Abstract
In this paper, we prove some common fixed point theorems for weakly compatible mappings in metric spaces satisfying generalized (ψ,φ)contractive conditions under the common limit range property. We present a fixed point theorem for four finite families of selfmappings which can be utilized to derive common fixed point theorems involving any number of finite mappings. Our results improve and extend the corresponding results of Radenović et al. (Bull. Iranian Math. Soc. 38(3):625–645, 2012). We also furnish some illustrative examples to support our main results.
Keywords:
Metric space; Weakly compatible mappings; Property (E.A); Generalized weak contraction; Common limit range property; Fixed pointIntroduction and preliminaries
The famous Banach Contraction Principle which is also referred as the Banach fixed point theorem continues to be a very popular and powerful tool in solving existence problems in pure and applied sciences which include biology, medicine, physics, and computer science. It evidently plays a crucial role in nonlinear analysis. This theorem states that if (X, d) is a complete metric space and T:X → X is a contraction mapping, i.e.,
for all x, y ∈ X, and k is a nonnegative real number such that k < 1, then T has a unique fixed point in X. Moreover, this fixed point can be explicitly obtained as a limit of repeated iteration of the mapping, initiating at any point of the underlying space. Obviously, every contraction is a continuous function but not conversely. Many mathematicians (e.g., [16]) proved several fixed point theorems to explore some new contractiontype mappings in order to generalize the classical Banach Contraction Principle.
The concept of weak contraction was introduced by Alber and GuerreDelabriere [7] in 1997, wherein the authors introduced the following notion for mappings defined on a Hilbert space X.
Consider the following set of real functions Φ = { φ:[0, + ∞) → [0, + ∞):φ is lower semicontinuous and φ^{−1}({0}) = {0} }.
A mapping T:X → X is called a φweak contraction if there exists a function φ ∈ Φ such that
for all x, y ∈ X.
Alber and GuerreDelabriere [7] also showed that each φweak contraction on a Hilbert space has a unique fixed point. Thereafter, Rhoades [8] showed that the results contained in [7] are also valid for any Banach space. In particular, he generalized the Banach Contraction Principle which follows in case one chooses φ(t) = (1 − k)t.
Zhang and Song [9] proved a common fixed point theorem for two mappings using φweak contraction. This result was extended by Dorić [10] and Dutta and Choudhury [11] to a pair of (ψ, φ)weak contractive mappings. However, the main fixed point theorem for a selfmapping satisfying (ψ, φ)weak contractive condition contained in Dutta and Choudhury [11] runs as follows:
Let us consider the following set of real functions: Ψ = { ψ:[0, + ∞) → [0, + ∞):ψ is continuous nondecreasing and ψ^{−1}({0}) = {0} }.
Theorem 1.1
Let (X, d) be a complete metric space and let T:X → X be a selfmapping satisfying
for some ψ ∈ Ψ and φ ∈ Φ and all x, y ∈ X. Then, T has a unique fixed point in X.
In recent years, many researchers utilized (ψ, φ)weak contractive conditions to prove a number of metrical fixed point theorems (e.g., [1219]). In an important paper, Jachymski [20] showed that some of the results involving two functions ψ ∈ Ψ and φ ∈ Φ can be reduced to one function φ^{′} ∈ Φ. Popescu [21] proved a fixed point theorem in a complete metric space and showed that the conditions on functions ψ and φ can be weakened. His result improved the corresponding results of Dutta and Choudhury [11] and Dorić [10].
A common fixed point result generally involves conditions on commutativity, continuity, and contraction along with a suitable condition on the containment of range of one mapping into the range of the other. Hence, one is always required to improve one or more of these conditions in order to prove a new common fixed point theorem.
It can be observed that in the case of two mappings A, S:X → X, one can consider the following classes of mappings for the existence and uniqueness of common fixed points:
where F is some function and m(x, y) is the maximum of one of the sets:
A further possible generalization is to consider four mappings instead of two and ascertain analogous common fixed point theorems. In the case of four mappings A, B, S, T:X → X, the corresponding sets take the form
In this case (1.1) is usually replaced by
where m(x, y) is the maximum of one of the M sets.
Similarly, we can define the M sets for six mappings A, B, H, R, S, T:X → X as
and the contractive condition is again in the form (1.2). Using different arguments of control functions, Radenović et al. [22] proved some common fixed point results for two and three mappings using (ψ, φ)weak contractive conditions and improved several known metrical fixed point theorems.
Motivated by these results, we prove some common fixed point theorems for two pairs of weakly compatible mappings with common limit range property satisfying generalized (ψ, φ)weak contractive conditions. Many known fixed point results are improved, especially the ones proved in [22] and also contained in the references cited therein. We also obtain a fixed point theorem for four finite families of selfmappings. Some related results are also derived besides furnishing illustrative examples.
Definition 1.1
Let A and S be two mappings from a metric space (X, d) into itself. Then, the mappings are said to
1. be commuting if ASx = SAx for all x ∈ X,
2. be compatible [23] if for each sequence {x_{n}} in X such that ,
3. be noncompatible [5] if there exists a sequence {x_{n}} in X such that but is either nonzero or nonexistent,
4. be weakly compatible [24] if they commute at their coincidence points, that is, ASx = SAx whenever Ax = Sx, for some x ∈ X,
5. satisfy the property (E.A) [25] e {x_{n}} in X such that , for some t ∈ X.
For further details, comparisons, and illustrations on systematic spaces, we refer to Singh and Tomar [26] and Murthy [4].
Any pair of compatible as well as noncompatible selfmappings of a metric space (X, d) satisfies the property (E.A), but a pair of mappings satisfying the property (E.A) need not be noncompatible (see Example 1 of [27]).
