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Common fixed point and invariant approximation in hyperbolic ordered metric spaces
Fixed Point Theory and Applications volume 2011, Article number: 25 (2011)
Abstract
We prove a common fixed point theorem for four mappings defined on an ordered metric space and apply it to find new common fixed point results. The existence of common fixed points is established for two or three noncommuting mappings where T is either ordered Scontraction or ordered asymptotically Snonexpansive on a nonempty ordered starshaped subset of a hyperbolic ordered metric space. As applications, related invariant approximation results are derived. Our results unify, generalize, and complement various known comparable results from the current literature.
2010 Mathematics Subject Classification:
47H09, 47H10, 47H19, 54H25.
1 Introduction
Metric fixed point theory has primary applications in functional analysis. The interplay between geometry of Banach spaces and fixed point theory has been very strong and fruitful. In particular, geometric conditions on underlying spaces play a crucial role for finding solution of metric fixed point problems. Although, it has purely metric flavor, it is still a major branch of nonlinear functional analysis with close ties to Banach space geometry, see for example [1–4] and references mentioned therein. Several results regarding existence and approximation of a fixed point of a mapping rely on convexity hypotheses and geometric properties of the Banach spaces. Recently, Khamsi and Khan [5] studied some inequalities in hyperbolic metric spaces, which lay foundation for a new mathematical field: the application of geometric theory of Banach spaces to fixed point theory. Meinardus [6] was the first to employ fixed point theorem to prove the existence of invariant approximation in Banach spaces. Subsequently, several interesting and valuable results have appeared about invariant approximations [7–9].
Existence of fixed points in ordered metric spaces was first investigated in 2004 by Ran and Reurings [10], and then by Nieto and Lopez [11].
In 2009, Dorić [12] proved some fixed point theorems for generalized (ψ, φ)weakly contractive mappings in ordered metric spaces. Recently, Radenović and Kadelburg [13] presented a result for generalized weak contractive mappings in ordered metric spaces (see also, [14, 15] and references mentioned theirin). Several authors studied the problem of existence and uniqueness of a fixed point for mappings satisfying different contractive conditions (e.g., [16–18, 13, 19]). The aim of this article is to study common fixed points of (i) four mappings on an ordered metric space (ii) ordered C_{ q }commuting mappings in the frame work of hyperbolic ordered metric spaces. Some results on invariant approximation for these mappings are also established which in turn extend and strengthen various known results.
2 Preliminaries
Let (X, d) be a metric space. A path joining x ∈ X to y ∈ X is a map c from a closed interval [0, l] ⊂ ℝ to X such that c(0) = x, c(l) = y, and d(c(t), c(t')) = t  t' for all t, t' ∈ [0, l]. In particular, c is an isometry and d(x, y) = l. The image of c is called a metric segment joining x and y. When it is unique the metric segment is denoted by [x, y]. We shall denote by (1  λ)x ⊕ λy the unique point z of [x, y] which satisfies
Such metric spaces are usually called convex metric spaces (see Takahashi [20] and Khan at el. [21]). Moreover, if we have for all p, x, y in X
then X is called a hyperbolic metric space. It is easy to check that in this case for all x, y, z, w in X and λ ∈ [0, 1]
Obviously, normed linear spaces are hyperbolic spaces [5]. As nonlinear examples one can consider Hadamard manifolds [2], the Hilbert open unit ball equipped with the hyperbolic metric [3] and CAT(0) spaces [4].
Let X be a hyperbolic ordered metric space. Throughout this article, we assume that (1  λ)x ⊕ λy ≤ (1  λ)z ⊕ λw for all x, y, z, w in X with x ≤ z and y ≤ w. A subset Y of X is said to be ordered convex if Y includes every metric segment joining any two of its comparable points. The set Y is said to be an ordered qstarshaped if there exists q in Y such that Y includes every metric segment joining any of its point comparable with q.
Let Y be an ordered qstarshaped subset of X and f, g : Y → Y. Put,
Set, for each x in X comparable with q in Y, .
Definition 2.1. A selfmap f on an ordered convex subset Y of a hyperbolic ordered metric space X is said to be affine if
for all comparable elements x, y ∈ Y , and λ ∈ [0, 1].
Let f and g be two selfmaps on X. A point x ∈ X is called (1) a fixed point of f if f(x) = x; (2) coincidence point of a pair (f, g) if fx = gx; (3) common fixed point of a pair (f, g) if x = fx = gx. If w = fx = gx for some x in X, then w is called a point of coincidence of f and g. A pair (f, g) is said to be weakly compatible if f and g commute at their coincidence points.
We denote the set of fixed points of f by Fix(f).
Definition 2.2. Let (X, ≤) be an ordered set. A pair (f, g) on X is said:

