Ciric-type δ-contractions in metric spaces endowedwithagraph

Transcription

1 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 R E S E A R C H Open Access Ciric-type δ-contractions in metric spaces endowedwithagraph Cristian Chifu 1* and Adrian Petruşel 2 * Correspondence: cristian.chifu@tbs.ubbcluj.ro 1 Department of Business, Babeş-Bolyai University Cluj-Napoca, Horea street, 7, Cluj-Napoca, , Romania Full list of author information is available at the end of the article Abstract The purpose of this paper is to present some fixed-point and strict fixed-point results in a metric space endowed with a graph, using a contractive condition of Ćirić type with respect to the functional δ. The data dependence of the fixed-point set, the well-posedness of the fixed-point problem, and the limit shadowing property are also studied. MSC: 47H10; 54H25 Keywords: fixed point; strict fixed point; metric space; connected graph; well-posed problem; limit shadowing property 1 Preliminaries A recent research direction in fixed-point theory is the study of the fixed-point problem for single-valued and multivalued operators in the context of a metric space endowed with a graph. This approach was recently considered by Jachymski in [1], Gwóźdź-Lukawska and Jachymski in [2],andthenitwasdevelopedinmanyotherpapers[3 5], etc.. On the other hand, fixed points and strict fixed points also called end-points are important elements in mathematical economics and game theory. It represents optimal preferencesinsomearrow-debreutypemodelsornashtypeequilibriumpointsforsome abstract noncooperative games, see, for example, [6] and[7]. From this perspective, it is important to give fixed and strict fixed-point theorems for multivalued operators. We shall begin by presenting some notions and notations that will be used throughout the paper. Let X, d be a metric space and be the diagonal of X X. LetG be a directed graph such that the set V G of its vertices coincides with X and EG, EGbeingthesetof the edges of the graph. Assuming that G has no parallel edges, we will suppose that G can be identified with the pair V G, EG. If x and y are vertices of G,thenapathinG from x to y of length k N is a finite sequence x n n {0,1,2,...,k} of vertices such that x 0 = x, x k = y and x i 1, x i EG, for i {1,2,...,k}. Let us denote by G the undirected graph obtained from G by ignoring the direction of edges. Notice that a graph G is connected if there is a path between any two vertices and it is weakly connected if G is connected. Let us consider the following families of subsets of a metric space X, d: PX:= { Y PX Y }, P b X:= { Y PX Y is bounded }, P cl X:= { Y PX Y is closed }, P cp X:= { Y PX Y is compact } Chifu and Petruşel; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 2 of 11 Let usdefine thegap functionalbetween the sets A and B in the metric space X, d as: D : PX PX R + {+ }, DA, B=inf { da, b a A, b B } and the generalized Pompeiu-Hausdorff functional as H : PX PX R + {+ }, { HA, B=max sup a A Da, B, sup DA, b b B }. The generalized diameter functional is denoted by δ : PX PX R + { },and defined by δa, B=sup { da, b a A, b B }. Denote by diama:=δa, A the diameter of the set A. Let T : X PX beamultivaluedoperatorandgraphict :={x, y y Tx} the graphic of T. x X is called fixed point for T if and only if x Tx, and it is called strict fixed point if and only if {x} = Tx. The set FixT :={x X x Tx} is called the fixed-point set of T, whiles FixT = {x X {x} = Tx} is called the strict fixed-point set of T.NoticethatS FixT FixT. We will write EG IT Tifandonlyifforeveryx, y X with x, y EGwehave Tx Ty EG. For the particular case of a single-valued operator t : X X the above notations should be considered accordingly. In particular, the condition EG It t means that the operator t is edge preserving in the sense of the Jachymski s definition of a Banach contraction, i.e. for each x, y X with x, y EG we havetx,ty EG see[1]. We will also denote by Ox 0, n:={x 0, tx 0, t 2 x 0,...,t n x 0 } the orbit of order n of the operator t corresponding to x 0 X. In this paper we prove some fixed-point and strict fixed-point theorems for singlevalued and multivalued operators satisfying a contractive condition of Ćirić type with respect to the functional δ. Our results also generalize and extend some fixed-point theorems in partially ordered complete metric spaces given in Harjani, Sadarangani [8],Nieto, Rodríguez-López [9] and[10], Nieto et al. [11], O Regan, Petruşel [12], Petruşel, Rus [13] and Ran, Reurings [14]. For other general results concerning Ćirić type fixed-point theorems see Rus [15]. In the main section of the paper we give results concerning the existence and uniqueness of the fixed point and of the strict fixed point of a Ćirić type single-valued and multivalued contraction. Then the well-posedness of the fixed-point problem, the data dependence of the fixed-point set, and the limit shadowing property are also studied. Our results complement and extend some recent theorems given in [16] for multivalued Reich-type operators. 2 Main results In this section we present the main results of the paper concerning the fixed-point problem and, respectively, the strict fixed-point problem for a single-valued, respectively, of a multivalued Ćirić type contraction.

