Fixed Point Theorem for Cyclic (µ, ψ, φ)-weakly Contractions via a New Function

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1 DOI: /awutm Analele Universităţii de Vest, Timişoara Seria Matematică Informatică LV, 2, 2017), 3 15 Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Contractions via a New Function Muaadh Almahalebi, Amir Hojat Ansari, and Sumit Chandok Abstract. In this paper, we introduce a generalization of cyclic µ, ψ, φ)-weakly contraction via a new function and derive the existence of fixed point for such mappings in the setup of complete metric spaces. Our results extend and improve some fixed point theorems in the literature. AMS Subject Classification 2010). 54H25; 47H10 Keywords. complete metric space, fixed point, cyclic µ, ψ, φ)- weakly contraction mapping. 1 Introduction It is well known that the fixed point theorem of Banach, for contraction mappings, is one of the pivotal result in analysis. It has been used in different fields of mathematics. Fixed point problems involving different types of contractive inequalities have been studied by many authors see [1]-[18] and references cited therein). Ya. I. Alber and S. Guerre-Delabriere [1] introduced the concept of weakly contractive mappings and proved the existence of fixed points for singlevalued weakly contractive mappings in Hilbert spaces. Thereafter, in 2001, B. E. Rhoades [18] proved the fixed point theorem which is one of the generalizations of Banach s Contraction Mapping Principle, because the weakly

2 4 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. contractions contains contractions as a special case and he also showed that some results of [1] are true for any Banach space. In fact, weakly contractive mappings are closely related to the mappings of D. W. Boyd and T. S. W. Wong [2] and of S. Reich types [17]. In [15], W. A. Kirk et al. introduced the following notion of cyclic representation and characterized the Banach Contraction Principle in the context of cyclic mapping. Definition 1.1. [15] Let X be a non-empty set and T : X X an operator. By definition, X = m i=1x i is a cyclic representation of X with respect to T if: 1. X i with i = 1,..., m are non-empty sets, 2. T X 1 ) X 2,..., T X m 1 ) X m, T X m ) X 1. M. Pacurar and I.A. Rus [16] proved the following important result o in fixed point theory. We state an analogue of this result as follows. Theorem 1.1. [16] Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X with, A m+1 = A 1, Y = m i=1a i, φ: [0, ) [0, ) be monotone increasing continuous functions with { φt) > 0, if t > 0, φ0) = 0, and T : Y Y be an operator. Assume that: 2. dt x, T y) dx, y) φdx, y)), for any x A i, y A i+1 with i = 1, 2,..., m. Then T has a unique fixed point z n i=1a i. In 2013, S. Chandok and V. Popa [7] introduced the notion of cyclic µ, ψ, φ)-weakly contraction mappings and they also derived a fixed point theorem for such cyclic contractions, in the framework of complete metric spaces. In this paper, we introduce a generalization of cyclic µ, ψ, φ)-weakly contraction and derive the existence of a fixed point for such mappings in the setup of complete metric spaces. Our results extend and improve some fixed point theorems in the literature.

3 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 5 2 Background Throughout this paper, N stands for the set of all positive integers and N 0 := N {0}. We introduce our notion of cyclic µ, ψ, φ)-weakly contraction mappings in metric space. Let θ denote the set of all monotone increasing continuous functions µ: [0, ) [0, ), with µt) > 0, if t > 0, µ0) = 0, µt 1 + t 2 ) µt 1 ) + µt 2 ), for all t 1, t 2 [0, ). Let Φ denote the set of all continuous functions φ: [0, ) [0, ) such that { φt) > 0, if t > 0, φ0) 0. Let C class denote the set of all functions f : [0, ) 2 [0, ) such that 1. f is continuous which is increasing in first variable, 2. for all t, s [0, ), fs, t, ) s, 3. fs, t) = s = s = 0 or t = 0. Let Ψ denote the set of all functions ψ : [0, ) 5 [0, ) such that 1. ψ is continuous, 2. ψ is strictly increasing in all the variables, 3. For all t 0, ), we have ψt, t, t, 0, 2t) t, ψt, t, t, 2t, 0) t, ψ0, 0, t, t, 0) t, ψ0, t, 0, 0, t) t and ψt, 0, 0, t, t) t. Example 2.1. Let s, t [0, ), then the following functions are of C class: 1. fs, t) = s t, fs, t) = s t = 0, 2. fs, t) = s t, fs, t) = s t = 0, 1+t 3. fs, t) = s, fs, t) = s s = 0 or t = 0, 1+t 4. fs, t) = s t, fs, t) = s t = 0, 1+t 5. fs, t) = s log a+t a, a 1, ), fs, t) = s s = 0 or t = 0,