(i)
weakly increasing if for all x ∈ X, we have fx ≤ gfx and gx ≤ fgx, ([22])

(ii)
partially weakly increasing if fx ≤ gfx, for all x ∈ X.
Remark 2.3. A pair (f, g) is weakly increasing if and only if ordered pair (f, g) and (g, f) are partially weakly increasing.
Example 2.4. Let X = [0, 1] be endowed with usual ordering. Let f, g : X → X be defined by fx = x^{2} and . Then fx = x^{2} ≤ x = gfx for all x ∈ X. Thus (f, g) is partially weakly increasing. But for x ∈ (0, 1). So (g, f) is not partially weakly increasing.
Definition 2.5. Let (X, ≤) be an ordered set. A mapping f is called weak annihilator of g if fgx ≤ x for all x ∈ X.
Example 2.6. Let X = [0, 1] be endowed with usual ordering. Define f, g : X → X by fx = x^{2} and gx = x^{3}. Then fgx = x^{6} ≤ x for all x ∈ X. Thus f is a weak annihilator of g.
Definition 2.7. Let (X, ≤) be an ordered set. A selfmap f on X is called dominating map if x ≤ fx for each x in X.
Example 2.8. Let X = [0, 1] be endowed with usual ordering. Let f : X → X be defined by . Then for all x ∈ X. Thus f is a dominating map.
Example 2.9. Let X = [0, ∞) be endowed with usual ordering. Define f : X → X by
n ∈ N. Then for all x ∈ X, x ≤ fx so that f is a dominating map.
Definition 2.10. Let (X, ≤) be a ordered set and f and g be selfmaps on X. Then the pair (f, g) is said to be order limit preserving if
for all sequences {x_{ n }} in X with gx_{ n } ≤ fx_{ n } and x_{ n } → x_{0}.
Definition 2.11. Let X be a hyperbolic ordered metric space, Y an ordered qstarshaped subset of X, f and g be selfmaps on X and q ∈ Fix(g). Then f is said to be:

(1)
ordered gcontraction if there exists k ∈ (0, 1) such that
for x, y ∈ Y with x ≤ y.

(2)
ordered asymptotically Snonexpansive if there exists a sequence {k_{ n }}, k_{ n } ≥ 1, with such that
for each x, y in Y with x ≤ y and each n ∈ N. If k_{ n } = 1, for all n ∈ N , then f is known as ordered gnonexpansive mapping. If g = I (identity map), then f is ordered asymptotically nonexpansive mapping;

(3)
Rweakly commuting if there exists a real number R > 0 such that
for all x in Y.

(4)
ordered Rsubweakly commuting [23] if there exists a real number R > 0 such that
for all x ∈ Y.

(5)
ordered uniformly Rsubweakly commuting [23] if there exists a real number R > 0 such that
for all x ∈ Y.

(6)
ordered C_{ q }commuting [24], if gfx = fgx for all x ∈ C_{ q }(f, g), where C_{ q }(f, g) = U {C(g, fk) : 0 ≤ k ≤ 1} and f_{ k }x = (1  k)q ⊕ kfx.

(7)
ordered uniformly C_{ q }commuting, if gf ^{n}x = f ^{n}gx for all x ∈ C_{ q }(g, f ^{n}) and n ∈ N.

