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# An AQCQ-functional equation in matrix Banach spaces

Choonkil Park1, Jung Rye Lee2 and Dong Yun Shin3*

Author Affiliations

1 Department of Mathematics, Research Institute for Natural Sciences, Hanyang University, Seoul, 133-791, Korea

2 Department of Mathematics, Daejin University, Pocheon, Kyeonggi, 487-711, Korea

3 Department of Mathematics, University of Seoul, Seoul, 130-743, Korea

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Advances in Difference Equations 2013, 2013:146  doi:10.1186/1687-1847-2013-146

 Received: 2 April 2013 Accepted: 8 May 2013 Published: 23 May 2013

© 2013 Park et al.; licensee Springer

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

Using the fixed point method, we prove the Hyers-Ulam stability of an additive-quadratic-cubic-quartic functional equation in matrix normed spaces.

MSC: 47L25, 47H10, 39B82, 46L07, 39B52.

##### Keywords:
operator space; fixed point; Hyers-Ulam stability; additive-quadratic-cubic-quartic functional equation

### 1 Introduction and preliminaries

The abstract characterization given for linear spaces of bounded Hilbert space operators in terms of matricially normed spaces[1] implies that quotients, mapping spaces and various tensor products of operator spaces may again be regarded as operator spaces. Owing in part to this result, the theory of operator spaces is having an increasingly significant effect on operator algebra theory (see [2]).

The proof given in [1] appealed to the theory of ordered operator spaces [3]. Effros and Ruan [4] showed that one can give a purely metric proof of this important theorem by using the technique of Pisier [5] and Haagerup [6] (as modified in [7]).

The stability problem of functional equations originated from a question of Ulam [8] concerning the stability of group homomorphisms.

The functional equation

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

is called the Cauchy additive functional equation. In particular, every solution of the Cauchy additive functional equation is said to be an additive mapping. Hyers [9] gave the first affirmative partial answer to the question of Ulam for Banach spaces. Hyers’ theorem was generalized by Aoki [10] for additive mappings and by TM Rassias [11] for linear mappings by considering an unbounded Cauchy difference. A generalization of the TM Rassias theorem was obtained by Găvruta [12] by replacing the unbounded Cauchy difference by a general control function in the spirit of TM Rassias’ approach.

In 1990, TM Rassias [13] during the 27th International Symposium on Functional Equations asked the question whether such a theorem can also be proved for p 1 . In 1991, Gajda [14], following the same approach as in TM Rassias [11], gave an affirmative solution to this question for p > 1 . It was shown by Gajda [14], as well as by TM Rassias and Šemrl [15], that one cannot prove a TM Rassias’ type theorem when p = 1 (cf. the books of Czerwik [16], Hyers et al.[17]).

In 1982, JM Rassias [18] followed the innovative approach of the TM Rassias’ theorem [11] in which he replaced the factor x p + y p by x p y q for p , q R with p + q 1 .

The functional equation

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

is called a quadratic functional equation. In particular, every solution of the quadratic functional equation is said to be a quadratic mapping. A Hyers-Ulam stability problem for the quadratic functional equation was proved by Skof [19] for mappings f : X Y , where X is a normed space and Y is a Banach space. Cholewa [20] noticed that the theorem of Skof is still true if the relevant domain X is replaced by an Abelian group. Czerwik [21] proved the Hyers-Ulam stability of the quadratic functional equation.

In [22], Jun and Kim considered the following cubic functional equation:

f ( 2 x + y ) + f ( 2 x y ) = 2 f ( x + y ) + 2 f ( x y ) + 12 f ( x ) . (1.1)

It is easy to show that the function f ( x ) = x 3 satisfies functional equation (1.1), which is called a cubic functional equation and every solution of the cubic functional equation is said to be a cubic mapping.

In [23], Lee et al. considered the following quartic functional equation:

f ( 2 x + y ) + f ( 2 x y ) = 4 f ( x + y ) + 4 f ( x y ) + 24 f ( x ) 6 f ( y ) . (1.2)

It is easy to show that the function f ( x ) = x 4 satisfies functional equation (1.2), which is called a quartic functional equation, and every solution of the quartic functional equation is said to be a quartic mapping. 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 [24-33]).

We will use the following notations:

e j = ( 0 , , 0 , 1 , 0 , , 0 ) ;

E i j is that ( i , j ) -component is 1 and the other components are zero;

E i j x is that ( i , j ) -component is x and the other components are zero;

For x M n ( X ) , y M k ( X ) ,

x y = ( x 0 0 y ) .

