CUBO, A Mathematical Journal Vol.22, N◦02, (203–214). August 2020 http://dx.doi.org/10.4067/S0719-06462020000200203 Received: 14 February, 2020 | Accepted: 22 June, 2020 Contractive mapping theorems in Partially ordered metric spaces N.Seshagiri Rao1, K.Kalyani2 and Kejal Khatri3 1Department of Applied Mathematics, School of Applied Natural Sciences Adama Science and Technology University Post Box No.1888, Adama, Ethiopia. 2Department of Mathematics Vignan’s Foundation for Science, Technology & Research Vadlamudi-522213, Andhra Pradesh, India. 3Department of Mathematics Government College Simalwara Dungarpur 314403, Rajasthan, India. seshu.namana@gmail.com, kalyani.namana@gmail.com, kejal0909@gmail.com ABSTRACT The purpose of this paper is to establish some coincidence, common fixed point the- orems for monotone f-non decreasing self mappings satisfying certain rational type contraction in the context of a metric spaces endowed with partial order. Also, the re- sults involving an integral type of such classes of mappings are discussed in application point of view. These results generalize and extend well known existing results in the literature. RESUMEN El propósito de este art́ıculo es establecer teoremas de coincidencia y de punto fijo común para auto mapeos monótonos f-no decrecientes satisfaciendo ciertas contrac- ciones de tipo racional en el contexto de espacios métricos dotados de un orden parcial. Adicionalmente, resultados que involucran clases de mapeos de tipo integral son dis- cutidos desde un punto de vista de las aplicaciones. Estos resultados generalizan y extienden resultados bien conocidos, existentes en la literatura. Keywords and Phrases: Partially ordered metric spaces; Rational contractions; Compatible mappings; Weakly compatible mappings. 2020 AMS Mathematics Subject Classification: 47H10; 54H25; 26A42; 46T99. c©2020 by the author. This open access article is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. http://dx.doi.org/10.4067/S0719-06462020000200203 204 N.Seshagiri Rao, K.Kalyani & Kejal Khatri CUBO 22, 2 (2020) 1 Introduction Ever since in Fixed point theory and Approximation theory, the classical Banach contraction principle plays a vital role to obtain an unique solution of the results. Of course, it is very important and popular tool in different fields of mathematics to solve the existing problems in nonlinear analysis. Since then a lot of variety of generalizations of this Banach contraction principle [1] have been taken place in a metric fixed point theory by improving the underlying contraction condition [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Thereafter vigorous research work has been obtained by weakening its hypotheses on various spaces such as rectangular metric spaces, pseudo metric spaces, fuzzy metric spaces, quasi metric spaces, quasi semi-metric spaces, probabilistic metric spaces, D-metric spaces, G-metric spaces, F-metric spaces, cone metric spaces, and so on to prove the existing results. Prominent work on the existence and uniqueness of a fixed point and common fixed point theorems involving monotone mappings on cone metric spaces, partially ordered metric spaces and others spaces [12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30] generate natural interest to establish usable fixed point results. The aim of this paper is to prove some coincidence, common fixed point results in the frame work of partially ordered metric spaces for a pair of self-mappings satisfying a generalized contrac- tive condition of rational type. These results generalize and extend the results of Harjani et al.[19] and Chandok [28] in ordered metric space. Also the applications of these results are presented on taking integral type contractions in the same space. 