(8)
uniformly asymptotically regular on Y if, for each η > 0, there exists N(η) = N such that d(f ^{n}x, f ^{n+1}x) < η for all η ≥ N and all x ∈ Y .
For other related notions of noncommuting maps, we refer to [7]; in particular, here Example 2.2 and Remark 3.10(2) provide two maps which are not C_{ q }commuting. Also, uniformly C_{ q }commuting maps on X are C_{ q }commuting and uniformly Rsubweakly commuting maps are uniformly C_{ q }commuting but the converse statements do not hold, in general [23, 25]. Fixed point theorems in a hyperconvex metric space (an example of a convex metric space) have been established by Khamsi [26] and Park [27].
Let Y be a closed subset of an ordered metric space X. Let x ∈ X. Define d(x, Y ) = inf{d(x, y) : y ∈ Y, y ≤ x or x ≤ y}. If there exists an element y_{0} in Y comparable with x such that d(x, y_{0}) = d(x, Y ), then y_{0} is called an ordered best approximation to X out of Y. We denote by P_{ Y } (x), the set of all ordered best approximation to x out of Y. The reader interested in the interplay of fixed points and approximation theory in normed spaces is referred to the pioneer work of Park [28] and Singh [9].
3 Common fixed point in ordered metric spaces
We begin with a common fixed point theorem for two pairs of partially weakly increasing functions on an ordered metric space. It may regarded as the main result of this article.
Theorem 3.1. Let (X, ≤, d) be an ordered metric space. Let f, g, S, and T be selfmaps on X, (T, f) and (S, g) be partially weakly increasing with f(X) ⊆ T(X), g(X) ⊆ S(X), and dominating maps f and g be weak annihilator of T and S, respectively. Also, for every two comparable elements x, y ∈ X,
where
for h ∈ [0, 1) is satisfied. If one of f(X), g(X), S(X), or T(X) is complete subspace of X, then {f, S} and {g, T} have unique point of coincidence in X provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u. Moreover, if {f, S} and {g, T } are weakly compatible, then f, g, S, and T have a common fixed point.
Proof. For any arbitrary point x_{0} in X, construct sequences {x_{ n }} and {y_{ n }} in X such that
Since dominating maps f and g are weak annihilator of T and S, respectively so for all n ≥ 1,
and
Thus, we have x_{ n } ≤ x_{n+1}for all n ≥ 1. Now (3.1) gives that.
for n = 1, 2, 3,..., where
Now if M(x_{2n}, x_{2n+1}) = d(y_{2n}, y_{2n+1}), then d(y_{2n+1}, y_{2n+2}) ≤ h d(y_{2n}, y_{2n+1}).
And if M(x_{2n}, x_{2n+1}) = d(y_{2n+1}, y_{2n+2}), then d(y_{2n+1}, y_{2n+2}) ≤ h d(y_{2n+1}, y_{2n+2})
which implies that d(y_{2n+1}, y_{2n+2}) = 0, and y_{2n+1}= y_{2n+2}. Hence
Therefore
for all n ∈ ℕ. Then, for m > n,
and so d(y_{ n }, y_{ m }) → 0 as n, m → ∞. Hence {y_{ n }} is a Cauchy sequence. Suppose that S(X) is complete. Then there exists u in S(X), such that Sx_{2n}= y_{2n}→ u as n → ∞. Consequently, we can find v in X such that Sv = u. Now we claim that fv = u. Since, x_{2n2}≤ x_{2n1}≤ gx_{2n1}= Sx_{2n}and Sx_{2n}→ Sv. So that x_{2n1}≤ Sv and since, Sv ≤ gSv and gSv ≤ v, implies x_{2n1}≤ v. Consider
where
for all n ∈ ℕ. Now we have four cases:
If M(v, x_{2n1}) = d(Sv, Tx_{2n1}), then d(fv, u) ≤ h d(Sv, Tx_{2n1})+d(gx_{2n1,}u) → 0 as n → ∞ implies that fv = u.
If M(v, x_{2n1}) = d(fv, Sv), then d(fv, u) ≤ h d(fv, Sv) + d(gx_{2n1,}u). Taking limit as n → ∞ we get d(fv, u) ≤ h d(fv, u). Since h < 1, so that fv = u.
If M(v, x_{2n1}) = d(gx_{2n1,}Tx_{2n1}), then d(fv, u) ≤ h d(gx_{2n1,}Tx_{2n1}) + d(gx_{2n1,}u) → 0 as n → ∞ implies that fv = u.
If , then
Taking limit as n → ∞ we get . Since h < 1, so that fv = u. Therefore, in all the cases fv = Sv = u.
