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A system of additive functional equations in complex Banach algebras

  • Siriluk Paokanta , Mehdi Dehghanian EMAIL logo , Choonkil Park EMAIL logo and Yamin Sayyari
Published/Copyright: February 13, 2023
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Abstract

In this article, we solve the system of additive functional equations:

2 f ( x + y ) g ( x ) = g ( y ) , g ( x + y ) 2 f ( y x ) = 4 f ( x )

and prove the Hyers-Ulam stability of the system of additive functional equations in complex Banach spaces. Furthermore, we prove the Hyers-Ulam stability of f -hom-ders in Banach algebras.

MSC 2010: 47B47; 17B40; 39B72; 47H10

1 Introduction

Let be a complex Banach algebra and f : be a C -linear mapping. Mirzavaziri and Moslehian [1] introduced the concept of f -derivation g : as follows:

(1.1) g ( x y ) = f ( x ) g ( y ) + g ( x ) f ( y )

for all x , y .

Park et al. [2] introduced the concept of hom-derivation on , i.e., g : is a homomorphism and f satisfies (1.1) for all x , y . Dehghanian et al. [3] introduced the concept of hom-der g : as follows:

g ( x ) g ( y ) = x g ( y ) + g ( x ) y

for all x , y . Kheawborisuk et al. [4] defined and studied hom-ders in fuzzy Banach algebras.

Definition 1.1

Let be a complex Banach algebra and f : be a homomorphism. A C -linear mapping g : is called an f -hom-der if it satisfies

g ( x ) g ( y ) = f ( x ) g ( y ) + g ( x ) f ( y )

for all x , y .

Example 1.2

Let M n be the collection of all n × n complex matrices and g : M n M n defined by g ( X ) = 2 X and f : M n M n defined by f ( X ) = X . Then f is a homomorphism, and g is an f -hom-der.

We say that an equation is stable if any function satisfying the equation approximately is near to an exact solution of the equation.

The stability analysis of functional equations emanated from a question of Ulam [5], which was raised in 1940 about the stability of group homomorphisms and then was extended by Hyers [6]. Recently, results on the so-called Hyers-Ulam stability have comfortable the stability conditions. Many mathematicians developed the Hyers results in various directions [7,8,9, 10,11,12, 13,14].

The method provided by Hyers [6], which produces the additive function, will be called a direct method. This method is the most significant and strong tool to concerning the stability of different functional equations. That is, the exact solution of the functional equation is explicitly constructed as a limit of a sequence, starting from the given approximate solution [10,15]. The other significant method is fixed-point theorem, that is, the exact solutions of functional equations and differential equations are explicitly created as fixed points of some certain mappings [16,17,18, 19,20,21, 22,23,24, 25,26,27, 28,29,30, 31,32,33, 34,35].

We remember a fixed-point alternative theorem.

Theorem 1.3

[36] If ( , d ) is a complete generalized metric space and J : is a strictly contractive mapping, that is,

d ( J u , J v ) L d ( u , v )

for all u , v and a Lipschitz constant L < 1 . Then for each given element u , either

d ( J n u , J n + 1 u ) = + , n 0 ,

or

d ( J n u , J n + 1 u ) < + , n n 0 ,

for some positive integer n 0 . Furthermore, if the second alternative holds, then:

  1. the sequence ( J n u ) is convergent to a fixed point v of J ;

  2. v is the unique fixed point of J in the set V { v , d ( J n 0 u , v ) < + } ;

  3. d ( v , v ) 1 1 L d ( v , J v ) for all u , v V .

In this article, we consider the following system of additive functional equations

(1.2) 2 f ( x + y ) g ( x ) = g ( y ) , g ( x + y ) 2 f ( y x ) = 4 f ( x )

for all x , y . The aim of the present article is to solve the system of additive functional equations and prove the Hyers-Ulam stability of f -hom-ders in complex Banach algebras by using the fixed point method.

Throughout this article, assume that is a complex Banach algebra.

2 Stability of system of additive functional equations

We solve and investigate the system of additive functional equations (1.2) in complex Banach algebras.

Lemma 2.1

Let f , g : be mappings satisfying (1.2) for all x , y . Then the mappings f , g : are additive.

Proof

By substituting x = y = 0 in (1.2), we obtain

f ( 0 ) = g ( 0 ) = 0 .

By substituting y = 0 in (1.2), we have

(2.1) 2 f ( x ) = g ( x ) , f ( x ) = f ( x )

for all x . So

g ( x + y ) = 2 f ( x + y ) = g ( x ) + g ( y )

for all x , y . Hence, the mapping g : is additive, and thus by (2.1), the mapping f : is additive.□

By using the fixed-point technique, we prove the Hyers-Ulam stability of the system of additive functional equations (1.2) in complex Banach algebras.