Note that ( X , { n } ) is a matrix normed space if and only if ( M n ( X ) , n ) is a normed space for each positive integer n and A x B k A B x n holds for A M k , n , x = ( x i j ) M n ( X ) and B M n , k , and that ( X , { n } ) is a matrix Banach space if and only if X is a Banach space and ( X , { n } ) is a matrix normed space.

Let E, F be vector spaces. For a given mapping h : E F and a given positive integer n, define h n : M n ( E ) M n ( F ) by

h n ( [ x i j ] ) = [ h ( x i j ) ]

for all [ x i j ] M n ( E ) .

Let X be a set. A function d : X × X [ 0 , ] is called a generalized metric on X if d satisfies

(1) d ( x , y ) = 0 if and only if x = y ;

(2) d ( x , y ) = d ( y , x ) for all x , y X ;

(3) d ( x , z ) d ( x , y ) + d ( y , z ) for all x , y , z X .

We recall a fundamental result in fixed point theory.

Theorem 1.1[34,35]

Let ( X , d ) be a complete generalized metric space and let J : X X be a strictly contractive mapping with Lipschitz constant α < 1 . Then, for each given element x X , either

d ( J n x , J n + 1 x ) =

for all nonnegative integersnor there exists a positive integer n 0 such that

(1) d ( J n x , J n + 1 x ) < , n n 0 ;

(2) the sequence { J n x } converges to a fixed point y ofJ;

(3) y is the unique fixed point ofJin the set Y = { y X d ( J n 0 x , y ) < } ;

(4) d ( y , y ) 1 1 α d ( y , J y ) for all y Y .

In 1996, Isac and Rassias [36] 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 [37-43]).

In this paper, we prove the Hyers-Ulam stability of the following additive-quadratic-cubic-quartic functional equation:

f ( x + 2 y ) + f ( x 2 y ) = 4 f ( x + y ) + 4 f ( x y ) 6 f ( x ) + f ( 2 y ) + f ( 2 y ) 4 f ( y ) 4 f ( y ) (1.3)

in matrix normed spaces by using the fixed point method.

One can easily show that an odd mapping f : X Y satisfies (1.3) if and only if the odd mapping f : X Y is an additive-cubic mapping, i.e.,

f ( x + 2 y ) + f ( x 2 y ) = 4 f ( x + y ) + 4 f ( x y ) 6 f ( x ) .

It was shown in [[44], Lemma 2.2] that g ( x ) : = f ( 2 x ) 2 f ( x ) and h ( x ) : = f ( 2 x ) 8 f ( x ) are cubic and additive, respectively, and that f ( x ) = 1 6 g ( x ) 1 6 h ( x ) .

One can easily show that an even mapping f : X Y satisfies (1.3) if and only if the even mapping f : X Y is a quadratic-quartic mapping, i.e.,

f ( x + 2 y ) + f ( x 2 y ) = 4 f ( x + y ) + 4 f ( x y ) 6 f ( x ) + 2 f ( 2 y ) 8 f ( y ) .

It was shown in [[45], Lemma 2.1] that g ( x ) : = f ( 2 x ) 4 f ( x ) and h ( x ) : = f ( 2 x ) 16 f ( x ) are quartic and quadratic, respectively, and that f ( x ) = 1 12 g ( x ) 1 12 h ( x ) .

Throughout this paper, let ( X , { n } ) be a matrix normed space and ( Y , { n } ) be a matrix Banach space.

### 2 Hyers-Ulam stability of AQCQ-functional equation (1.3) in matrix normed spaces: odd mapping case

In this section, we prove the Hyers-Ulam stability of AQCQ-functional equation (1.3) in matrix normed spaces for an odd mapping case.

Lemma 2.1Let ( X , { n } ) be a matrix normed space. Then:

(1) E k l x n = x for x X .

(2) x k l [ x i j ] n i , j = 1 n x i j for [ x i j ] M n ( X ) .

(3) lim n x n = x if and only if lim n x i j n = x i j for x n = [ x i j n ] , x = [ x i j ] M k ( X ) .

Proof (1) Since E k l x = e k x e l and e k = e l = 1 , E k l x n x . Since e k ( E k l x ) e l = x , x E k l x n . So E k l x n = x .