Since u ∈ f(X) ⊂ T(X), there exists w ∈ X such that Tw = u. Now we shall show that gw = u. As, x_{2n1}≤ x_{2n}≤ fx_{2n}= Tx_{2n+1}and Tx_{2n+1}→ Tw and so x_{2n}≤ Tw. Hence, Tw ≤ fTw and fTw ≤ w, imply x_{2n}≤ w. Consider
where
for all n ∈ ℕ.
Again we have four cases:
If M(x_{2n,}w) = d(Sx_{2n,}Tw), then d(gw, u) ≤ h d(Sx_{2n,}Tw) + d(fx_{2n,}u) → 0 as n → ∞.
If M(x_{2n},w) = d(fx_{2n,}Sx_{2n}), then d(gw, u) ≤ h d(fx_{2n,}Sx_{2n}) + d(fx_{2n,}u) → 0 as n → ∞.
If M(x_{2n,}w) = d(gw, Tw), then d(gw, u) ≤ h d(gw, Tw)+d(fx_{2n,}u) = h d(gw, u)+ d(fx_{2n,}u). Taking limit as n → ∞ we get d(gw, u) ≤ h d(gw, u) which implies that gw = u. If , then
Taking limit as n → ∞ we get which implies that gw = u. Following the arguments similar to those given above, we obtain gw = Tw = u. Thus {f, S} and {g, T} have a unique point of coincidence in X. Now, if {f, S} and {g, T} are weakly compatible, then fu = fSv = Sfv = Su = w_{1} (say) and gu = gTw = Tgw = Tu = w 2 (say). Now
where
Therefore d(w_{1}, w_{2}) ≤ h d(w_{1}, w_{2}) gives w_{1} = w_{2}. Hence
That is, u is a coincidence point of f, g, S,, and T. Now we shall show that u = gu. Since, v ≤ fv = u,
where
Thus, d(u, gu) ≤ h d(u, gu) implies that gu = u. In similar way, we obtain fu = u. Hence, u is a common fixed point of f, g, S, and T.
In the following result, we establish existence of a common fixed point for a pair of partially weakly increasing functions on an ordered metric space by using a control function r : R^{+} → R^{+}.
Theorem 3.2. Let (X, ≤, d) be an ordered metric space. Let f and g be Rweakly commuting selfmaps on X, (g, f) be partially weakly increasing with f(X) ⊆ g(X), dominating map f is weak annihilator of g. Suppose that for every two comparable elements x, y ∈ X,
where r : R^{+} → R^{+} is a continuous function such that r(t) < t for each t > 0. If either f or g is continuous and one of f(X) or g(X) is complete subspace of X, then f and g have a common fixed point provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. Let x_{0} be an arbitrary point in X. Choose a point x_{1} in X such that
Since dominating map f is weak annihilator of g, so that for all n ≥ 1,
Thus, we have x_{ n } ≤ x_{ n }+1 for all n ≥ 1. Now
Thus {d(fx_{ n }, fx_{n+1})} is a decreasing sequence of positive real numbers and, therefore, tends to a limit L. We claim that L = 0. For if L > 0, the inequality
on taking limit as n → ∞ and in the view of continuity of r yields L ≤ r(L) < L, a contradiction. Hence, L = 0.
For a given ε > 0, since r(ε) < ε, there is an integer k_{0} such that
For m, n ∈ N with m > n, we claim that
We prove inequality (3.3) by induction on m. Inequality (3.3) holds for m = n + 1, using inequality (3.2) and the fact that ε  r (ε) < ε. Assume inequality (3.3) holds for m = k. For m = k + 1, we have
By induction on m, we conclude that inequality (3.3) holds for all m ≥ n ≥ k_{0}.
So {fx_{ n }} is a Cauchy sequence. Suppose that g(X) is a complete metric space. Hence {fx_{ n }} has a limit z in g(X). Also gx_{ n } → z as n → ∞.
Let us suppose that the mapping f is continuous. Then ffx_{ n } → fz and fgx_{ n } → fz. Further, since f and g are R  weakly commuting, we have
Taking limit as n → ∞, the above inequality yields gffx_{ n } → fz. We now assert that z = fz. Otherwise, since x_{ n } ≤ fx_{ n }, so we have the inequality
Taking limit as n → ∞ gives d(z, fz) ≤ r(d(z, fz)) < d(z, fz), a contradiction.
Hence, z = fz. As f(X) ⊆ g(X), there exists z_{1} in X such that z = fz = gz_{1}.
Now, since fx_{ n } ≤ ffx_{ n } and ffxn → fz = gz_{1} and gz_{1} ≤ fgz_{1} ≤ z_{1} imply fx_{ n } ≤ z_{1}. Consider,