4 6 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. 6. fs, t) = s s, fs, t) = s s = 0. 1+t Definition 2.1. Let X, d) be a metric space, m be a natural number, A 1, A 2..., A m be non-empty subsets of X and Y = m i=1a i. An operator T : Y Y is called a cyclic µ, ψ, φ)-f-weakly contraction if 2. µ dt x, T y) ) f ψ µ dx, y) ), µ dx, T x) ), µ dy, T y) ), µ dx, T y) ), µ dy, T x) )), φ Mx, y) )), for all x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, µ θ, φ Φ, ψ Ψ and Mx, y) = max{dx, y), dx, T x), dy, T y)}. Definition 2.2. Let X, d) be a metric space, m be a natural number, A 1, A 2..., A m be non-empty subsets of X and Y = m i=1a i. An operator T : Y Y is called a cyclic µ, ψ, φ)-raitional-weakly contraction if 2. µ dt x, T y) ) ψ µ dx, y) ), µ dx, T x) ), µ dy, T y) ), µ dx, T y) ), µ dy, T x) )) 1 + φ Mx, y) ) for all x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, µ θ, φ Φ, ψ Ψ and Mx, y) = max{dx, y), dx, T x), dy, T y)}. 3 Main results To state and prove our main results, we need the following lemma in the sequel. Lemma 3.1. For every positive real number ɛ, there exists a natural number n such that if r, q n with r q 1 mod m), then dx r, x q ) < ɛ.

5 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 7 Proof. Assume the contrary. Thus there exists ɛ > 0 such that for any n N, we can find r n > q n n with r n q n 1 mod m) satisfying dx rn, x qn ) ɛ. Now, we take n > 2m. Then, corresponding to q n n, we can choose r n in such that it is a smallest integer with r n > q n satisfying r n q n 1 mod m) and dx rn, x qn ) ɛ. Therefore, dx rn m, x qn ) < ɛ. By using the triangular inequality, we have ɛ dx qn, x rn ) dx qn, x rn m ) + < ɛ + m dx rn i, x rn i+1 ) i=1 m dx rn i, x rn i+1 ). i=1 Letting n and using dx n+1, x n ) 0, we obtain Again, by the triangular inequality, ɛ dx qn, x rn ) dx qn, x qn+1 ) + dx qn+1, x rn+1 ) + dx rn+1, x rn ) lim dx qn, x rn ) = ɛ. 3.1) dx qn, x qn+1 ) + dx qn+1, x qn ) + dx qn, x rn ) + dx rn, x rn+1 ) + dx rn+1, x rn ). Letting n and using dx n+1, x n ) 0, we get and Consider lim dx qn+1, x rn+1 ) = ɛ. 3.2) dx qn, T x rn ) = dx qn, x rn+1 ) dx qn, x rn ) + dx rn, x rn+1 ), 3.3) dx rn, T x qn ) = dx rn, x qn+1 ) dx rn, x qn ) + dx qn, x qn+1 ). 3.4) Taking n in the inequalities 3.3) and 3.4), we have lim n dx q n, T x rn ) = ɛ, 3.5) and lim dx r n, T x qn ) = ɛ. 3.6) n