(2) Since e k x e l = x k l and e k = e l = 1 , x k l [ x i j ] n . Since [ x i j ] = i , j = 1 n E i j x i j ,

[ x i j ] n = i , j = 1 n E i j x i j n i , j = 1 n E i j x i j n = i , j = 1 n x i j .

(3) By

x k l n x k l [ x i j n x i j ] n = [ x i j n ] [ x i j ] n i , j = 1 n x i j n x i j ,

we get the result. □

For a mapping f : X Y , define D f : X 2 Y and D f n : M n ( X 2 ) M n ( Y ) by

D f ( a , b ) : = f ( a + 2 b ) + f ( a 2 b ) 4 f ( a + b ) 4 f ( a b ) + 6 f ( a ) f ( 2 b ) f ( 2 b ) + 4 f ( b ) + 4 f ( b ) , D f n ( [ x i j ] , [ y i j ] ) : = f n ( [ x i j ] + 2 [ y i j ] ) + f n ( [ x i j ] 2 [ y i j ] ) 4 f n ( [ x i j ] + [ y i j ] ) 4 f n ( [ x i j ] [ y i j ] ) + 6 f n ( [ x i j ] ) f n ( 2 [ y i j ] ) f n ( 2 [ y i j ] ) + 4 f n ( [ y i j ] ) + 4 f n ( [ y i j ] )

for all a , b X and all x = [ x i j ] , y = [ y i j ] M n ( X ) .

Theorem 2.2Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) 2 α φ ( a 2 , b 2 ) (2.1)

for all a , b X . Let f : X Y be an odd mapping satisfying

D f n ( [ x i j ] , [ y i j ] ) n i , j = 1 n φ ( x i j , y i j ) (2.2)

for all x = [ x i j ] , y = [ y i j ] M n ( X ) . Then there exists a unique additive mapping A : X Y such that

f n ( 2 [ x i j ] ) 8 f n ( [ x i j ] ) A n ( [ x i j ] ) n i , j = 1 n 1 1 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) ) (2.3)

for all x = [ x i j ] M n ( X ) .

Proof Let x i j = 0 and y i j = 0 except for ( i , j ) = ( s , t ) in (2.2).

Putting y s t = x s t in (2.2), we get

f ( 3 y s t ) 4 f ( 2 y s t ) + 5 f ( y s t ) φ ( y s t , y s t ) (2.4)

for all y s t X .

Replacing x s t by 2 y s t in (2.2), we get

f ( 4 y s t ) 4 f ( 3 y s t ) + 6 f ( 2 y s t ) 4 f ( y s t ) φ ( 2 y s t , y s t ) (2.5)

for all y s t X .

By (2.4) and (2.5),

f ( 4 y s t ) 10 f ( 2 y s t ) + 16 f ( y s t ) 4 ( f ( 3 y s t ) 4 f ( 2 y s t ) + 5 f ( y s t ) ) + f ( 4 y s t ) 4 f ( 3 y s t ) + 6 f ( 2 y s t ) 4 f ( y s t ) = 4 f ( 3 y s t ) 4 f ( 2 y s t ) + 5 f ( y s t ) + f ( 4 y s t ) 4 f ( 3 y s t ) + 6 f ( 2 y s t ) 4 f ( y s t ) 4 φ ( y s t , y s t ) + φ ( 2 y s t , y s t ) (2.6)

for all y s t X . Replacing y s t by x s t and letting g ( x s t ) : = f ( 2 x s t ) 8 f ( x s t ) in (2.6), we get

g ( 2 x s t ) 2 g ( x s t ) 4 φ ( x s t , x s t ) + φ ( 2 x s t , x s t )

for all x s t X . So

g ( x s t ) 1 2 g ( 2 x s t ) 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) (2.7)

for all x s t X .

Consider the set

S : = { h : X Y }

and introduce the generalized metric on S:

d ( g , h ) = inf { μ R + : g ( a ) h ( a ) μ ( 2 φ ( a , a ) + 1 2 φ ( 2 a , a ) ) , a X } ,

where, as usual, inf ϕ = + . It is easy to show that ( S , d ) is complete (see [46,47]).

Now we consider the linear mapping J : S S such that

J g ( a ) : = 1 2 g ( 2 a )

for all a X .

