have
for all This means that satisfies (1.6), that is,
Now putting in (2.3), we get . Setting in (2.3), by evenness of we obtain
Replacing by in (2.3), we obtain
Comparing (2.4) with (2.5), we get
By utilizing (2.5) with (2.6), we obtain
Hence, according to (2.6) and (2.7), (2.3) can be written as
With the substitution in (2.8), we have
Replacing by in above relation, we obtain
Setting instead of in (2.8), we get
Interchanging and in (2.11), we get
If we subtract (2.12) from (2.11) and use (2.10), we obtain
which, by putting and using (2.7), leads to
Let us interchange and in (2.14). Then we see that
and by adding (2.14) and (2.15), we arrive at
Replacing by in (2.8), we obtain
Let us Interchange and in (2.17). Then we see that
Thus by adding (2.17) and (2.18), we have
Replacing by in (2.11) and using (2.7) we have
and interchanging and in (2.20) yields
If we add (2.20) to (2.21), we have
Interchanging and in (2.8), we get
and by adding the last equation and (2.8) with (2.19), we get
Now according to (2.22) and (2.24), it follows that
From the substitution in (2.25) it follows that
Replacing by in (2.25) we have
and interchanging and yields
By adding (2.27) and (2.28) and then using (2.25) and (2.26), we lead to
If we compare (2.16) and (2.29), we conclude that
This means that is quadratic. Thus there exists a unique quadratic function such that for all On the other hand we can show that satisfies (1.6), that is,
Now we show that the mapping defined by is additive and the mapping defined by is cubic. Putting in (2.31), then by oddness of we have
Hence (2.31) can be written as
From the substitution in (2.33) it follows that
Interchange and in (2.33), and it follows that
With the substitutions and in (2.35), we have
Replace by in (2.34). Then we have
Replacing by in (2.37) gives
Interchanging and in (2.38), we get
If we add (2.38) to (2.39), we have
Replacing by in (2.36) gives
By comparing (2.40) with (2.41), we arrive at
Replacing by in (2.42) gives
With the substitution in (2.43), we have
and replacing by gives
Let us interchange and in (2.45). Then we see that
If we add (2.45) to (2.46), we have
Adding (2.42) to (2.47) and using (2.33) and (2.35), we obtain
for all The last equality means that
for all Therefore the mapping is additive. With the substitutions and in (2.35), we have
Let be the additive mapping defined above. It is easy to show that is cubicadditive function. Then there exists a unique function and a unique additive function such that for all and is symmetric and 3additive. Thus for all , we have
This completes the proof of theorem.
The following corollary is an alternative result of Theorem 2.1.
Corollary 2.2.
Let , be vector spaces, and let be a function satisfying (1.6). Then the following assertions hold.

(a)
If is even function, then is quadratic.

(b)
If is odd function, then is cubicadditive.
3. Stability
We now investigate the generalized HyersUlamRassias stability problem for functional equation (1.6). From now on, let be a real vector space, and let be a Banach space. Now before taking up the main subject, given , we define the difference operator by
for all We consider the following functional inequality:
for an upper bound
Theorem 3.1.
Let be fixed. Suppose that an even mapping satisfies and
for all If the upper bound is a mapping such that
and that
for all then the limit
exists for all and is a unique quadratic function satisfying (1.6), and
for all
Proof.
Let Putting in (3.3), we get
for all On the other hand by replacing by in (3.3), it follows that
for all Combining (3.8) and (3.9), we lead to
for all With the substitution in (3.10) and then dividing both sides of inequality by 2, we get
Now, using methods similar as in [8, 34, 35], we can easily show that the function defined by for all is unique quadratic function satisfying (1.6) and (3.7). Let Then by (3.10) we have
for all And analogously, as in the case , we can show that the function defined by is unique quadratic function satisfying (1.6) and (3.7).
Theorem 3.2.