2 Preliminaries The following definitions are frequently used in results given in upcoming sections. Definition 1. The triple (X, d, �) is called a partially ordered metric space, if (X, �) is a partially ordered set together with (X, d) is a metric space. Definition 2. If (X, d) is a complete metric space, then the triple (X, d, �) is called a partially ordered complete metric space. Definition 3. Let (X, �) be a partially ordered set. A self-mapping f : X → X is said to be strictly increasing, if f(x) ≺ f(y), for all x, y ∈ X with x ≺ y and is also said to be strictly decreasing, if f(x) ≻ f(y), for all x, y ∈ X with x ≺ y. Definition 4. A point x ∈ A, where A is a non-empty subset of a metric space (X, d) is called a common fixed (coincidence) point of two self-mappings f and T if fx = T x = x(fx = T x). Definition 5. The two self-mappings f and T defined over a subset A of a metric space (X, d) are called commuting if fT x = T fx for all x ∈ A. CUBO 22, 2 (2020) Contractive mapping theorems in Partially ordered metric spaces 205 Definition 6. Two self-mappings f and T defined over A ⊂ X are compatible, if for any sequence {xn} with lim n→+∞ fxn = lim n→+∞ T xn = µ, for some µ ∈ A then lim n→+∞ d(T fxn, fT xn) = 0. Definition 7. Two self-mappings f and T defined over A ⊂ X are said to be weakly compatible, if they commute at their coincidence points. i.e., if fx = T x then fT x = T fx. Definition 8. Let f and T be two self-mappings defined over a partially ordered set (X, �). A mapping T is called a monotone f non-decreasing if fx � fy implies T x � T y, for all x, y ∈ X. Definition 9. Let A be a non-empty subset of a partially ordered set (X, �). If very two elements of A are comparable then it is called well ordered set. Definition 10. A partially ordered metric space (X, d, �) is called an ordered complete, if for each convergent sequence {xn} +∞ n=0 ⊂ X, one of the following condition holds • if {xn} is a nondecreasing sequence in X such that xn → x implies xn � x, for all n ∈ N that is, x = sup{xn} or • if {xn} is a nonincreasing sequence in X such that xn → x implies x � xn, for all n ∈ N that is, x = inf{xn}. 3 Main Results In this section, we prove some coincidence, common fixed point theorems in the context of ordered metric space. Theorem 1. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition d(T x, T y) ≤ α d(fx, T x) d(fy, T y) d(fx, fy) + β [d(fx, T x) + d(fy, T y)] + γd(fx, fy) (3.1) for all x, y in X with f(x) 6= f(y) are comparable, where α, β, γ ∈ [0, 1) with 0 ≤ α + 2β + γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Proof. Let x0 ∈ X such that f(x0) � T (x0). Since from hypotheses, we have T (X) ⊆ f(X) then, we can choose a point x1 ∈ X such that fx1 = T x0. But T x1 ∈ f(X) then, again there exists another point x2 ∈ X such that fx2 = T x1. By continuing the same way, we can construct a sequence {xn} in X such that fxn+1 = T xn, for all n. 206 N.Seshagiri Rao, K.Kalyani & Kejal Khatri CUBO 22, 2 (2020) Again, by hypotheses, we have f(x0) � T (x0) = f(x1) and T is a monotone f-nondecreasing mapping then, we get T (x0) � T (x1). Similarly, we obtain T (x1) � T (x2), since f(x1) � f(x2) and then by continuing the same procedure, we obtain that T (x0) � T (x1) � T (x2) � .........T (xn) � T (xn+1) � ........ The equality T (xn+1) = T (xn) is impossible because f(xn+2) 6= f(xn+1) for all n ∈ N. Thus d(T (xn), T (xn+1)) > 0 for all n ≥ 0 therefore, from contraction condition (3.1), we have d(T xn+1, T xn) ≤ α d(fxn+1, T xn+1) d(fxn, T xn) d(fxn+1, fxn) + β [d(fxn+1, T xn+1) + d(fxn, T xn)] +γd(fxn+1, fxn) which intern implies that d(T xn+1, T xn) ≤ αd(T xn, T xn+1) + β [d(T xn, T xn+1) + d(T xn−1, T xn)] + γd(T xn, T xn−1) Finally, we arrive at d(T xn+1, T xn) ≤ ( β + γ 1 − α − β ) d(T xn, T xn−1) Continuing the same process up to (n − 1) times, we get d(T xn+1, T xn) ≤ ( β + γ 1 − α − β )n d(T x1, T x0) Let k = β+γ 1−α−β ∈ [0, 1), then from triangular inequality for m ≥ n, we have d(T xm, T xn) ≤ d(T xm, T xm−1) + d(T xm−1, T xm−2) + ........ + d(T xn+1, T xn) ≤ ( km−1 + km−2 + .......... + kn ) d(T x1, T x0) ≤ kn 1 − k d(T x1, T x0) as m, n → +∞, d(T xm, T xn) → 0, which shows that the sequence {T xn} is a Cauchy sequence in X. So, by the completeness of X, there exists a point µ ∈ X such that T xn → µ as n → +∞. Again, by the continuity of T , we have lim n→+∞ T (T xn) = T ( lim n→+∞ T xn ) = T µ. But fxn+1 = T xn, then fxn+1 → µ as n → +∞ and from the compatibility for T and f, we have lim n→+∞ d(T (fxn), f(T xn)) = 0. Further by triangular inequality, we have d(T µ, fµ) = d(T µ, T (fxn)) + d(T (fxn), f(T xn)) + d(f(T xn), fµ) CUBO 22, 2 (2020) Contractive mapping theorems in Partially ordered metric spaces 207 On taking limit as n → +∞ in both sides of the above equation and using the fact that T and f are continuous then, we get d(T µ, fµ) = 0. Thus, T µ = fµ. Hence, µ is a coincidence point of T and f in X. Corollary 1. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition d(T x, T y) ≤ α d(fx, T x) d(fy, T y) d(fx, fy) + β [d(fx, T x) + d(fy, T y)] for all x, y in X with f(x) 6= f(y) are comparable and for some α, β ∈ [0, 1) with 0 ≤ α + 2β < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Proof. Set γ = 0 in Theorem 1. Corollary 2. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition d(T x, T y) ≤ β [d(fx, T x) + d(fy, T y)] + γd(fx, fy) for all x, y in X with f(x) 6= f(y) are comparable and for some β, γ ∈ [0, 1) with 0 ≤ 2β + γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Proof. The proof can be obtained by setting α = 0 in Theorem 1. We may remove the continuity criteria of T in Theorem 1 is still valid by assuming the following hypothesis in X: If {xn} is a nondecreasing sequence in X such that xn → x, then xn � x for all n ∈ N. Theorem 2. Let (X, d, �) be a complete partially ordered metric space. Suppose that f and T are self-mappings on X, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying d(T x, T y) ≤ α d(fx, T x) d(fy, T y) d(fx, fy) + β [d(fx, T x) + d(fy, T y)] + γd(fx, fy) (3.2) for all x, y in X with f(x) 6= f(y) are comparable and for some α, β, γ ∈ [0, 1) with 0 ≤ α+2β+γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and {xn} is a nondecreasing sequence in X such that xn → x, then xn � x for all n ∈ N. If f(X) is a complete subset of X, then T and f have a coincidence point in X. Further, if T and f are weakly compatible, then T and f have a common fixed point in X. Moreover, the set 208 N.Seshagiri Rao, K.Kalyani & Kejal Khatri CUBO 22, 2 (2020) of common fixed points of T and f is well ordered if and only if T and f have one and only one common fixed point in X. Proof. Suppose f(X) is a complete subset of X. As we know from the proof of Theorem 1, the sequence {T xn} is a Cauchy sequence and hence {fxn} is also a Cauchy sequence in (f(X), d) as fxn+1 = T xn and T (X) ⊆ f(X). Since f(X) is complete then there exists some fu ∈ f(X) such that lim n→+∞ T (xn) = lim n→+∞ f(xn) = f(u). Also note that the sequences {T xn} and {fxn} are nondecreasing and from hypotheses, we have T (xn) � f(u) and f(xn) � f(u) for all n ∈ N. But T is a monotone f-nondecreasing then, we get T (xn) � T (µ) for all n. Letting n → +∞, we obtain that f(u) � T (u). Suppose that f(u) ≺ T (u) then define a sequence {un} by u0 = u and fun+1 = T un for all n ∈ N. An argument similar to that in the proof of Theorem 1 yields that {fun} is a nondecreasing sequence and lim n→+∞ f(un) = lim n→+∞ T (un) = f(v) for some v ∈ X. So from hypotheses, it is clear that sup f(un) � f(v) and sup T (un) � f(v), for all n ∈ N. Notice that f(xn) � f(u) � f(u1) � ........ � f(un) � .... � f(v). Case:1 Suppose if there exists some n0 ≥ 1 such that f(xn0) = f(un0) then, we have f(xn0) = f(u) = f(un0) = f(u1) = T (u). Hence, u is a coincidence point of T and f in X. Case:2 Suppose that f(xn0) 6= f(un0) for all n then, from (3.2), we have d(fxn+1, fun+1) =d(T xn, T un) ≤α d(fxn, T xn) d(fun, T un) d(fxn, fun) + β [d(fxn, T xn) + d(fun, T un)] +γd(fxn, fun) Taking limit as n → +∞ on both sides of the above inequality, we get d(fu, fv) ≤γ d(fu, fv) < d(fu, fv), since γ < 1. Thus, we have f(u) = f(v) = f(u1) = T (u). Hence, we conclude that u is a coincidence point of T and f in X. CUBO 22, 2 (2020) Contractive mapping theorems in Partially ordered metric spaces 209 Now, suppose that T and f are weakly compatible. Let w be a coincidence point then, T (w) = T (f(z)) = f(T (z)) = f(w), since w = T (z) = f(z), for some z ∈ X. Now by contraction condition, we have d(T (z), T (w)) ≤α d(fz, T z) d(fw, T w) d(fz, fw) + β [d(fz, T z) + d(fw, T w)] + γd(fz, fw) ≤γ d(T (z), T (w)) as γ < 1, then d(T (z), T (w)) = 0. Therefore, T (z) = T (w) = f(w) = w. Hence, w is a common fixed point of T and f in X. Now suppose that the set of common fixed points of T and f is well ordered, we have to show that the common fixed point of T and f is unique. Let u and v be two common fixed points of T and f such that u 6= v then from (3.2), we have d(u, v) ≤α d(fu, T u) d(fv, T v) d(fu, fv) + β [d(fu, T u) + d(fv, T v)] + γd(fu, fv) ≤γ d(u, v) < d(u, v), since γ < 1, which is a contradiction. Thus, u = v. Conversely, suppose T and f have only one common fixed point then the set of common fixed points of T and f being a singleton is well ordered. This completes the proof. Corollary 3. Let (X, d, �) be a complete partially ordered metric space. Suppose that f and T are self-mappings on X, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying d(T x, T y) ≤ α d(fx, T x) d(fy, T y) d(fx, fy) + β [d(fx, T x) + d(fy, T y)] for all x, y in X with f(x) 6= f(y) are comparable and for some α, β ∈ [0, 1) with 0 ≤ α + 2β < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and {xn} is a nondecreasing sequence in X such that xn → x, then xn � x for all n ∈ N. If f(X) is a complete subset of X, then T and f have a coincidence point in X. Further, if T and f are weakly compatible, then T and f have a common fixed point in X. Moreover, the set of common fixed points of T and f is well ordered if and only if T and f have one and only one common fixed point in X. Proof. Set γ = 0 in Theorem 2. Corollary 4. Let (X, d, �) be a complete partially ordered metric space. Suppose that f and T are self-mappings on X, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying d(T x, T y) ≤β [d(fx, T x) + d(fy, T y)] + γd(fx, fy) 210 N.Seshagiri Rao, K.Kalyani & Kejal Khatri CUBO 22, 2 (2020) for all x, y in X for which f(x) 6= f(y) are comparable and for some β, γ ∈ [0, 1) with 0 ≤ 2β +γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and {xn} is a nondecreasing sequence in X such that xn → x, then xn � x for all n ∈ N. If f(X) is a complete subset of X, then T and f have a coincidence point in X. Further, if T and f are weakly compatible, then T and f have a common fixed point in X. Moreover, the set of common fixed points of T and f is well ordered if and only if T and f have one and only one common fixed point in X. Proof. Set α = 0 in Theorem 2. Remark 1. (i). If β = 0, in Theorem 1 and Theorem 2, we obtain Theorem 2.1 and Theorem 2.3 of Chandok [28]. (ii). If f = I and β = 0, in Theorem 1 and Theorem 2, then we get Theorem 2.1 and Theorem 2.3 of Harjani et al. [19]. 