Let g , h S be given such that d ( g , h ) = ε . Then

g ( a ) h ( a ) 2 φ ( a , a ) + 1 2 φ ( 2 a , a )

for all a X . Hence

J g ( a ) J h ( a ) = 1 2 g ( 2 a ) 1 2 h ( 2 a ) α ( 2 φ ( a , a ) + 1 2 φ ( 2 a , a ) )

for all a X . So d ( g , h ) = ε implies that d ( J g , J h ) α ε . This means that

d ( J g , J h ) α d ( g , h )

for all g , h S .

It follows from (2.7) that d ( g , J g ) 1 .

By Theorem 1.1, there exists a mapping A : X Y satisfying the following:

(1) A is a fixed point of J, i.e.,

A ( 2 a ) = 2 A ( a ) (2.8)

for all a X . The mapping A is a unique fixed point of J in the set

M = { g S : d ( h , g ) < } .

This implies that A is a unique mapping satisfying (2.8) such that there exists a μ ( 0 , ) satisfying

g ( a ) A ( a ) μ ( 2 φ ( a , a ) + 1 2 φ ( 2 a , a ) )

for all a X ;

(2) d ( J l g , A ) 0 as l . This implies the equality

lim l 1 2 l g ( 2 l a ) = A ( a )

for all a X ;

(3) d ( g , A ) 1 1 α d ( g , J g ) , which implies the inequality

d ( g , A ) 1 1 α .

So

g ( a ) A ( a ) 1 1 α ( 2 φ ( a , a ) + 1 2 φ ( 2 a , a ) ) (2.9)

for all a X .

It follows from (2.1) and (2.2) that

D A ( a , b ) = lim l 1 2 l D g ( 2 l a , 2 l b ) lim l 1 2 l ( φ ( 2 l + 1 a , 2 l + 1 b ) + 8 φ ( 2 l a , 2 l b ) ) lim l 2 l α l 2 l ( φ ( 2 a , 2 b ) + 8 φ ( a , b ) ) = 0

for all a , b X . Hence D A ( a , b ) = 0 for all a, b. So A : X Y is additive.

By Lemma 2.1 and (2.9),

f n ( 2 [ x i j ] ) 8 f n ( [ x i j ] ) A n ( [ x i j ] ) n i , j = 1 n f ( 2 x i j ) 8 f ( x i j ) A ( x i j ) i , j = 1 n 1 1 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) . Thus A : X Y is a unique additive mapping satisfying (2.3), as desired. □

Corollary 2.3Letr, θbe positive real numbers with r < 1 . Let f : X Y be an odd mapping such that

D f n ( [ x i j ] , [ y i j ] ) n i , j = 1 n θ ( x i j r + y i j r ) (2.10)

for all x = [ x i j ] , y = [ y i j ] M n ( X ) . Then there exists a unique additive mapping A : X Y such that

f n ( 2 [ x i j ] ) 8 f n ( [ x i j ] ) A n ( [ x i j ] ) n i , j = 1 n 9 + 2 r 2 2 r θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 2.2 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 r 1 and we get the desired result. □

Theorem 2.4Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) α 2 φ ( 2 a , 2 b )

for all a , b X . Let f : X Y be an odd mapping satisfying (2.2). Then there exists a unique additive mapping A : X Y such that

f n ( 2 [ x i j ] ) 8 f n ( [ x i j ] ) A n ( [ x i j ] ) n i , j = 1 n α 1 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

Now we consider the linear mapping J : S S such that

J g ( a ) : = 2 g ( a 2 )

for all a X .

It follows from (2.7) that

g ( x s t ) 2 g ( x s t 2 ) 4 φ ( x s t 2 , x s t 2 ) + φ ( x s t , x s t 2 ) α ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) )

for all x s t X . Thus d ( g , J g ) α . So

d ( g , A ) α 1 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 2.5Letr, θbe positive real numbers with r > 1 . Let f : X Y be an odd mapping satisfying (2.10). Then there exists a unique additive mapping A : X Y such that

f n ( 2 [ x i j ] ) 8 f n ( [ x i j ] ) A n ( [ x i j ] ) n i , j = 1 n 2 r + 9 2 r 2 θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 2.4 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 1 r and we get the desired result. □

Theorem 2.6Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) 8 α φ ( a 2 , b 2 )

for all a , b X . Let f : X Y be an odd mapping satisfying (2.2). Then there exists a unique cubic mapping C : X Y such that

f n ( 2 [ x i j ] ) 2 f n ( [ x i j ] ) C n ( [ x i j ] ) n i , j = 1 n 1 4 4 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