6 8 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. As x qn and x rn lie in different adjacently labeled sets A i and A i+1 for certain 1 i m, using the fact T is a cyclic µ, ψ, φ)-weakly contraction, we obtain µɛ) µ dx qn+1, x rn+1 ) ) = µ dt x qn, T x rn ) ) f ψ µ dx qn, x rn ) ), µ dx qn, T x qn ) ), µ dx rn, T x rn ) ), µ dx qn, T x rn ) ), µ dx rn, T x qn ) )), φ Mx qn, x rn ) )) = f ψ µ dx qn, x rn ) ), µ dx qn, x qn+1 ) ), µ dx rn, x rn+1 ) ), µ dx qn, x rn+1 ) ), µ dx rn, x qn+1 ) )), φ Mx qn, x rn ) )), where Mx qn, x rn ) = max { dx qn, x rn ), dx qn, T x qn ), dx rn, T x rn ) } Letting n in the last inequality, by using 3.5), 3.6), the continuity of µ and φ and the property of ψ, we get that µε) f ψ µε), µ0), µ0), µε), µε) ) ), φε) f µε), φε) ). Consequently, we obtain that µε) = 0 and φε) = 0 which is a contradiction with ε > 0. Hence, the result is proved. Theorem 3.2. Let X, d) be a complete metric space, f : [0, ) 2 R be a function of C-class, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T is a cyclic µ, ψ, φ)-f-weakly contraction. Then T has a fixed point z n i=1a i. Proof. We can construct a sequence x n+1 = T x n with n N 0. If there exists n 0 N such that x n0 +1 = x n0, hence the result. Indeed, we can see that T x n0 = x n0 +1 = x n0. Now, we assume that x n+1 x n for any n N 0. As X = m i=1a i, for any n > 0, there exists i n {1, 2,..., m} such that x n 1 A in and x n A in+1. Since T is a cyclic µ, ψ, φ)-weakly contraction, we have µ dx n+1, x n ) ) = µ dt x n, T x n 1 ) ) f ψ µ dx n, x n 1 ) ), µ dx n, T x n ) ), µ dx n 1, T x n 1 ) ), µ dx n, T x n 1 ) ), µ dx n 1, T x n ) )) )), φ Mx n, x n 1 ) = f ψ µ dx n, x n 1 ) ), µ dx n, x n+1 ) ), µ dx n 1, x n ) ), µ dx n, x n ) ), µ dx n 1, x n+1 ) )) )), φ Mx n, x n 1 )

7 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 9 where Mx n, x n 1 ) = max { dx n, x n 1 ), dx n, x n+1 ), dx n 1, x n ) }. If Mx n, x n 1 ) = dx n, x n+1 ), then µ dx n+1, x n ) ) f ψ µ dx n, x n+1 ) ), µ dx n, x n+1 ) ), µ dx n, x n+1 ) ), 0, 2µ dx n, x n+1 ) )) )), φ dx n, x n+1 ) f µ dx n, x n+1 ) ), φ dx n, x n+1 ) )) µ dx n, x n+1 ) ), which is a contradiction. Hence, µ dx n+1, x n ) ) f µ dx n, x n 1 ) ), φ dx n, x n 1 ) )) 3.7) and dx n+1, x n ) dx n, x n 1 ). Thus, {dx n+1, x n )} is a monotone decreasing sequence of non-negative real numbers and hence is convergent. Therefore, there exists r 0 such that dx n+1, x n ) r. Letting n in 3.7) and using the continuity of µ and φ, we obtain that µr) f µr), φr) ). This implies that φr) = 0 and r = 0. Thus, we have dx n+1, x n ) ) To prove that {x n } is a Cauchy sequence, we use Lemma 3.1. Indeed, we show that {x n } is a Cauchy sequence in Y. Fix ɛ > 0. By Lemma 3.1, we can find n 0 N such that r, q n 0 with r q 1 mod m) dx r, x q ) ɛ ) Since lim dx n, x n+1 ) = 0, we can also find n 1 N such that dx n, x n+1 ) ɛ 2m, 3.10) for any n n 1. Assume that r, s max{n 0, n 1 } and s > r. Then there exists k {1, 2,..., m} such that s r k mod m). Hence, s r + t = 1 mod m), for t = m k + 1. So, we have dx r, x s ) dx r, x s+j ) + dx s+j, x s+j 1 ) dx s+1, x s ). 3.11)

