Abstract
We define generalized additive set-valued functional equations, which are related with the following generalized additive functional equations: ,
for a fixed integer l with , and they prove the Hyers-Ulam stability of the generalized additive set-valued functional equations by using the fixed point method.
MSC: 39B52, 54C60, 91B44.
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1 Introduction and preliminaries
After the pioneering papers were 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 theory of set-valued functions has been much related with the control theory and the mathematical economics.
Let Y be a Banach space. We define the following:
: the set of all subsets of Y;
: the set of all closed bounded subsets of Y;
: the set of all closed convex subsets of Y;
: the set of all closed convex bounded subsets of Y;
: the set of all closed compact subsets of Y.
We can consider the addition and the scalar multiplication on as follows:
where and . Further, if , then we denote by
We can easily check that
where and . Furthermore, when C is convex, we obtain
for all .
For a given set , the distance function and the support function are, respectively, defined by
For every pair , we define the Hausdorff distance between C and by
where is the closed unit ball in Y.
The following proposition is related with some properties of the Hausdorff distance.
Proposition 1.1 For every and , the following properties hold:
-
(a)
;
-
(b)
.
Let be endowed with the Hausdorff distance h. Since Y is a Banach space, is a complete metric semigroup (see [7]). Debreu [2] proved that is isometrically embedded in a Banach space as follows.
Lemma 1.2 [2]
Let be the Banach space of continuous real-valued functions on endowed with the uniform norm . Then the mapping , given by , satisfies the following properties:
-
(a)
;
-
(b)
;
-
(c)
;
-
(d)
is closed in
for all and all .
Let be a set-valued function from a complete finite measure space into . Then f is Debreu integrable if the composition is Bochner integrable (see [10]). In this case, the Debreu integral of f in Ω is the unique element such that is the Bochner integral of . The set of Debreu integrable functions from Ω to will be denoted by . Furthermore, on , we define for all . Then we find that is an abelian semigroup.
The stability problem of functional equations originated from a question of Ulam [11] concerning the stability of group homomorphisms. Hyers [12] gave a first affirmative partial answer to the question of Ulam for Banach spaces. Hyers’ Theorem was generalized by Aoki [13] for additive mappings and by Rassias [14] for linear mappings by considering an unbounded Cauchy difference. A generalization of the Rassias theorem was obtained by Găvruta [15] by replacing the unbounded Cauchy difference by a general control function in the spirit of 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 [12, 14, 16–20]).
Let X be a set. A function is called a generalized metric on X if d satisfies
-
(1)
if and only if ;
-
(2)
for all ;
-
(3)
for all .
Note that the distinction between the generalized metric and the usaul metric is that the range of the former includes the infinity.
Let be a generalized metric space. An operator satisfies a Lipschitz condition with Lipschitz constant L if there exists a constant such that for all . If the Lipschitz constant is less than 1, then the operator T is called a strictly contractive operator. We recall a fundamental result in the fixed point theory.
Let be a complete generalized metric space and let be a strictly contractive mapping with Lipschitz constant . Then for each given element , either
for all nonnegative integers n or there exists a positive integer such that
-
(1)
, ;
-
(2)
the sequence converges to a fixed point of J;
-
(3)
is the unique fixed point of J in the set ;
-
(4)
for all .
In 1996, Isac and Rassias started to use the fixed point theory for the proof of stability theory of functional equations. Afterwards the stability problems of several functional equations by using the fixed point methods have been extensively investigated by a number of authors [19, 20, 23].
Set-valued functional equations have been studied by a number of authors and there are many interesting results concerning this problem (see [24–31]). In this paper, we define generalized additive set-valued functional equations and prove the Hyers-Ulam stability of generalized additive set-valued functional equations by using the fixed point method.
Throughout this paper, let X be a real vector space and Y a Banach space.
2 Stability of a generalized additive set-valued functional equation
Definition 2.1 Let be a set-valued function. The generalized additive set-valued functional equation is defined by
for all . Every solution of the generalized additive set-valued functional equation is called a generalized additive set-valued mapping.
Theorem 2.2 Let be a function such that there exists an with
for all . Suppose that is a mapping satisfying
for all . Then there exists a unique generalized additive set-valued mapping such that
for all .