Replacing y s t by x s t and letting g ( x s t ) : = f ( 2 x s t ) 2 f ( x s t ) in (2.6), we get

g ( 2 x s t ) 8 g ( x s t ) 4 φ ( x s t , x s t ) + φ ( 2 x s t , x s t )

for all x s t X . So

g ( x s t ) 1 8 g ( 2 x s t ) 1 4 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) ) (2.11)

for all x s t X . Thus d ( g , J g ) 1 4 . So

d ( g , A ) 1 4 4 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 2.7Letr, θbe positive real numbers with r < 3 . Let f : X Y be an odd mapping satisfying (2.10). Then there exists a unique cubic mapping C : X Y such that

f n ( 2 [ x i j ] ) 2 f n ( [ x i j ] ) C n ( [ x i j ] ) n i , j = 1 n 9 + 2 r 8 2 r θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 2.6 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 r 3 and we get the desired result. □

Theorem 2.8Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) α 8 φ ( 2 a , 2 b )

for all a , b X . Let f : X Y be an odd mapping satisfying (2.2). Then there exists a unique cubic mapping C : X Y such that

f n ( 2 [ x i j ] ) 2 f n ( [ x i j ] ) C n ( [ x i j ] ) n i , j = 1 n α 4 4 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

It follows from (2.11) that

g ( x s t ) 8 g ( x s t 2 ) 4 φ ( x s t 2 , x s t 2 ) + φ ( x s t , x s t 2 ) α 4 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) )

for all x s t X . Thus d ( g , J g ) α 4 . So

d ( g , A ) α 4 4 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 2.9Letr, θbe positive real numbers with r > 3 . Let f : X Y be an odd mapping satisfying (2.10). Then there exists a unique cubic mapping C : X Y such that

f n ( 2 [ x i j ] ) 2 f n ( [ x i j ] ) C n ( [ x i j ] ) n i , j = 1 n 2 r + 9 2 r 8 θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 2.8 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 3 r and we get the desired result. □

### 3 Hyers-Ulam stability of AQCQ-functional equation (1.3) in matrix normed spaces: even mapping case

In this section, we prove the Hyers-Ulam stability of AQCQ-functional equation (1.3) in matrix normed spaces for an even mapping case.

Theorem 3.1Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) 4 α φ ( a 2 , b 2 )

for all a , b X . Let f : X Y be an even mapping satisfying f ( 0 ) = 0 and (2.2). Then there exists a unique quadratic mapping Q : X Y such that

f n ( 2 [ x i j ] ) 16 f n ( [ x i j ] ) Q n ( [ x i j ] ) n i , j = 1 n 1 2 2 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

Let x i j = 0 and y i j = 0 except for ( i , j ) = ( s , t ) in (2.2).

Putting y s t = x s t in (2.2), we get

f ( 3 y s t ) 6 f ( 2 y s t ) + 15 f ( y s t ) φ ( y s t , y s t ) (3.1)

for all y s t X .

Replacing x s t by 2 y s t in (2.2), we get

f ( 4 y s t ) 4 f ( 3 y s t ) + 4 f ( 2 y s t ) + 4 f ( y s t ) φ ( 2 y s t , y s t ) (3.2)

for all y s t X .

By (3.1) and (3.2),

f ( 4 y s t ) 20 f ( 2 y s t ) + 64 f ( y s t ) 4 ( f ( 3 y s t ) 6 f ( 2 y s t ) + 15 f ( y s t ) ) + f ( 4 y s t ) 4 f ( 3 y s t ) + 4 f ( 2 y s t ) + 4 f ( y s t ) = 4 f ( 3 y s t ) 6 f ( 2 y s t ) + 15 f ( y s t ) + f ( 4 y s t ) 4 f ( 3 y s t ) + 4 f ( 2 y s t ) + 4 f ( y s t ) 4 φ ( y s t , y s t ) + φ ( 2 y s t , y s t ) (3.3)

for all y s t X . Replacing y s t by x s t and letting g ( x s t ) : = f ( 2 x s t ) 16 f ( x s t ) in (3.3), we get

g ( 2 x s t ) 4 g ( x s t ) 4 φ ( x s t , x s t ) + φ ( 2 x s t , x s t )

for all x s t X . So

g ( x s t ) 1 4 g ( 2 x s t ) 1 2 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) ) (3.4)

for all x s t X . Thus d ( g , J g ) 1 2 . So

d ( g , A ) 1 2 2 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 3.2Letr, θbe positive real numbers with r < 2 . Let f : X Y be an even mapping satisfying (2.10). Then there exists a unique quadratic mapping Q : X Y such that

f n ( 2 [ x i j ] ) 16 f n ( [ x i j ] ) Q n ( [ x i j ] ) n i , j = 1 n 9 + 2 r 4 2 r θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 3.1 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 r 2 and we get the desired result. □