8 10 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. Using 3.9), 3.10) and 3.11), we obtain dx r, x s ) ɛ 2 + j ɛ 2m ɛ 2 + m ɛ = ɛ. 3.12) 2m Therefore, {x n } is a Cauchy sequence in Y. Since Y is closed in X, then Y is also complete space and there exists x Y such that lim x n = x. Next, we prove that x is a fixed point of T. For this purpose, we have Y = m i=1a i is a cyclic representation of Y with respect to T. So, the sequence {x n } has infinite terms in each A i for i = {1, 2,..., m}. Suppose that x A i, T x A i+1 and take a subsequence {x nk } of {x n } with x nk A i. By using the contractive condition, we obtain that µ dx nk +1, T x) ) = µ dt x nk, T x) ) f ψ µ dx nk, x) ), µ dx nk, T x nk ) ), µ dx, T x) ), µ dx nk, T x) ), µ dx, T x nk ) )), φ Mx nk, x) )) = f ψ µ dx nk, x) ), µ dx nk, x nk +1) ), µ dx, T x) ), µdx nk, T x) ), µ dx, x nk +1) )), φ Mx nk, x) )), where Mx nk, x) = max { dx nk, x), dx nk, x nk +1), dx, T x) }. Letting n and using continuity of µ and φ, we have µ dx, T x) ) f ψ µ0), µ0), µ dx, T x) ), µ dx, T x) ), µ0) ), φ dx, T x) )) f µ dx, T x) ), φ dx, T x) )), which is a contradiction unless dx, T x) = 0. Thus, x is a fixed point of T. For the uniqueness of the fixed point, we suppose that x 1 and x 2 x 1 x 2 ) are two fixed points of T. Using the contractive condition and the continuity of µ and ψ, we get µ dx 1, x 2 ) ) = µ dt x 1, T x 2 ) ) f ψ µ dx 1, x 2 ) ), µ dx 1, T x 1 ) ), µ dx 2, T x 2 ) ), µ dx 1, T x 2 ) ), µ dx 2, T x 1 ) )), φ Mx 1, x 2 ) )) = f ψ µ dx 1, x 2 ) ), µ dx 1, x 1 ) ), µ dx 2, x 2 ) ), µ dx 1, x 2 ) ), µ dx 2, x 1 ) )), φ Mx 1, x 2 ) )) = f ψ µ dx 1, x 2 ) ), µ0), µ0), µ dx 1, x 2 ) ), µ dx 2, x 1 ) )), φ Mx 1, x 2 ) )),

9 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 11 where Mx 1, x 2 ) = max { dx 1, x 2 ), dx 1, x 1 ), dx 2, x 2 ) }. Therefore, we deduce that µ dx 1, x 2 ) ) f ψ µ dx 1, x 2 ) ), 0, 0, µ dx 1, x 2 ) ), µ dx 2, x 1 ) )), φ dx 1, x 2 ) )) f µ dx 1, x 2 ) ), φ dx 1, x 2 ) )) which is a contradiction unless x 1 = x 2. 4 Applications Form Theorem 3.2, we can obtain the following corollaries as natural results. Corollary 4.1. Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T is a cyclic µ, ψ, φ)-rational-weakly contraction. Then T has a fixed point z n i=1a i. Proof. Taking fs, t) = s 1 + t for all s, t [0, ) in Theorem 3.2, we get the desired result. Corollary 4.2. [[7] Theorem 2.2] Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T is a cyclic µ, ψ, φ)-weakly contraction. Then T has a fixed point z n i=1a i. Proof. The proof follows from Theorem 3.2 by taking fs, t) = s t for all s, t [0, ) and we get the desired result. By similar method, we can prove the following consequences. Corollary 4.3. Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T : Y Y is an operator such that

10 12 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. 2. µ dt x, T y) ) ψ µ dx, y) ), µ dx, T x) ), µ dy, T y) ), µ dx, T y) ), µ dy, T x) )) log ) a, a+φ Mx,y) for any x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, µ θ, φ Φ, ψ Ψ and { } Mx, y) = max dx, y), dx, T x), dy, T y). Then T has a fixed point z n i=1a i. Corollary 4.4. Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T : Y Y is an operator such that 2. µ dt x, T y) ) ψ µ dx, y) ), µ dx, T x) ), µ dy, T y) ), µ dx, T y) ), µ dy, T x) )) ψ µ dx, y) ), µ dx, T x) ), µ dy, T y) ), µ dx, T y) ), µ dy, T x) )) 1 + φ Mx, y) ), for any x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, µ θ, φ Φ, ψ Ψ and { } Mx, y) = max dx, y), dx, T x), dy, T y). Then T has a fixed point z n i=1a i. If µa) = a in Corollary 4.2, then we have the following corollary. Corollary 4.5. Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T : Y Y is an operator such that ) ) 2. dt x, T y) ψ dx, y), dx, T x), dy, T y), dx, T y), dy, T x) φ Mx, y),