Taking limit as n → ∞ implies that fz = fz_{1}. This in turn implies that
i.e., z = fz = gz. Thus z is a common fixed point of f and g. The same conclusion is found when g is assumed to be continuous since continuity of g implies continuity of f.
4 Results in hyperbolic ordered metric spaces
In this section, existence of common fixed points of ordered C_{ q }commuting and ordered uniformly C_{ q }commuting mappings is established in hyperbolic ordered metric spaces by utilizing the notions of ordered Scontractions and ordered asymptotically Snonexpansive mappings.
Theorem 4.1. Let Y be a nonempty closed ordered subset of a hyperbolic ordered metric space X. Let T and S be ordered R subweakly commuting selfmaps on Y such that T(Y ) ⊂ S(Y ), cl(T(Y )) is compact, q ∈ Fix(S) and S(Y ) is complete and qstarshaped where each x in X is comparable with q. Let (T, S) be partially weakly increasing, order limit preserving and weakly compatible pair such that dominating map T is weak annihilator of S. If T is continuous, Sordered nonexpansive and S is affine, then Fix(T) ∩ Fix(S) is nonempty provided that for a nondecreasing sequence {x_{ n }} with x_{ n } → u implies that x_{ n } ≤ u.
Proof. Define T_{ n } : Y → Y by
for each n ≥ 1, where λ_{ n } ∈ (0, 1) with . Then T_{ n } is a selfmap on Y for each n ≥ 1. Since S is ordered affine and T(Y ) ⊂ S(Y ), therefor we obtain T_{ n }(Y ) ⊂ S(Y ). Note that,
This implies that the pair {T_{ n }, S} is ordered λ_{ n }Rweakly commuting for each n. Also for any two comparable elements x and y in X, we get
Now following lines of the proof of Theorem 3.2, there exists x_{ n } in Y such that x_{ n } is a common fixed point of S and T_{ n } for each n ≥ 1. Note that
Since cl(T(Y )) is compact, there exists a positive integer M such that
The compactness of cl(T_{ n }(Y )) implies that there exists a subsequence {x_{ k }} of {x_{ n }} such that x_{ k } → x_{0} ∈ Y as k → ∞. Now,
and continuity of T give that x_{0} ∈ Fix(T). Since, T is dominating map, therefore Sx_{ k } ≤ TSx_{ k }. As T is weak annihilator of S and T is dominating, so TSx_{ k } ≤ x_{ k } ≤ Tx_{ k }. Thus Sx_{ k } ≤ Tx_{ k } and order limit preserving property of (T, S) implies that Sx_{0} ≤ Tx_{0} = x_{0}. Also x_{0} ≤ Sx_{0}. Consequently, Sx_{0} = Tx_{0} = x_{0}. Hence the result follows.
Theorem 4.2. Let Y be a nonempty closed subset of a complete hyperbolic ordered metric space X and let T and S be mappings on Y such that T(Y  {u}) ⊂ S(Y  {u}), where u ∈ Fix(S). Suppose that T is an Scontraction and continuous. Let (T, S) be partially weakly increasing, dominating maps T is weak annihilator of S. If T is continuous, and S and T are Rweakly commuting mappings on Y  {u}, then Fix(T)∩Fix(S) is nonempty provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. Similar to the proof of Theorem 3.2.
Theorem 3.1 yields a common fixed point result for a pair of maps on an ordered startshaped subset Y of a hyperbolic ordered metric space as follows.
Theorem 4.3. Let Y be a nonempty closed q starshaped subset of a complete hyperbolic ordered metric space X and let T and S be uniformly C_{ q } commuting selfmapps on Y  {q} such that S(Y ) = Y and T(Y  {q}) ⊂ S(Y  {q}), where q ∈ Fix(S). Let (T, S) be partially weakly increasing, order limit preserving and weakly compatible pair, dominating map T is weak annihilator of S, T is continuous and asymptotically S nonexpansive with sequence {k_{ n }}, as in Definition 2.11 (2), and S is an affine mapping. For each n ≥ 1, define a mapping T_{ n } on Y by T_{ n }x = (1  α_{ n })q ⊕ α_{ n }T ^{n}x, where and {λ_{ n }} is a sequence in (0, 1) with . Then for each n ∈ N, F (T_{ n }) ∩ Fix(S) is nonempty provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. For all x, y ∈ Y, we have