Theorem 2.2

Suppose that Δ : 2 [ 0 , ) is a function such that there exists an L < 1 with

(2.2) Δ x 2 , y 2 L 2 Δ ( x , y )

for all x , y . Let f , g : be mappings satisfying

(2.3) 2 f ( x + y ) g ( x ) g ( y ) Δ ( x , y ) , g ( x + y ) 2 f ( y x ) 4 f ( x ) Δ ( x , y )

for all x , y . Then there exist unique additive mappings F , G : such that

(2.4) F ( x ) f ( x ) L 2 + L 4 ( 1 L ) Δ ( x , x ) ,

(2.5) G ( x ) g ( x ) L 2 + L 2 ( 1 L ) Δ ( x , x )

for all x .

Proof

By substituting x = y = 0 in (2.3), we obtain

2 f ( 0 ) 2 g ( 0 ) Δ ( 0 , 0 ) = 0 , g ( 0 ) 6 f ( 0 ) Δ ( 0 , 0 ) = 0

and so f ( 0 ) = g ( 0 ) = 0 .

By substituting y = x in (2.3), we obtain

2 f ( 2 x ) 2 g ( x ) Δ ( x , x ) , g ( 2 x ) 4 f ( x ) Δ ( x , x )

and so

(2.6) g ( x ) 4 g x 4 2 Δ x 4 , x 4 + Δ x 2 , x 2 L 2 + L 2 Δ ( x , x ) , f ( x ) 4 f x 4 Δ x 4 , x 4 + 1 2 Δ x 2 , x 2 L 2 + L 4 Δ ( x , x )

for all x .

Let Γ = { γ : : γ ( 0 ) = 0 } . We define a generalized metric on Γ as follows: d : Γ × Γ [ 0 , ] by

d ( δ , γ ) = inf { μ R + : δ ( x ) γ ( x ) μ Δ ( x , x ) , x } ,

and we consider inf = + . Then d is a complete generalized metric on Γ (see [37]).

Now, we define the mapping J : ( Γ , d ) ( Γ , d ) such that

J δ ( x ) 4 δ x 4

for all x .

Actually, let δ , γ ( Γ , d ) be given such that d ( δ , γ ) = μ . Then

δ ( x ) γ ( x ) μ Δ ( x , x )

for all x . Hence,

J δ ( x ) J γ ( x ) = 4 δ x 4 4 γ x 4 4 μ Δ x 4 , x 4 L 2 μ Δ ( x , x )

for all x . It follows that d ( J δ ( x ) , J γ ( x ) ) L 2 μ . So

d ( J δ ( x ) , J γ ( x ) ) L 2 d ( δ , γ )

for all x and all δ , γ Γ .

It follows from (2.6) that d ( f , J f ) L 2 + L 4 and d ( g , J g ) L 2 + L 2 .

By using the fixed-point alternative we deduce the existence of a unique fixed point of J and a unique fixed point of J , that is, the existence of mappings F , G : , respectively, such that

F ( x ) = 4 F x 4 , G ( x ) = 4 G x 4

with the following property: there exist μ , η ( 0 , ) satisfying

f ( x ) F ( x ) μ Δ ( x , x ) , g ( x ) G ( x ) η Δ ( x , x )

for all x .

Since lim n d ( J n f , F ) = 0 and lim n d ( J n g , G ) = 0 ,

lim n 4 n f x 4 n = F ( x ) , lim n 4 n g x 4 n = G ( x )

for all x .

Next, d ( f , F ) 1 1 L d ( f , J f ) and d ( g , G ) 1 1 L d ( g , J g ) , which imply

f ( x ) F ( x ) L 2 + L 4 ( 1 L ) Δ ( x , x ) , g ( x ) G ( x ) L 2 + L 2 ( 1 L ) Δ ( x , x )

for all x .

By using (2.2) and (2.3), we conclude that

2 F ( x + y ) G ( x ) G ( y ) = lim n 4 n 2 f x + y 4 n + g x 4 n g y 4 n lim n 4 n Δ x 4 n , y 4 n lim n L 2 n Δ ( x , y ) = 0

and

G ( x + y ) 2 F ( y x ) 4 F ( x ) = lim n 4 n g x + y 4 n 2 f y x 4 n 4 f x 4 n lim n 4 n Δ x 4 n , y 4 n lim n L 2 n Δ ( x , y ) = 0

for all x , y , since L < 1 . Hence,

2 F ( x + y ) G ( x ) = G ( y ) , G ( x + y ) 2 F ( y x ) = 4 F ( x )

for all x , y . Therefore, by Lemma 2.1, the mappings F , G : are additive.□

Corollary 2.3

Let f , g : be mappings satisfying

2 f ( x + y ) g ( x ) g ( y ) x y , g ( x + y ) 2 f ( y x ) 4 f ( x ) x y

for all x , y . Then there exist unique additive mappings F , G : such that

F ( x ) f ( x ) 3 8 x 2 , G ( x ) g ( x ) 3 4 x 2

for all x .