11 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 13 for any x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, φ Φ, ψ Ψ and { } Mx, y) = max dx, y), dx, T x), dy, T y). Then T has a fixed point z n i=1a i. If µa) = a in Theorem 3.2, then we have the following corollary. Corollary 4.6. Let X, d) be a complete metric space, m N, A 1, A 2,..., A m be non-empty closed subsets of X and Y = m i=1a i. Suppose that T : Y Y is an operator such that ) )) 2. dt x, T y) f ψ dx, y), dx, T x), dy, T y), dx, T y), dy, T x), φ Mx, y) for any x A i, y A i+1, i = 1, 2,..., m where A m+1 = A 1, φ Φ, ψ Ψ and { } Mx, y) = max dx, y), dx, T x), dy, T y). Then T has a fixed point z n i=1a i. Acknowledgement The authors are very thankful to the referee for the valuable comments and suggestions. References [1] Ya. I. Alber and S. Guerre-Delabriere, Principles of weakly contractive maps in Hilbert spaces, New results in Operator Theory, Advances Appl. I. Gohberg and Yu. Lyubich, eds.), Birkhauser, Basel, 8, 1997), [2] D. W. Boyd and T. S. W. Wong, On nonlinear contractions, Proc. Amer. Math. Soc., 20, 1969), [3] S. Chandok, Some common fixed point theorems for generalized nonlinear contractive mappings, Comput. Math. Appl., 62, 2011), [4] S. Chandok, Common fixed points, invariant approximation and generalized weak contractions, Internat. J. Math. Math. Sci., 2012, 2012), Article ID [5] S. Chandok, Common fixed points for generalized nonlinear contractive mappings in metric spaces, Mat. Vesnik, 65, 2013),

12 14 M. Almahalebi and A. H. Ansari and S. ChandokAn. U.V.T. [6] S. Chandok, A fixed point result for weakly Kannan type cyclic contractions, Internat. J. Pure Appl. Math., 82, 2), 2013), [7] S. Chandok and V. Popa, Fixed point theorem for cyclic µ, ψ, φ)-weakly contractions, Vasile Alecsandri University of Bacău, Scientific Studies and Research, Series Mathematics and Informatics, 23, 2), 2013), [8] S. Chandok, Some common fixed point results for generalized weak contractive mappings in partially ordered metric spaces, J. Nonlinear Anal. Optim., 4, 1), 2013), [9] S. Chandok, M. S. Khan, and K. P. R. Rao, Some coupled common fixed point theorems for a pair of mappings satisfying a contractive condition of rational type without monotonicity, Internat. J. Math. Anal., 7, 9), 2013), [10] S. Chandok and M. Postolache, Fixed point theorem for weakly Chatterjea type cyclic contractions, Fixed Point Theory Appl., 2013, 2013), 9 pages. [11] Chi-Ming Chen, Fixed point theorems for ψ-contractive mappings in ordered metric spaces, J. Appl. Math., 2012, 2012), Article ID , 10 pages. [12] E. Karapinar and K. Sadarangani, Fixed point theory for cyclic φ-ψ)- contractions, Fixed Point Theory Appl., 2011: 69, 2011), doi.org/ / [13] E. Karapinar and I. M. Erhan, Best proximity on different type contractions, Appl. Math. Information Sci., 5, 2011), [14] E. Karapinar and I. M. Erhan, Cyclic contractions and fixed point theorems, Filomat, 26, 2012), [15] W. A. Kirk, P. S. Srinivasan, and P. Veeramani, Fixed points for mappings satisfying cyclical contractive conditions, Fixed Point Theory, 4, 1), 2003), [16] M. Pacurar and I. A. Rus, Fixed point theory for cyclic φ-contractions, Nonlinear Anal., 72, 2010), [17] S. Reich, Some fixed point problems, Atti Acad. Naz. Lincei Ren. Cl. Sci. Fis. Mat. Natur., 57, 1975), [18] B. E. Rhoades, Some theorems on weakly contractive maps, Nonlinear Anal., 47, 2001), Muaadh Almahalebi Department of Mathematics, Ibn Tofail University, Kenitra, Morocco. muaadh1979@hotmail.fr Amir Hojat Ansari Department of Mathematics, Karaj Branch, Islamic Azad University, Karaj, Iran. analsisamirmath2@gmail.com

13 Vol. LV 2017) Fixed Point Theorem for Cyclic µ, ψ, φ)-weakly Cont. 15 Sumit Chandok School of Mathematics, Thapar University, Patiala , India. Received: Accepted: Revised: