# Hyers-Ulam stability of a generalized additive set-valued functional equation

- Sun Young Jang
^{1}, - Choonkil Park
^{2}and - Young Cho
^{3}Email author

**2013**:101

https://doi.org/10.1186/1029-242X-2013-101

© Jang et al.; licensee Springer. 2013

**Received: **9 November 2012

**Accepted: **24 February 2013

**Published: **13 March 2013

## Abstract

In this paper, we define a generalized additive set-valued functional equation, which is related to the following generalized additive functional equation:

for a fixed integer *l* with $l>1$, and prove the Hyers-Ulam stability of the generalized additive set-valued functional equation.

**MSC:**39B52, 54C60, 91B44.

## Keywords

## 1 Introduction and preliminaries

The theory of set-valued functions has been much related to the control theory and the mathematical economics. After the pioneering papers written by Aumann [1] and Debreu [2], set-valued functions in Banach spaces have been developed in the last decades. We can refer to the papers by Arrow and Debreu [3], McKenzie [4], the monographs by Hindenbrand [5], Aubin and Frankowska [6], Castaing and Valadier [7], Klein and Thompson [8] and the survey by Hess [9].

The stability problem of functional equations originated from a question of Ulam [10] concerning the stability of group homomorphisms. Hyers [11] gave the first affirmative partial answer to the question of Ulam for Banach spaces. Hyers’ theorem was generalized by Aoki [12] for additive mappings and by Th.M. Rassias [13] for linear mappings by considering an unbounded Cauchy difference. A generalization of the Th.M. Rassias theorem was obtained by Găvruta [14] by replacing the unbounded Cauchy difference with a general control function in the spirit of Th.M. Rassias’ approach. 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 [15–17]).

Let *Y* be a Banach space. We define the following:

${2}^{Y}$: the set of all subsets of *Y*;

${C}_{b}(Y)$: the set of all closed bounded subsets of *Y*;

${C}_{c}(Y)$: the set of all closed convex subsets of *Y*;

${C}_{cb}(Y)$: the set of all closed convex bounded subsets of *Y*.

where $C,{C}^{\prime}\in {2}^{Y}$ and $\lambda \in \mathbb{R}$. Further, if $C,{C}^{\prime}\in {C}_{c}(Y)$, then we denote $C\oplus {C}^{\prime}=\overline{C+{C}^{\prime}}$.

Furthermore, when *C* is convex, we obtain $(\lambda +\mu )C=\lambda C+\mu C$ for all $\lambda ,\mu \in {\mathbb{R}}^{+}$.

*C*and ${C}^{\prime}$ by

where ${B}_{Y}$ is the closed unit ball in *Y*.

The following proposition reveals some properties of the Hausdorff distance.

**Proposition 1.1**

*For every*$C,{C}^{\prime},K,{K}^{\prime}\in {C}_{cb}(Y)$

*and*$\lambda >0$,

*the following properties hold*:

- (a)
$h(C\oplus {C}^{\prime},K\oplus {K}^{\prime})\le h(C,K)+h({C}^{\prime},{K}^{\prime})$;

- (b)
$h(\lambda C,\lambda K)=\lambda h(C,K)$.

Let $({C}_{cb}(Y),\oplus ,h)$ be endowed with the Hausdorff distance *h*. Since *Y* is a Banach space, $({C}_{cb}(Y),\oplus ,h)$ is a complete metric semigroup (see [7]). Debreu [2] proved that $({C}_{cb}(Y),\oplus ,h)$ is isometrically embedded in a Banach space as follows.

**Lemma 1.2** [2]

*Let*$C({B}_{{Y}^{\ast}})$

*be the Banach space of continuous real*-

*valued functions on*${B}_{{Y}^{\ast}}$

*endowed with the uniform norm*${\parallel \cdot \parallel}_{u}$.

*Then the mapping*$j:({C}_{cb}(Y),\oplus ,h)\to C({B}_{{Y}^{\ast}})$,

*given by*$j(A)=s(\cdot ,A)$,

*satisfies the following properties*:

- (a)
$j(A\oplus B)=j(A)+j(B)$;

- (b)
$j(\lambda A)=\lambda j(A)$;

- (c)
$h(A,B)={\parallel j(A)-j(B)\parallel}_{u}$;

- (d)
$j({C}_{cb}(Y))$

*is closed in*$C({B}_{{Y}^{\ast}})$

*for all* $A,B\in {C}_{cb}(Y)$ *and all* $\lambda \ge 0$.

Let $f:\mathrm{\Omega}\to ({C}_{cb}(Y),h)$ be a set-valued function from a complete finite measure space $(\mathrm{\Omega},\mathrm{\Sigma},\nu )$ into ${C}_{cb}(Y)$. Then *f* is *Debreu integrable* if the composition $j\circ f$ is Bochner integrable (see [18]). In this case, the Debreu integral of *f* in Ω is the unique element $(D){\int}_{\mathrm{\Omega}}f\phantom{\rule{0.2em}{0ex}}d\nu \in {C}_{cb}(Y)$ such that $j((D){\int}_{\mathrm{\Omega}}f\phantom{\rule{0.2em}{0ex}}d\nu )$ is the Bochner integral of $j\circ f$. The set of Debreu integrable functions from Ω to ${C}_{cb}(Y)$ will be denoted by $D(\mathrm{\Omega},{C}_{cb}(Y))$. Furthermore, on $D(\mathrm{\Omega},{C}_{cb}(Y))$, we define $(f+g)(\omega )=f(\omega )\oplus g(\omega )$ for all $f,g\in D(\mathrm{\Omega},{C}_{cb}(Y))$. Then we obtain that $((\mathrm{\Omega},{C}_{cb}(Y)),+)$ is an abelian semigroup.

Set-valued functional equations have been extensively investigated by a number of authors, and there are many interesting results concerning this problem (see [19–27]).

In this paper, we define a generalized additive set-valued functional equation and prove the Hyers-Ulam stability of the generalized additive set-valued functional equation.

Throughout this paper, let *X* be a real vector space and *Y* be a Banach space.

## 2 Stability of a generalized additive set-valued functional equation

**Definition 2.1**Let $f:X\to {C}_{cb}(Y)$. The generalized additive set-valued functional equation is defined by

for all ${x}_{1},\dots ,{x}_{l}\in X$. Every solution of the generalized additive set-valued functional equation is called a *generalized additive set-valued mapping*.

Note that there are some examples in [28].

**Theorem 2.2**

*Let*$\phi :{X}^{l}\to [0,\mathrm{\infty})$

*be a function such that*

*for all*${x}_{1},\dots ,{x}_{l}\in X$.

*Suppose that*$f:X\to ({C}_{cb}(Y),h)$

*is a mapping satisfying*

*for all*${x}_{1},\dots ,{x}_{l}\in X$.

*Then there exists a unique generalized additive set*-

*valued mapping*$A:X\to ({C}_{cb}(Y),h)$

*such that*

*for all* $x\in X$.

*Proof*Let ${x}_{1}=\cdots ={x}_{l}=x$ in (3). Since $f(x)$ is convex, we get

*x*by ${l}^{n}x$, $n\in \mathbb{N}$ in (5), then we obtain

for all integers *n*, *m* with $n\ge m$. It follows from (2) and (6) that $\{\frac{f({l}^{n}x)}{{l}^{n}}\}$ is a Cauchy sequence in $({C}_{cb}(Y),h)$.

*A*is a generalized additive set-valued mapping. Note that

which tends to zero as $n\to \mathrm{\infty}$. So, *A* is a generalized additive set-valued mapping. Letting $m=0$ and passing the limit $m\to \mathrm{\infty}$ in (6), we get the inequality (4).

which tends to zero as $n\to \mathrm{\infty}$ for all $x\in X$. So, we can conclude that $A(x)=T(x)$ for all $x\in X$, which proves the uniqueness of *A*, as desired. □

**Corollary 2.3**

*Let*$1>p>0$

*and*$\theta \ge 0$

*be real numbers*,

*and let*

*X*

*be a real normed space*.

*Suppose that*$f:X\to ({C}_{cb}(Y),h)$

*is a mapping satisfying*

*for all*${x}_{1},\dots ,{x}_{l}\in X$.

*Then there exists a unique generalized additive set*-

*valued mapping*$A:X\to Y$

*satisfying*

*for all* $x\in X$.

*Proof*

for all ${x}_{1},\dots ,{x}_{l}\in X$. □

**Theorem 2.4**

*Let*$\phi :{X}^{l}\to [0,\mathrm{\infty})$

*be a function such that*

*for all*${x}_{1},\dots ,{x}_{l}\in X$.

*Suppose that*$f:X\to ({C}_{cb}(Y),h)$

*is a mapping satisfying*(3).

*Then there exists a unique generalized additive set*-

*valued mapping*$A:X\to ({C}_{cb}(Y),h)$

*such that*

*for all* $x\in X$.

*Proof*It follows from (5) that

for all $x\in X$.

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

**Corollary 2.5**

*Let*$p>1$

*and*$\theta \ge 0$

*be real numbers*,

*and let*

*X*

*be a real normed space*.

*Suppose that*$f:X\to ({C}_{cb}(Y),h)$

*is a mapping satisfying*(7).

*Then there exists a unique generalized additive set*-

*valued mapping*$A:X\to Y$

*satisfying*

*for all* $x\in X$.

*Proof*

for all ${x}_{1},\dots ,{x}_{l}\in X$. □

## Declarations

### Acknowledgements

SYJ was supported by the 2012 Research Fund of University of Ulsan and had written this paper during visiting the Research Institute of Mathematics, Seoul National University. CP was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2012R1A1A2004299).

## Authors’ Affiliations

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