Moreover, since T and S are uniformly C_{ q }commuting and S is affine on Y with Sq = q, for each x ∈ C_{ n }(S, T ) ⊆ C_{ q }(S, T ), we have
Thus S and T_{ n } are weakly compatible for all n. Now, the result follows from Theorem 3.1.
The above theorem leads to the following result.
Theorem 4.4. Let Y be a nonempty closed q starshaped subset of a hyperbolic ordered metric space X and let T and S be selmaps on Y such that S(Y ) = Y and T(Y  {q}) ⊂ S(Y  {q}), q ∈ Fix(S). Let (T, S) be partially weakly increasing, order limit preserving, T is continuous, uniformly asymptotically regular, asymptotically Snonexpansive and S is an affine mapping. If cl(Y  {q}) is compact and S and T are uniformly C_{ q }commuting selfmaps on Y  {q}, then Fix(T) ∩ Fix(S) is nonempty provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ yn for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. By Theorem 4.3, for each n ∈ N, F(T_{ n }) ∩ Fix(S) is singleton in Y. Thus,
Also,
Since T(Y  {q}) is bounded so d(x_{ n }, T ^{n}x_{ n }) → 0 as n → ∞. Note that,
Consequently, d(x_{ n }, Tx_{ n }) → 0, when n → ∞. Since cl(Y  {q}) is compact and Y is closed, therefore there exists a subsequence of {x_{ n }} such that as i → ∞. By the continuity of T , we have T(x_{0}) = x_{0}. Since, T is dominating map, therefore Sx_{ k } ≤ TSx_{ k }. As T is weak annihilator of S and T is dominating, so TSx_{ k } ≤ x_{ k } ≤ Tx_{ k }. Thus, Sx_{ k } ≤ Tx_{ k } and order limit preserving property of (T, S) implies that Sx_{0} ≤ Tx_{0} = x_{0}. Also x_{0} ≤ Sx_{0}. Consequently, Sx_{0} = Tx_{0} = x_{0}. Hence, the result follows.
As another application of Theorem 3.1, we obtain yet an other result for two maps satisfying a very general contractive condition on the set Y.
Theorem 4.5. Let Y be a nonempty qstarshaped complete subset of a hyperbolic ordered metric space and T, f, and g be selfmaps on Y . Suppose that T is continuous, cl(T(Y )) is compact and f and g are affine and continuous and T(Y ) ⊂ f(Y ) ∩g(Y ). Let (T, f) and (T, g) be partially weakly increasing, and dominating maps f and g be weak annihilators of T. If the pairs {T, f} and {T, g} are C_{ q }commuting and satisfy for all x, y ∈ Y,
then T, f, and g have a common fixed point provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. Define T_{ n } : Y → Y by
where λ_{ n } ∈ (0, 1) with . Then T_{ n } is a selfmap on Y for each n ≥ 1. Since f and g are affine and T(Y ) ⊂ f(Y ) ∩ g(Y ), therefore we obtain T_{ n }(Y ) ⊂ f (Y ) ∩ g(Y ). Now f and T are C_{ q }commuting and f is affine on Y with fq = q, for each x ∈ C_{ n }(f, T ) ⊆ C_{ q }(f, T ), so we have
Thus, f and T_{ n } are weakly compatible for all n. Also since g and T are C_{ q }commuting and g is affine on Y with gq = q, therefore, g and T_{ n } are weakly compatible for all n. Moreover using (4.1) we have
By Theorem 3.1, for each n ≥ 1, there exists x_{ n } in Y such that x_{ n } is a common fixed point of f, g and T_{ n }. The compactness of cl(T (Y )) implies that there exists a subsequence {Tx_{ k }} of {Tx_{ n }} such that Tx_{ k } → y as k → ∞. Now, the definition of T_{ k }x_{ k } gives that x_{ k } → y and the result follows using continuity of T, f, and g.
5 Invariant approximation