Theorem 3.3Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) α 4 φ ( 2 a , 2 b )

for all a , b X . Let f : X Y be an even mapping satisfying f ( 0 ) = 0 and (2.2). Then there exists a unique quadratic mapping Q : X Y such that

f n ( 2 [ x i j ] ) 16 f n ( [ x i j ] ) Q n ( [ x i j ] ) n i , j = 1 n α 2 2 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

It follows from (3.4) that

g ( x s t ) 4 g ( x s t 2 ) 4 φ ( x s t 2 , x s t 2 ) + φ ( x s t , x s t 2 ) α 2 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) )

for all x s t X . Thus d ( g , J g ) α 2 . So

d ( g , A ) α 2 2 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 3.4Letr, θbe positive real numbers with r > 2 . Let f : X Y be an even mapping satisfying (2.10). Then there exists a unique quadratic mapping Q : X Y such that

f n ( 2 [ x i j ] ) 16 f n ( [ x i j ] ) Q n ( [ x i j ] ) n i , j = 1 n 2 r + 9 2 r 4 θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 3.3 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 2 r and we get the desired result. □

Theorem 3.5Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) 16 α φ ( a 2 , b 2 )

for all a , b X . Let f : X Y be an even mapping satisfying f ( 0 ) = 0 and (2.2). Then there exists a unique quartic mapping R : X Y such that

f n ( 2 [ x i j ] ) 4 f n ( [ x i j ] ) R n ( [ x i j ] ) n i , j = 1 n 1 8 8 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

Replacing y s t by x s t and letting g ( x s t ) : = f ( 2 x s t ) 4 f ( x s t ) in (3.3), we get

g ( 2 x s t ) 16 g ( x s t ) 4 φ ( x s t , x s t ) + φ ( 2 x s t , x s t )

for all x s t X . So

g ( x s t ) 1 16 g ( 2 x s t ) 1 8 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) ) (3.5)

for all x s t X . Thus d ( g , J g ) 1 8 . So

d ( g , A ) 1 8 8 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 3.6Letr, θbe positive real numbers with r < 4 . Let f : X Y be an even mapping satisfying (2.10). Then there exists a unique quartic mapping R : X Y such that

f n ( 2 [ x i j ] ) 4 f n ( [ x i j ] ) R n ( [ x i j ] ) n i , j = 1 n 9 + 2 r 16 2 r θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 3.5 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 r 4 and we get the desired result. □

Theorem 3.7Let φ : X 2 [ 0 , ) be a function such that there exists an α < 1 with

φ ( a , b ) α 16 φ ( 2 a , 2 b )

for all a , b X . Let f : X Y be an even mapping satisfying f ( 0 ) = 0 and (2.2). Then there exists a unique quartic mapping R : X Y such that

f n ( 2 [ x i j ] ) 4 f n ( [ x i j ] ) R n ( [ x i j ] ) n i , j = 1 n α 8 8 α ( 2 φ ( x i j , x i j ) + 1 2 φ ( 2 x i j , x i j ) )

for all x = [ x i j ] M n ( X ) .

Proof Let ( S , d ) be the generalized metric space defined in the proof of Theorem 2.2.

It follows from (3.5) that

g ( x s t ) 16 g ( x s t 2 ) 4 φ ( x s t 2 , x s t 2 ) + φ ( x s t , x s t 2 ) α 8 ( 2 φ ( x s t , x s t ) + 1 2 φ ( 2 x s t , x s t ) )

for all x s t X . Thus d ( g , J g ) α 8 . So

d ( g , A ) α 8 8 α .

The rest of the proof is similar to the proof of Theorem 2.2. □

Corollary 3.8Letr, θbe positive real numbers with r > 4 . Let f : X Y be an even mapping satisfying (2.10). Then there exists a unique quartic mapping R : X Y such that

f n ( 2 [ x i j ] ) 4 f n ( [ x i j ] ) R n ( [ x i j ] ) n i , j = 1 n 2 r + 9 2 r 16 θ x i j r

for all x = [ x i j ] M n ( X ) .