4 Applications In this section, we state some applications of the main result for a self mapping involving the integral type contractions. Let us denote τ, a set of all functions ϕ defined on [0, +∞) satisfying the following conditions: (1) each ϕ is Lebesgue integrable mapping on each compact subset of [0, +∞) and (2) for any ǫ > 0, we have ∫ ǫ 0 ϕ(t)dt > 0. Theorem 3. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition ∫ d(T x,T y) 0 ϕ(t)dt ≤ α ∫ d(fx,T x) d(fy,T y) d(fx,fy) 0 ϕ(t)dt + β ∫ d(fx,T x)+d(fy,T y) 0 ϕ(t)dt + γ ∫ d(fx,fy) 0 ϕ(t)dt (4.1) for all x, y in X with f(x) 6= f(y) are comparable, ϕ(t) ∈ τ and for some α, β, γ ∈ [0, 1) such that 0 ≤ α + 2β + γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Similarly, we can obtain the following results in complete partially ordered metric space, by putting γ = 0 and α = 0 in an integral contraction of Theorem 3. CUBO 22, 2 (2020) Contractive mapping theorems in Partially ordered metric spaces 211 Theorem 4. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition ∫ d(T x,T y) 0 ϕ(t)dt ≤ α ∫ d(fx,T x) d(fy,T y) d(fx,fy) 0 ϕ(t)dt + β ∫ d(fx,T x)+d(fy,T y) 0 ϕ(t)dt (4.2) for all x, y in X with f(x) 6= f(y) are comparable, ϕ(t) ∈ τ and where α, β ∈ [0, 1) such that 0 ≤ α + 2β < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Theorem 5. Let (X, d, �) be a complete partially ordered metric space. Suppose that the self- mappings f and T on X are continuous, T is a monotone f-nondecreasing, T (X) ⊆ f(X) and satisfying the following condition ∫ d(T x,T y) 0 ϕ(t)dt ≤ β ∫ d(fx,T x)+d(fy,T y) 0 ϕ(t)dt + γ ∫ d(fx,fy) 0 ϕ(t)dt (4.3) for all x, y in X with f(x) 6= f(y) are comparable, ϕ(t) ∈ τ and for some β, γ ∈ [0, 1) such that 0 ≤ 2β + γ < 1. If there exists a point x0 ∈ X such that f(x0) � T (x0) and the mappings T and f are compatible, then T and f have a coincidence point in X. Corollary 5. By replacing β = 0 in Theorem 3, we obtain the Corollary 2.5 of Chandok [28]. We illustrate the usefulness of the obtained results for the existence of the coincidence point in the space. Example 1. Define a metric d : X × X → [0, +∞) by d(x, y) = |x − y|, where X = [0, 1] with usual order ≤. Suppose that T and f be two self mappings on X such that T x = x 2 2 and fx = 2x 2 1+x , then T and f have a coincidence in point X. Proof. By definition of a metric d, it is clear that (X, d) is a complete metric space. Obviously, (X, d, ≤) is a partially ordered complete metric space with usual order. Let x0 = 0 ∈ X then f(x0) ≤ T (x0) and also by definition; T , f are continuous, T is a monotone f- nondecreasing and T (X) ⊆ f(X). Now for any distinct x, y in X, we have d(T x, T y) = 1 2 |x2 − y2| = 1 2 (x + y)|x − y| < α 4 x2y2 (x + y + xy) |x − 3||y − 3| |x − y| + β 2 x2(1 + y)|x − 3| + y2(1 + x)|y − 3| (1 + x)(1 + y) + γ 2(x + y + xy) (1 + x)(1 + y) |x − y| 212 N.Seshagiri Rao, K.Kalyani & Kejal Khatri CUBO 22, 2 (2020) < α x 2|x−3| 2(1+x) . y 2|y−3| 2(1+y) 2|x − y| x+y+xy (1+x)(1+y) + β [ x2|x − 3| 2(1 + x) + y2|y − 3| 2(1 + y) ] + γ 2(x + y + xy) (1 + x)(1 + y) |x − y| < α d(fx, T x) d(fy, T y) d(fx, fy) + β [d(fx, T x) + d(fy, T y)] + γd(fx, fy) Then, the contraction condition in Theorem 1 holds by selecting proper values of α, β, γ in [0, 1) such that 0 ≤ α + 2β + γ < 1. Therefore T , f have a coincidence point 0 ∈ X. Similarly the following is one more example of main Theorem 1. Example 2. A distance function d : X × X → [0, +∞) by d(x, y) = |x − y|, where X = [0, 1] with usual order ≤. 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