Proof Let in (2.3). Since is a convex set, we get
and if we replace x by in (2.6), then we obtain
for all . Consider
and introduce the generalized metric on X,
where, as usual, . It is easy to show that is complete (see [23], Theorem 2.5). Now we consider the linear mapping such that
for all . Let be given such . Then
for all . Hence
for all . So implies the . This means that
for all . Furthermore we can have from (2.6). By Theorem 1.3, there exists a mapping satisfying the following:
-
(1)
A is a fixed point of J, i.e.,
(2.7)
for all . The mapping A is a unique fixed point of J in the set
This implies that A is a unique mapping satisfying (2.7) such that there exists a satisfying
for all ;
-
(2)
as . This implies the equality
for all ;
-
(3)
, which implies the inequality
This implies that the inequality (2.4) holds. By (2.3),
which tends to zero as for all . Thus
as desired. □
Corollary 2.3 Let and be real numbers, and let X be a real normed space. Suppose that is a mapping satisfying
for all . Then there exists a unique generalized additive set-valued mapping satisfying
for all .
Proof The proof follows from Theorem 2.2 by taking
for all . □
Theorem 2.4 Let be a function such that there exists an with
for all . Suppose that is a mapping satisfying (2.3). Then there exists a unique generalized additive set-valued mapping such that
for all .
Proof It follows from (2.5) that
for all .
The rest of the proof is similar to the proof of Theorem 2.2. □
Corollary 2.5 Let and be real numbers, and let X be a real normed space. Suppose that is a mapping satisfying (2.8). Then there exists a unique generalized additive set-valued mapping satisfying
for all .
Proof The proof follows from Theorem 2.4 by taking
for all . □
3 Stability of a generalized Cauchy-Jensen type additive set-valued functional equation
Definition 3.1 Let be a set-valued function. The generalized Cauchy-Jensen type additive set-valued functional equation is defined by
for all . Every solution of the generalized Cauchy-Jensen type additive set-valued functional equation is called a generalized Cauchy-Jensen type additive set-valued mapping.
Theorem 3.2 Let be a function such that there exists an with
for all . Suppose that is a mapping satisfying
for all . Then
exists for each and defines a unique generalized Cauchy-Jensen type additive set-valued mapping such that
and
for all .
Proof Let in (3.2). Since is a convex set, we get
and so
for all . Consider
and introduce the generalized metric on X,
where, as usual, . Then is complete. Now we consider the linear mapping such that
for all . Let be given such that . Then
for all . Hence
for all . So implies the . This means that
for all . It follows from (3.6) that . By Theorem 1.3, there exists a mapping satisfying the following:
-
(1)
A is a fixed point of J, i.e.,
(3.7)
for all . The mapping A is a unique fixed point of J in the set
This implies that A is a unique mapping satisfying (3.7) such that there exists a satisfying
for all ;
-
(2)
as . This implies the equality
for all ;
-
(3)
, which implies the inequality
This implies that the inequality (3.4) holds. By (3.3),
which tends to zero as for all . Thus we can have
as desired. □
Corollary 3.3 Let and be real numbers, and let X be a real normed space. Suppose that is a mapping satisfying
for all . Then there exists a unique generalized Cauchy-Jensen type additive set-valued mapping satisfying (3.3) and
for all .
Proof The proof follows from Theorem 3.2 by taking
for all . □
Theorem 3.4 Let be a function such that there exists an with
for all . Suppose that is a mapping satisfying (3.2). Then there exists a unique generalized Cauchy-Jensen type additive set-valued mapping satisfying (3.3) and
for all .
Proof It follows from (3.10) that
for all .
The rest of the proof is similar to the proof of Theorem 3.2. □
Corollary 3.5 Let and be real numbers and let X be a real normed space. Suppose that is a mapping satisfying (3.2). Then there exists a unique generalized Cauchy-Jensen type additive set-valued mapping satisfying (3.3)
for all .
Proof The proof follows from Theorem 3.4 by taking
for all . □
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Acknowledgements
SY Jang was supported by NRF Research Fund 2013-007226 and had worked during a visit to the Research Institute of Mathematics, Seoul National University.
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Jang, S.Y. The fixed point alternative and Hyers-Ulam stability of generalized additive set-valued functional equations. Adv Differ Equ 2014, 127 (2014). https://doi.org/10.1186/1687-1847-2014-127
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DOI: https://doi.org/10.1186/1687-1847-2014-127