Proof The proof follows from Theorem 3.7 by taking φ ( a , b ) = θ ( a r + b r ) for all a , b X . Then we can choose α = 2 4 r and we get the desired result. □

### Competing interests

The authors declare that they have no competing interests.

### Authors’ contributions

All authors conceived of the study, participated in its design and coordination, drafted the manuscript, participated in the sequence alignment, and read and approved the final manuscript.

### Acknowledgements

CP was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2012R1A1A2004299), and DYS was supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2010-0021792).

### References

1. Ruan, ZJ: Subspaces of C -algebras. J. Funct. Anal.. 76, 217–230 (1988). Publisher Full Text

2. Effros, E, Ruan, ZJ: On approximation properties for operator spaces. Int. J. Math.. 1, 163–187 (1990). Publisher Full Text

3. Choi, MD, Effros, E: Injectivity and operator spaces. J. Funct. Anal.. 24, 156–209 (1977). Publisher Full Text

4. Effros, E, Ruan, ZJ: On the abstract characterization of operator spaces. Proc. Am. Math. Soc.. 119, 579–584 (1993). Publisher Full Text

5. Pisier, G: Grothendieck’s theorem for non-commutative C -algebras with an appendix on Grothendieck’s constants. J. Funct. Anal.. 29, 397–415 (1978). PubMed Abstract | Publisher Full Text

6. Haagerup, U: Decomp. of completely bounded maps (unpublished manuscript)

7. Effros, E: On Multilinear Completely Bounded Module Maps, pp. 479–501. Am. Math. Soc., Providence (1987)

8. Ulam, SM: A Collection of the Mathematical Problems, Interscience, New York (1960)

9. Hyers, DH: On the stability of the linear functional equation. Proc. Natl. Acad. Sci. USA. 27, 222–224 (1941). PubMed Abstract | Publisher Full Text | PubMed Central Full Text

10. Aoki, T: On the stability of the linear transformation in Banach spaces. J. Math. Soc. Jpn.. 2, 64–66 (1950). Publisher Full Text

11. Rassias, TM: On the stability of the linear mapping in Banach spaces. Proc. Am. Math. Soc.. 72, 297–300 (1978). Publisher Full Text

12. Gǎvruta, P: A generalization of the Hyers-Ulam-Rassias stability of approximately additive mappings. J. Math. Anal. Appl.. 184, 431–436 (1994). Publisher Full Text

13. Rassias, TM: Problem 16; 2. Report of the 27th international symp. on functional equations. Aequ. Math.. 39, 292–293 309 (1990)

14. Gajda, Z: On stability of additive mappings. Int. J. Math. Math. Sci.. 14, 431–434 (1991). Publisher Full Text

15. Rassias, TM, Šemrl, P: On the behaviour of mappings which do not satisfy Hyers-Ulam stability. Proc. Am. Math. Soc.. 114, 989–993 (1992). Publisher Full Text

16. Czerwik, S: Functional Equations and Inequalities in Several Variables, World Scientific, Singapore (2002)

17. Hyers, DH, Isac, G, Rassias, TM: Stability of Functional Equations in Several Variables, Birkhäuser, Basel (1998)

18. Rassias, JM: On approximation of approximately linear mappings by linear mappings. J. Funct. Anal.. 46, 126–130 (1982). Publisher Full Text

19. Skof, F: Proprietà locali e approssimazione di operatori. Rend. Semin. Mat. Fis. Milano. 53, 113–129 (1983). Publisher Full Text

20. Cholewa, PW: Remarks on the stability of functional equations. Aequ. Math.. 27, 76–86 (1984). Publisher Full Text

21. Czerwik, S: On the stability of the quadratic mapping in normed spaces. Abh. Math. Semin. Univ. Hamb.. 62, 59–64 (1992). Publisher Full Text

22. Jun, K, Kim, H: The generalized Hyers-Ulam-Rassias stability of a cubic functional equation. J. Math. Anal. Appl.. 274, 867–878 (2002). Publisher Full Text

23. Lee, S, Im, S, Hwang, I: Quartic functional equations. J. Math. Anal. Appl.. 307, 387–394 (2005). Publisher Full Text

24. Aczel, J, Dhombres, J: Functional Equations in Several Variables, Cambridge University Press, Cambridge (1989)

25. Amyari, M, Park, C, Moslehian, MS: Nearly ternary derivations. Taiwan. J. Math.. 11, 1417–1424 (2007)

26. Chou, CY, Tzeng, JH: On approximate isomorphisms between Banach ∗-algebras or C -algebras. Taiwan. J. Math.. 10, 219–231 (2006)

27. Eshaghi Gordji, M, Savadkouhi, MB: Stability of a mixed type cubic-quartic functional equation in non-Archimedean spaces. Appl. Math. Lett.. 23, 1198–1202 (2010). Publisher Full Text

28. Isac, G, Rassias, TM: On the Hyers-Ulam stability of ψ-additive mappings. J. Approx. Theory. 72, 131–137 (1993). Publisher Full Text

29. Jun, K, Lee, Y: A generalization of the Hyers-Ulam-Rassias stability of the Pexiderized quadratic equations. J. Math. Anal. Appl.. 297, 70–86 (2004). Publisher Full Text

30. Jung, S: Hyers-Ulam-Rassias Stability of Functional Equations in Mathematical Analysis, Hadronic Press, Palm Harbor (2001)

31. Park, C: Homomorphisms between Poisson J C -algebras. Bull. Braz. Math. Soc.. 36, 79–97 (2005). Publisher Full Text

32. Park, C, Ghaleh, SG, Ghasemi, K: n-Jordan ∗-homomorphisms in C -algebras. Taiwan. J. Math.. 16, 1803–1814 (2012)

33. Rassias, JM: Solution of a problem of Ulam. J. Approx. Theory. 57, 268–273 (1989). Publisher Full Text

34. Cădariu, L, Radu, V: Fixed points and the stability of Jensen’s functional equation. J. Inequal. Pure Appl. Math.. 4(1), Article ID 4 (2003)

35. Diaz, J, Margolis, B: A fixed point theorem of the alternative for contractions on a generalized complete metric space. Bull. Am. Math. Soc.. 74, 305–309 (1968). Publisher Full Text

36. Isac, G, Rassias, TM: Stability of ψ-additive mappings: applications to nonlinear analysis. Int. J. Math. Math. Sci.. 19, 219–228 (1996). Publisher Full Text

37. Cădariu, L, Radu, V: On the stability of the Cauchy functional equation: a fixed point approach. Grazer Math. Ber.. 346, 43–52 (2004)

38. Cădariu, L, Radu, V: Fixed point methods for the generalized stability of functional equations in a single variable. Fixed Point Theory Appl.. 2008, Article ID 749392 (2008)

39. Jung, Y, Chang, I: The stability of a cubic type functional equation with the fixed point alternative. J. Math. Anal. Appl.. 306, 752–760 (2005). Publisher Full Text

40. Mirzavaziri, M, Moslehian, MS: A fixed point approach to stability of a quadratic equation. Bull. Braz. Math. Soc.. 37, 361–376 (2006). Publisher Full Text

41. Park, C: Fixed points and Hyers-Ulam-Rassias stability of Cauchy-Jensen functional equations in Banach algebras. Fixed Point Theory Appl.. 2007, Article ID 50175 (2007)

42. Park, C: Generalized Hyers-Ulam stability of functional equations: a fixed point approach. Taiwan. J. Math.. 14, 1591–1608 (2010)

43. Radu, V: The fixed point alternative and the stability of functional equations. Fixed Point Theory. 4, 91–96 (2003)

44. Eshaghi Gordji, M, Kaboli-Gharetapeh, S, Park, C, Zolfaghari, S: Stability of an additive-cubic-quartic functional equation. Adv. Differ. Equ.. 2009, Article ID 395693 (2009)

45. Eshaghi Gordji, M, Abbaszadeh, S, Park, C: On the stability of a generalized quadratic and quartic type functional equation in quasi-Banach spaces. J. Inequal. Appl.. 2009, Article ID 153084 (2009)

46. Miheţ, D, Radu, V: On the stability of the additive Cauchy functional equation in random normed spaces. J. Math. Anal. Appl.. 343, 567–572 (2008). Publisher Full Text

47. Park, C: Generalized Hyers-Ulam-Rassias stability of quadratic functional equations: a fixed point approach. Fixed Point Theory Appl.. 2008, Article ID 493751 (2008)