3 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 3 of 11 Definition 2.1 Let X, dbeametricspaceandt : X P cl Xbeamultivaluedoperator. By definition, the fixed-point problem is well-posed for T with respect to H if: i S Fix T = {x }; d ii If x n n N is a sequence in X such that Hx n, Tx n 0,asn,thenx n x, as n. Definition 2.2 Let X, d be a metric space and T : X PX beamultivaluedoperator. By definition T has the limit shadowing property if for any sequence y n n N from X such that Dy n+1, Ty n 0, as n,thereexistsx n n N, a sequence of successive approximation of T,suchthatdx n, y n 0, as n. In order to prove the limit shadowing property we shall need Cauchy s lemma. Lemma 2.1 Cauchy s lemma Let a n n N and b n n N be two sequences of non-negative real numbers, such that + k=0 a k <+ and b n 0, as n. Then n a n k b k 0, k=0 as n. The first main result of this paper is the following result for the case of single-valued operators. The proof of this result is inspired by the proof of Ćirić s fixed-point theorem in [17] and the approach introduced for metric spaces endowed with a graph by Jachymski in [1]. Theorem 2.1 Let X, d be a complete metric space and G be a directed graph such that the triple X, d, G satisfies the following property: P for any sequence x n n N Xwithx n xasn, there exists a subsequence x kn n N of x n n N such that x kn, x EG. Let t : X Xbeasingle-valuedoperator. Suppose the following assertions hold: i there exists a [0, 1[ such that d tx, ty a max { dx, y, d x, tx, d y, ty, d x, ty, d y, tx }, for all x, y EG; ii there exists x 0 X such that x 0, tx 0 EG; iii EG It t; iv if x, y EG and y, z EG, then x, z EG. In these conditions we have: a existence Fixt ; b uniqueness If, in addition, the following implication holds x, y Fixt x, y EG, then Fixt={x }.

4 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 4 of 11 Proof a Let x 0 X such that x 0, tx 0 EG. By iii we obtain t i x 0, t i+1 x 0 EG, for all i N.Then,byiv,weget t i x 0, t j x 0 EG, for all i, j N with i < j. Since EGweget t i x 0, t j x 0 EG, for all i, j N with i j. 2.1 For n N,leti, j N with 1 i < j n.then d t i x 0, t j x 0 a diam Ox 0 ; n. Notice now that, from the above relation, it follows that there exists k N with k n such that d x 0, t k x 0 = diam Ox 0 ; n. 2.2 In order to show that the sequence t n x 0 n N is Cauchy, let us consider n, m N with n < m.thenwehave d t n x 0, t m x 0 a diam O t n 1 x 0 ; m n +1. By 2.2thereexistsp N with p m n +1suchthat diam O t n 1 x 0 ; m n +1 = d t n 1 x 0, t p t n 1 x 0. Then d t n 1 x 0, t p t n 1 x 0 = d t n 1 x 0, t p+n 1 x 0 = d t t n 2 x 0, t p+1 t n 2 x 0 a diam O t n 2 x 0 ; p +1 a diam O t n 2 x 0 ; m n +2. Thus, we get diam O t n 1 x 0 ; m n +1 a diam O t n 2 x 0 ; m n +2 a n diam Ox 0 ; m. Hence d t n x 0, t m x 0 a n diam Ox 0 ; m. Notice now that diam Ox 0 ; m 1 1 a d x 0, tx 0, for all m N. 2.3