Proof

The proof follows from Theorem 2.2 by taking L = 1 2 and Δ ( x , y ) = x y for all x , y .□

Corollary 2.4

Let p and θ be nonnegative real numbers with p > 1 and f , g : be mappings satisfying

2 f ( x + y ) g ( x ) g ( y ) θ ( x p + y p ) , g ( x + y ) 2 f ( y x ) 4 f ( x ) θ ( x p + y p )

for all x , y . Then there exist unique additive mappings F , G : such that

F ( x ) f ( x ) 2 p + 2 2 p ( 2 p 2 ) θ x p , G ( x ) g ( x ) 2 ( 2 p + 2 ) 2 p ( 2 p 2 ) θ x p

for all x .

Proof

The proof follows from Theorem 2.2 by taking L = 2 2 p and Δ ( x , y ) = θ ( x p + y p ) for all x , y .□

3 Stability of F -hom-ders in Banach algebras

In this section, by using the fixed-point technique, we prove the Hyers-Ulam stability of F -hom-ders in complex Banach algebras.

Lemma 3.1

[38] Let be a complex Banach algebra and : be an additive mapping such that ( α x ) = α ( x ) for all α T 1 { ζ C : ζ = 1 } and all x . Then is C -linear.

Lemma 3.2

Let f , g : be mappings satisfying

(3.1) 2 f ( λ ( x + y ) ) g ( λ x ) = λ g ( y ) , g ( λ ( x + y ) ) 2 f ( λ ( y x ) ) = 4 λ f ( x )

for all x , y and all λ T 1 . Then the mappings f , g : are C -linear.

Proof

If we substitute λ = 1 in (3.1), then f and g are additive by Lemma 2.1.

By substituting y = 0 in (3.1), we have

(3.2) 2 f ( λ x ) = λ g ( x ) , g ( λ x ) = 2 λ f ( x )

for all x and all λ T 1 , since the mappings f and g are additive. By substituting λ = 1 in (3.2), we obtain

2 f ( x ) = g ( x ) , g ( x ) = 2 f ( x ) ,

and so

2 f ( λ x ) = 2 λ f ( x ) , g ( λ x ) = λ g ( x )

for all x and all λ T 1 . So by Lemma 3.2, the mappings f and g are C -linear.□

Theorem 3.3

Suppose that Δ : 2 [ 0 , ) is a function such that there exists an L < 1 with

(3.3) Δ ( x , y ) L 4 Δ ( 2 x , 2 y ) L 2 Δ ( 2 x , 2 y )

for all x , y . Let f , g : be mappings satisfying

(3.4) 2 f ( λ ( x + y ) ) g ( λ x ) λ g ( y ) Δ ( x , y ) , g ( λ ( x + y ) ) 2 f ( λ ( y x ) ) 4 λ f ( x ) Δ ( x , y ) ,

(3.5) f ( x y ) f ( x ) f ( y ) Δ ( x , y ) ,

(3.6) g ( x ) g ( y ) f ( x ) g ( y ) g ( x ) f ( y ) Δ ( x , y )

for all x , y , and all λ T 1 . Then there exist unique mappings F , G : satisfying (2.4) and (2.5) and F is a homomorphism and G is an F-hom-der.

Proof

Let λ = 1 in (3.4). Since

Δ ( x , y ) L 4 Δ ( 2 x , 2 y ) L 2 Δ ( 2 x , 2 y )

for all x , y , by Theorem 2.2, there exist unique mappings F , G : satisfying (2.4) and (2.5), which are given by

lim n 4 n f x 4 n = F ( x ) , lim n 4 n g x 4 n = G ( x )

for all x .