In this section, we obtain results on best approximation as a fixed point of Rsubweakly and uniformly Rsubweakly commuting mappings in the setting of hyperbolic ordered metric spaces. In particular, as an application of Theorem 4.4 (respectively Theorem 4.5), we demonstrate the existence of common fixed point for one pair (respectively two pairs) of maps from the set of best approximation.
Theorem 5.1. Let M be a nonempty subset of a hyperbolic ordered metric space X, T, and S be continuous selfmaps on X such that T(∂M ∩ M) ⊂ M, ∂M stands for boundary of M, and u ∈ Fix(S) ∩ Fix(T) for some u in X, where u is comparable with all x ∈ X. Let (T, S) be partially weakly increasing, order limit preserving, T is uniformly asymptotically regular, asymptotically Snonexpansive and S is affine on P_{ M } (u) with S(P_{ M } (u)) = P_{ M } (u), q ∈ Fix(S), and P_{ M } (u) is qstarshaped. If cl(P_{ M } (u)) is compact, P_{ M } (u) is complete and S and T are uniformly C_{ q }commuting mappings on P_{ M } (u) ∪ {u} satisfying d(Tx, Tu) ≤ d(Sx, Su), then P_{ M } (u) ∩ Fix(T ) ∩ Fix(S) ≠ ϕ provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. Let x ∈ P_{ M } (u). Then d(x, u) = d(u, M ). Note that for any λ ∈ (0, 1),
This shows that . So x ∈ ∂M ∩ M which further implies that Tx ∈ M. Since Sx ∈ P_{ M } (u), u is a common fixed point of S and T, therefore by the given contractive condition, we obtain
Thus, P_{ M } (u) is T invariant. Hence,
Now the result follows from Theorem 4.4.
Theorem 5.2. Let M be a nonempty subset of a hyperbolic ordered metric space X, T, f, and g be selfmaps on X such that u is common fixed point of f, g, and T and T(∂M ∩ M) ⊂ M. Suppose that f and g are continuous and affine on P_{ M } (u), q ∈ Fix(f ) ∩ Fix(g), and P_{ M } (u) is qstarshaped with f(P_{ M } (u)) = P_{ M } (u) = g(P_{ M } (u)). Let (T, f ) and (T, g) be partially weakly increasing, and dominating maps f and g be weak annihilator of T. Assume that the pairs {T, f} and {T, g} are C_{ q }commuting and satisfy for all x ∈ P_{ M } (u) ∪ {u}
If cl(P_{ M } (u)) is compact and P_{ M } (u) is complete, then P_{ M } (u)∩Fix(T )∩Fix(f )∩ Fix(g) ≠ ϕ provided that for a nondecreasing sequence {x_{ n }} with x_{ n } ≤ y_{ n } for all n and y_{ n } → u implies x_{ n } ≤ u.
Proof. Let x ∈ P_{ M } (u). Then d(x, u) = d(u, M ). Note that for any λ ∈ (0, 1)
which shows that M and are disjoint. So x ∈ ∂M ∩ M which further implies that Tx ∈ M. Since fx ∈ P_{ M } (u), u is a common fixed point of f, g, and T, therefore by the given contractive condition, we obtain
Thus P_{ M } (u) is T invariant. Hence,
The result follows from Theorem 4.5.
Remark 5.3.

(a)
Theorem 3.2 extends and improves Theorem 2.2 of AlThagafi [8] and Theorem 2.2(i) of Hussain and Jungck [25] in the setup of hyperbolic ordered metric spaces.

(b)
Theorems 4.4 and 4.5 extend the results in [23] to more general classes of mappings defined on a hyperbolic ordered metric space.

(c)
Theorems 5.1 and 5.2 set analogues of Theorems 2.11(i) and 2.12(i) in [25], respectively.
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The second and third authors are grateful to King Fahd University of Petroleum and Minerals and SABIC for supporting research project SB100012.
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Abbas, M., Khamsi, M.A. & Khan, A.R. Common fixed point and invariant approximation in hyperbolic ordered metric spaces. Fixed Point Theory Appl 2011, 25 (2011). https://doi.org/10.1186/16871812201125
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DOI: https://doi.org/10.1186/16871812201125
Keywords
 Hyperbolic metric space
 common fixed point
 Ordered uniformly C_{ q }commuting mapping
 ordered asymptotically Snonexpansive mapping
 Best approximation