5 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 5 of 11 Indeed, if we take m N arbitrary, then by 2.2, there exists r N such that 1 r m and dx 0, t r x 0 = diamox 0 ; m. Then, using 2.1, we get d x 0, t r x 0 d x 0, tx 0 + d tx 0, t r x 0 d x 0, tx 0 + a diam Ox 0 ; m = d x 0, tx 0 + ad x 0, t r x 0. Hence we have diam Ox 0 ; m = d x 0, t r x a d x 0, tx 0, proving 2.3. Now, we can conclude that d t n x 0, t m x 0 an 1 a d x 0, tx 0, for all n, m N with n < m. 2.4 Thus, 2.4 shows that the sequence t n x 0 n N is Cauchy and, as a consequence of the completeness of the space, it converges to an element x X. We will show now that x Fixt. Notice first that, by the property P, there exists a subsequence x kn n N of x n n N such that x kn, x EGforeachn N. Now, we can write d x, t x d x, x kn+1 + d txkn, t x d x, x kn+1 + a max { d x kn, x, d x kn, tx kn, d x, t x, d x kn, t x, d x, tx kn } d x, x kn+1 + a d xkn, x + d x kn, tx kn + d x, t x + d x, tx kn. Hence d x, t x 1 [ 1 + ad x, x kn+1 + ad xkn, x + adx kn, x kn+1 ]. 1 a Letting n + we get x Fixt. b Suppose that there exist x, y Fixtwithx y. Using i and the additional hypothesis of b, we obtain d x, y = d t x, t y a max { d x, y, d x, t x, d t y, y, d x, t y, d y, t x } = ad x, y, which is a contradiction. Remark 2.1 Notice that, from the proof of the above theorem, it follows that the sequence t n x 0 n N converges to x in X, d.

6 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 6 of 11 Remark 2.2 If in the above theorem, instead of property Pwesupposethatt has closed graphic, then we can reach the same conclusion. Moreover if we suppose that x, y Fixt x, y EG, then we get again Fixt={x }. Based on the above theorem, we can prove now our second main result. Theorem 2.2 Let X, d be a complete metric space and G be a directed graph such that the triple X, d, G satisfies the following property: P for any sequence x n n N Xwithx n xasn, there exists a subsequence x kn n N of x n n N such that x kn, x EG. Let T : X P b X be a multivalued operator. Suppose the following assertions hold: i there exists a [0, 1[ such that δ Tx, Ty a max { dx, y, δ x, Tx, δ y, Ty, D x, Ty, D y, Tx }, for all x, y EG; ii there exists x 0 X such that, for all y Tx 0, we have x 0, y EG; iii EG IT T; iv if x, y EG and y, z EG, then x, z EG. In these conditions we have: a existence FixT=SFixT ; b uniqueness If, in addition, the following implication holds: x, y FixT x, y EG, then FixT=SFixT={x }; c well-posedness of the fixed-point problem If T has closed graphic and for any sequence x n n N, x n X with Hx n, Tx n 0, as n, we have x n, x EG, then the fixed-point problem is well-posed for T with respect to H; d limit shadowing property of T If a < 1 3 and y n n N is a sequence in X such that the following implication holds: D y n+1, Ty n 0, as n y n, x EG, n N, then T has the limit shadowing property. Proof a Let 0 < q < 1 be an arbitrary real number. Notice first that, for any x X, there exists u Tx suchthata q δx, Tx dx, u. In this way, we get an operator t : X X which assigns to each x X the element tx Txwith a q δ x, Tx d x, tx.

7 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 7 of 11 Then, for x, y EG, we have d tx, ty δ Tx, Ty aa q max { a q dx, y, a q δ x, Tx, a q δ y, Ty, a q D x, Ty, a q D y, Tx } a 1 q max { dx, y, d x, tx, d y, ty, d x, ty, d y, tx }. Thus, the operator t satisfies all the hypotheses of Theorem 2.1 and, as a consequence, it has a fixed point x X. Thenx FixT. If we suppose now that there exists x FixT \ S FixT, then, since x, x, from the condition i with y = x, we get δtx aδtx, which implies since a <1thatδTx = 0. Thus Tx={x}.Thisisa contradiction with x FixT \ S FixT, proving that FixT=S FixT. b Suppose that there exist x, y FixT=S FixT. Using i we obtain d x, y = δ T x, T y a max { d x, y, δ x, T x, δ y, T y, D x, T y, D y, T x } = ad x, y, which implies that dx, y =0.HenceFixT=S FixT={x }. c Let x n n N, x n X, be a sequence with the property that Hx n, Tx n 0, as n. It is obvious that Hx n, Tx n = δx n, Tx n. Let x FixT=S FixT. Then Hence d x n, x = δ x n, T x δ x n, Tx n + δ Tx n, T x δ x n, Tx n + a max { d x n, x, δ x n, Tx n, d x n, x, D x, Tx n } δ x n, Tx n + aδ x n, Tx n + ad x n, x. d x n, x 1+a 1 a δ x n, Tx n 0, as n +, and, as a consequence, the fixed-point problem is well-posed for T with respect to H. d Let y n n N be a sequence in X such that Dy n+1, Ty n 0, as n and let x n n N be a sequence of successive approximation of T starting from x 0 X,constructedasinthe proof of Theorem 2.1. We shall prove that dx n, y n 0, as n. Let x FixT=S FixT. Then dx n, y n d x n, x + d x, y n. In what follows we shall prove that dx, y n 0, as n.thenwehave d x, y n+1 δ x, Ty n + D y n+1, Ty n = δ T x, Ty n + D y n+1, Ty n a max { d x, y n, δ yn, Ty n, D x, Ty n, d y n, x } + D y n+1, Ty n aδ y n, Ty n + ad x, y n + D yn+1, Ty n.