By using (3.3), we conclude that

2 F ( λ ( x + y ) ) G ( λ x ) λ G ( y ) = lim n 4 n 2 f λ ( x + y ) 4 n g λ x 4 n λ g y 4 n lim n 4 n Δ x 4 n , y 4 n lim n L 2 n Δ ( x , y ) = 0

and

G ( λ ( x + y ) ) 2 F ( λ ( y x ) ) 4 λ F ( x ) = lim n 4 n g λ ( x + y ) 4 n 2 f λ ( y x ) 4 n 4 λ f x 4 n lim n 4 n Δ x 4 n , y 4 n lim n L 2 n Δ ( x , y ) = 0

for all x , y , since L < 1 . Hence,

2 F ( λ ( x + y ) ) G ( λ x ) = λ G ( y ) , G ( λ ( x + y ) ) 2 F ( λ ( y x ) ) = 4 λ F ( x )

for all x , y and all λ T 1 . Therefore, by Lemma 3.2, the mappings F , G : are C -linear.

It follows from (3.5) that

F ( x y ) F ( x ) F ( y ) = lim n 4 n f x y 4 n f x 2 n f y 2 n lim n 4 n Δ x 2 n , y 2 n lim n L n Δ ( x , y ) = 0

for all x , y . So

F ( x y ) = F ( x ) F ( y )

for all x , y . Thus, F is a homomorphism.

It follows from (3.6) that

G ( x ) G ( y ) F ( x ) G ( y ) G ( x ) F ( y ) = lim n 4 n g x 2 n g y 2 n f x 2 n g y 2 n g x 2 n f y 2 n lim n 4 n Δ x 2 n , y 2 n lim n L n Δ ( x , y ) = 0

for all x , y . So

G ( x ) G ( y ) = F ( x ) G ( y ) + G ( x ) F ( y )

for all x , y . Thus, the C -linear mapping G is an F -hom-der.□

Corollary 3.4

Let p and q be nonnegative real numbers with p + q > 2 and f , g : be mappings satisfying

2 f ( λ ( x + y ) ) g ( λ x ) λ g ( y ) x p y q , g ( λ ( x + y ) ) 2 f ( λ ( y x ) ) 4 λ f ( x ) x p y q

and

f ( x y ) f ( x ) f ( y ) x p y q ,

g ( x y ) f ( x ) g ( y ) g ( x ) f ( y ) x p y q

for all x , y and all λ T 1 . Then there exist unique mappings F , G : such that F is a homomorphism and G is an F-hom-der and

F ( x ) f ( x ) 2 p + 1 + 4 2 p + q ( 2 p + q 4 ) x p + q , G ( x ) g ( x ) 2 ( 2 p + 1 + 4 ) 2 p + 1 ( 2 p + q 4 ) x p + q

for all x .

Proof

The proof follows from Theorem 3.3 by taking Δ ( x , y ) = x p y q for all x , y . By choosing L = 2 2 p q , we obtain the desired result.□

Corollary 3.5

Let p and θ be nonnegative real numbers with p > 2 and f , g : be mappings satisfying

2 f ( λ ( x + y ) ) g ( λ x ) λ g ( y ) θ ( x p + y p ) , g ( λ ( x + y ) ) 2 f ( λ ( y x ) ) 4 λ f ( x ) θ ( x p + y p )

and

f ( x y ) f ( x ) f ( y ) θ ( x p + y p ) ,

g ( x y ) f ( x ) g ( y ) g ( x ) f ( y ) θ ( x p + y p )

for all x , y and all λ T 1 . Then there exist unique mappings F , G : such that F is a homomorphism and G is an F-hom-der and

F ( x ) f ( x ) 2 ( 2 p + 4 ) 2 p ( 2 p 4 ) θ x p , G ( x ) g ( x ) 4 ( 2 p + 4 ) 2 p ( 2 p 4 ) θ x p

for all x .

Proof

The proof follows from Theorem 3.3 by taking Δ ( x , y ) = θ ( x p + y p ) for all x , y . By choosing L = 2 2 p , we obtain the desired result.□

4 Conclusion

In this article, we introduced a new system of additive functional equations in complex Banach spaces and the concept of f -hom-der in complex Banach algebras and proved the Hyers-Ulam stability of the new system of additive functional equations in complex Banach spaces and the Hyers-Ulam stability of f -hom-ders in complex Banach algebras.

Acknowledgements

We would like to express our sincere gratitude to the anonymous referee for his/her helpful comments that will help to improve the quality of the manuscript.

  1. Funding information: The authors declare that there is no funding available for this article.

  2. Author contributions: The authors equally participated in conception of the study, participated in its design and coordination, drafted the manuscript, participated in the sequence alignment, and read and approved the final manuscript.

  3. Conflict of interest: The authors declare that they have no competing interest.

  4. Human and animal rights: We would like to mention that this article does not contain any studies with animals and does not involve any studies over human being.

  5. Data availability statement: Not applicable.

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Received: 2022-07-14
Revised: 2022-08-19
Accepted: 2022-08-31
Published Online: 2023-02-13

© 2023 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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