Let be fixed. Let is a mapping such that
and that
for all
Suppose that an odd mapping satisfies
for all
Then the limit
exists, for all and is a unique additive function satisfying (1.6), and
for all
Proof.
Let set in (3.15). Then by oddness of we have
for all Replacing by in (3.15) we get
Combining (3.18) and (3.19), we lead to
for all Putting and for all Then we get
for all Now, in a similar way as in [8, 34, 35], we can show that the limit exists, for all and is the unique function satisfying (1.6) and (3.17). If , then the proof is analogous.
Theorem 3.3.
Let be fixed. Suppose that an odd mapping satisfies
for all If the upper bound is a mapping such that
and that for all then the limit
exists, for all and is a unique cubic function satisfying (1.6) and
for all
Proof.
We prove the theorem for When we have a similar proof. It is easy to see that satisfies (3.20). Set then by putting in (3.20), it follows that
for all By using (3.26), we may define a mapping as for all Similar to Theorem 3.1, we can show that is the unique cubic function satisfying (1.6) and (3.25).
Theorem 3.4.
Suppose that an odd mapping satisfies
for all If the upper bound is a mapping such that
and that for all then there exists a unique cubic function and a unique additive function such that
for all
Proof.
By Theorems 3.2 and 3.3, there exist an additive mapping and a cubic mapping such that
for all Combine the two equations of (3.30) to obtain
for all So we get (3.29) by letting and for all To prove the uniqueness of and let be another additive and cubic maps satisfying (3.29). Let , and let So
for all Since
then
for all Hence (3.32) implies that
for all On the other hand and are cubic, then Therefore by (3.35) we obtain that for all Again by (3.35) we have for all
Theorem 3.5.
Suppose that an odd mapping satisfies
for all If the upper bound is a mapping such that
and that for all then there exist a unique cubic function and a unique additive function such that
for all
Proof.
The proof is similar to the proof of Theorem 3.4.
Now we establish the generalized HyersUlamRassias stability of functional equation (1.6) as follows.
Theorem 3.6.
Suppose that a mapping satisfies and for all If the upper bound is a mapping such that
and that for all then there exist a unique additive function a unique quadratic function and a unique cubic function such that
for all .
Proof.
Let for all Then and for all Hence in view of Theorem 3.1 there exists a unique quadratic function satisfying (3.7). Let for all Then and for all From Theorem 3.4, it follows that there exist a unique cubic function and a unique additive function satisfying (3.29). Now it is obvious that (3.40) holds true for all and the proof of theorem is complete.
Corollary 3.7.
Let Suppose that a mapping satisfies and
for all Then there exist a unique additive function a unique quadratic function and a unique cubic function satisfying
for all
Proof.
It follows from Theorem 3.6 by taking for all .
Theorem 3.8.
Suppose that satisfies and for all If the upper bound is a mapping such that
and that for all then there exists a unique additive function a unique quadratic function and a unique cubic function such that
for all .
By Theorem 3.8, we are going to investigate the following stability problem for functional equation (1.6).
Corollary 3.9.
Let Suppose that satisfies and
for all then there exist a unique additive function a unique quadratic function and a unique cubic function satisfying
for all .
By Corollary 3.9, we solve the following HyersUlam stability problem for functional equation (1.6).
Corollary 3.10.
Let be a positive real number. Suppose that a mapping satisfies and for all then there exist a unique additive function a unique quadratic function and a unique cubic function such that
for all .
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Acknowledgments
The authors would like to express their sincere thanks to referees for their invaluable comments. The first author would like to thank the Semnan University for its financial support. Also, the fourth author would like to thank the office of gifted students at Semnan University for its financial support.
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Eshaghi Gordji, M., Kaboli Gharetapeh, S., Rassias, J.M. et al. Solution and Stability of a Mixed Type Additive, Quadratic, and Cubic Functional Equation. Adv Differ Equ 2009, 826130 (2009). https://doi.org/10.1155/2009/826130
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DOI: https://doi.org/10.1155/2009/826130