8 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 8 of 11 Now, we can write δ y n, Ty n d x, y n + δ x, Ty n = d x, y n + δ T x, Ty n d x, y n + aδ yn, Ty n + ad x, y n. Thus δ y n, Ty n 1+a 1 a d x, y n. If we replace this in the above relation, we get Hence d x, y n+1 2a 1 a d x, y n + D yn+1, Ty n. d y n+1, x 2a 1 a d y n, x + D y n+1, Ty n 2a 2 d y n 1, x + 2a 1 a 1 a D y n, Ty n 1 + D y n+1, Ty n. Continuing this process we shall obtain d y n+1, x 2a n+1 d y 0, x + 1 a n 2a n k D y k+1, Ty k. 1 a k=0 If we consider a n = 2a 1 a n and b n = Dy n+1, Ty n and using the fact that a < 1 3,weobtain, via Cauchy s lemma, dx, y n 0, as n.thusdx, y n 0, as n, and hence, the operator T has the limit shadowing property. Remark 2.3 If in Theorem 2.1, instead of property P, we suppose that every selection t of T has closed graphic, then we obtain FixT=S FixT. Moreoverifwesuppose that the following implication holds: x, y FixT x, y EG, then FixT=SFixT={x }. Proof Since every selection t of T has closed graphic, the conclusion follows by Theorem 2.1,viaRemark2.2. Definition 2.3 Let X, dbeametricspaceandt : X P cl Xbeamultivaluedoperator. By definition the fixed-point problem is well-posed in the generalized sense for T with respect to H if i S Fix T ; ii If x n n N is a sequence in X such that Hx n, Tx n 0,asn, then there d exists a subsequence x kn n N of x n n N such that x kn x,asn.

9 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 9 of 11 Remark 2.4 If in Theorem 2.1 instead of c we suppose the following assumption, c : c If every selection t of T has closed graphic and, in addition, we suppose that for any sequence x n n N X with Hx n, Tx n 0, asn,thereexistsasubsequence x kn n N such that x kn, x EG and Hx kn, Tx kn 0, then the fixed-point problem is well-posed in the generalized sense for T with respect to H. In what follows we shall present some examples of operators satisfying the hypotheses of our main results. Example 2.1 Let X = {0, 1} { 1 : k N } and f : X X given by 2 k x/2, if x { 1 : k N }, f x= 2 k 0, if x {0, 1}. 1 Let V G:=X and EG:={0, 1, 1, 0} {0,, 1 1, 0, 1,, 1,1:k N }. 2 k 2 k 2 k 2 k Then all the hypotheses of Theorem 2.1 are satisfied, Fixt={0} and, if we take x 0 = 1 2, then t n x 0 n N converges to x =0. Example 2.2 Let X = {0, 0, 0, 1, 1, 0} and T : X P b Xgivenby {0, 0}, x = 0, 0, Tx= {0, 0, 1, 0}, x = 0, 1, {0, 0}, x = 1, 0. Let EG:={0, 1; 1, 0, 1, 0; 0, 0}. Notice that FixT=S FixT={0, 0} and all the hypotheses in Theorem 2.2 are satisfied the condition i is verified for a 0, 71. Remark 2.5 It is also important to notice that, if we suppose that there exists x FixT \ S FixT, then, since x, x, from the condition i in the above theorem with y = x, we get δtx aδtx, which implies δtx = 0 since a < 1. This is a contradiction with x FixT \ S FixT, showing that we cannot get fixed points which are not strict fixed points in the presence of the condition i of the above theorem. It is an open problem to prove a similar theorem to the above one for a more general class of multivalued operators T. The next result presents the data dependence of the fixed-point set of a multivalued operator which satisfies a contractive condition of Ćirić type. Theorem 2.3 Let X, d be a complete metric space and G be a directed graph such that the triple X, d, G satisfies property P. Let T 1, T 2 : X P b X be two multivalued operators. Suppose the following assertions hold: i for i {1, 2}, there exist a i [0, 1 such that δ T i x, T i y a max { dx, y, δ x, T i x, δ y, T i y, D x, T i y, D y, T i x }, for all x, y EG;

10 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 10 of 11 ii for each x X and each y Tx we have x, y EG, for i {1, 2}; iii EG IT T; iv if x, y EG and y, z EG, then x, z EG; v there exists η >0such that HT 1 x, T 2 x η for all x X. Under these conditions we have: a FixT i =SFixT i, i {1, 2}; b If, in addition, for i {1, 2}, the following implication holds: x i, y i FixT i x i, y i EG, then FixT i =SFixT i ={x i }, for each i {1, 2}; c HS FixT 1, S FixT 2 η 1 max{a 1,a 2 }. Proof Conclusions a and b are immediate if we apply Theorem 2.2. For c notice that we can prove that for every x 1 S FixT 1, there exists x 2 S FixT 2, such that d x 1, η x 2. 1 a 2 A second relation of this type will be obtained by interchanging the role of T 1 and T 2. Hence, the conclusion follows by the properties of the functional H. Competing interests The authors declare that they have no competing interests. Authors contributions Both authors made equal contribution to this article regarding results, conception and drafting the manuscript. The authors read and approved the final manuscript. Author details 1 Department of Business, Babeş-Bolyai University Cluj-Napoca, Horea street, 7, Cluj-Napoca, , Romania. 2 Department of Mathematics, Babeş-Bolyai University Cluj-Napoca, Kogalniceanu street, 1, Cluj-Napoca, , Romania. Acknowledgements The work of the second author is supported by financial support of a grant of the Romanian National Authority for Scientific Research, CNCS-UEFISCDI, project number PN-II-ID-PCE Received: 15 October 2013 Accepted: 13 January 2014 Published: 17 Feb 2014 References 1. Jachymski, J: The contraction principle for mappings on a metric space with a graph. Proc. Am. Math. Soc. 136, Gwóźdź-Lukawska, G, Jachymski, J: IFS on a metric space with a graph structure and extensions of the Kelisky-Rivlin theorem. J. Math. Anal. Appl. 356, Nicolae, A, O Regan, D, Petruşel, A: Fixed point theorems for single-valued and multivalued generalized contractions in metric spaces endowed with a graph. Georgian Math. J. 18, Beg, I, Butt, AR, Radojevic, S: The contraction principles for set-valued mappings on a metric space with a graph. Comput. Math. Appl. 60, Dinevari, T, Frigon, M: Fixed point results for multivalued contractions on a metric space with a graph. J. Math. Anal. Appl. 405, Border, K: Fixed Point Theorems with Applications to Economic and Game Theory. Cambridge University Press, London Yuan, GX-Z: KKM Theory and Applications in Nonlinear Analysis. Dekker, New York Harjani, J, Sadarangani, K: Fixed point theorems for weakly contractive mappings in partially ordered sets. Nonlinear Anal. 71, Nieto, JJ, Rodríguez-López, R: Contractive mapping theorems in partially ordered sets and applications to ordinary differential equations. Order 22,

11 Chifu and Petruşel Journal of Inequalities and Applications 2014, 2014:77 Page 11 of Nieto, JJ, Rodríguez-López, R: Existence and uniqueness of fixed points in partially ordered sets and applications to ordinary differential equations. Acta Math. Sin. Engl. Ser. 23, Nieto, JJ, Pouso, RL, Rodríguez-López, R: Fixed point theorems in ordered abstract spaces. Proc. Am. Math. Soc. 135, O Regan, D, Petruşel, A: Fixed point theorems in ordered metric spaces. J. Math. Anal. Appl. 341, Petruşel, A, Rus, IA: Fixed point theorems in ordered L-spaces. Proc. Am. Math. Soc. 134, Ran, ACM, Reurings, MC: A fixed point theorem in partially ordered sets and some applications to matrix equations. Proc.Am.Math.Soc.132, Rus, IA: Generalized Contractions and Applications. Transilvania Press, Cluj-Napoca Chifu,C,Petruşel, G, Bota, M: Fixed points and strict fixed points for multivalued contractions of Reich type on metric spaces endowed with a graph. Fixed Point Theory Appl. 2013, doi: / Ćirić, LB: A generalization of Banach s contraction principle. Proc. Am. Math. Soc. 45, Petruşel, A, Rus, IA: Well-posedness of the fixed point problem for multivalued operators. In: Carja, O, Vrabie, II eds. Applied Analysis and Differential Equations, pp World Scientific, Hackensack Petruşel, A, Rus, IA, Yao, J-C: Well-posedness in the generalized sense of the fixed point problems for multivalued operators. Taiwan. J. Math. 11, / X Cite this article as: Chifu and Petruşel: Ciric-type δ-contractionsinmetricspacesendowedwithagraph.journal of Inequalities and Applications 2014, 2014:77