In 2005, Liu et al. [28] defined the notion of common property (E.A) for hybrid pairs of mappings, which contain the property (E.A).
Definition 1.2
Liu, 2005 [28] Two pairs (A, S) and (B, T) of selfmappings of a metric space (X, d) are said to satisfy the common property (E.A) if two sequences {x_{n}} and {y_{n}} in X exist such that
for some t ∈ X.
It can be observed that the fixed point results usually require closedness of the underlying subspaces for the existence of common fixed points under the property (E.A) and common property (E.A). In 2011, Sintunavarat and Kumam [29] coined the idea of ‘common limit range property’ (see also [30]). Most recently, Imdad et al. [31] extended the notion of common limit range property to two pairs of selfmappings which relax the closedness requirements of the underlying subspaces.
Definition 1.3
Sintunavarat, 2012 [29] A pair (A, S) of selfmappings of a metric space (X, d) is said to satisfy the common limit range property with respect to S, denoted by (CLR_{S}), if there exists a sequence {x_{n}} in X such that
where t ∈ S(X).
Thus, one can infer that a pair (A, S) satisfying the property (E.A) along with the closedness of the subspace S(X) always enjoys the (CLR_{S}) property with respect to the mapping S (see Examples 2.16–2.17 of [31]).
Definition 1.4
Imdad, 2012 [31] Two pairs (A, S) and (B, T) of selfmappings of a metric space (X, d) are said to satisfy the common limit range property with respect to mappings S and T, denoted by (CLR_{ST}) if two sequences {x_{n}} and {y_{n}} in X exist such that
where t ∈ S(X) ∩ T(X).
Definition 1.5
Imdad, 2009 [32] Two families of selfmappings and are said to be pairwise commuting if
1. A_{i}A_{j} = A_{j}A_{i} for all i, j ∈ {1, 2, …, m},
2. S_{k}S_{l} = S_{l}S_{k} for all k, l ∈ {1, 2, …, n},
3. A_{i}S_{k} = S_{k}A_{i} for all i ∈ {1, 2, …, m} and k ∈ {1, 2, …, n}.
Main results
Now, we state and prove our main results for four mappings employing the common limit range property in metric spaces. Firstly, we prove the following lemma.
Lemma 2.1
Let A, B, S and T be selfmappings of a metric space (X, d). Suppose that
1. the pair (A, S) satisfies the (CLR_{S}) property (resp. (B, T) satisfies the (CLR_{T}) property),
2. A(X) ⊂ T(X) (resp. B(X) ⊂ S(X)),
3. T(X) (resp. S(X)) is a closed subset of X,
4. {By_{n}} converges for every sequence {y_{n}} in X whenever {Ty_{n}} converges (resp. {Ax_{n}} converges for every sequence {x_{n}} inX whenever {Sx_{n}} converges),
5. there exist φ ∈ Φ and ψ ∈ Ψ such that
for all x, y ∈ X, where
Then, the pairs (A, S) and (B, T) share the (CLR_{ST}) property.
Proof
Since the pair (A, S) satisfies the (CLR_{S}) property, a sequence {x_{n}} in X exists such that
where t ∈ S(X). By Lemma 2.1 item (2), A(X) ⊂ T(X), and for each sequence {x_{n}}, there exists a sequence {y_{n}} in X such that Ax_{n} = Ty_{n}. Therefore, due to the closedness of T(X),
so that t ∈ T(X) and in all t ∈ S(X) ∩ T(X). Thus, we have Ax_{n} → t, Sx_{n} → t and Ty_{n} → t as n → ∞. By Lemma 2.1 item (4), the sequence {By_{n}} converges, and in all, we need to show that By_{n} → t as n → ∞. Let, on the contrary that By_{n} → z(≠ t) as n → ∞. On using inequality (2.1) with x = x_{n}, y = y_{n}, we have
where
Taking the limit as n → ∞ in (2.2), we get
where
From (2.3), we obtain
so that d(t, z) = 0, i.e., t = z which is a contradiction. Hence, By_{n} → t which shows that the pairs (A, S) and (B, T) share the (CLR_{ST}) property. This concludes the proof. □
Remark 2.1
In general, the converse of Lemma 2.1 is not true (see Example 3.5 of [31]).
Theorem 2.1
Let A, B, S and T be selfmappings of a metric space (X, d) satisfying the inequality (2.1) of Lemma 2.1. If the pairs (A, S) and (B, T) satisfy the (CLR_{ST}) property, then (A, S) and (B, T) have a coincidence point each. Moreover, A, B, S and T have a unique common fixed point provided both the pairs (A, S) and (B, T) are weakly compatible.
Proof
If the pairs (A, S) and (B, T) enjoy the (CLR_{ST}) property, then two sequences {x_{n}} and {y_{n}} in X exist such that
where t ∈ S(X) ∩ T(X). Since t ∈ S(X), a point u ∈ X exists such that Su = t. We assert that Au = Su. Using inequality (2.1) with x = u, y = y_{n}, we get
where
Taking the limit as n → ∞ in (2.4), we get
where
which in turn yields
so that φ(d(Au, t)) = 0, i.e., d(Au, t) = 0. Hence Au = Su = t. Therefore, u is a coincidence point of the pair (A, S).
As t ∈ T(X), there exists a point v ∈ X such that Tv = t. We show that Bv = Tv. Using inequality (2.1) with x = u, y = v, we get
where
which in turn yields
so that φ(d(t, Bv)) = 0, i.e., d(t, Bv) = 0. Hence, Bv = Tv = t, which shows that v is a coincidence point of the pair (B, T).
Since the pair (A, S) is weakly compatible, and Au = Su; therefore, At = ASu = SAu = St. Now, we assert that t is a common fixed point of the pair (A, S). Using inequality (2.1) with x = t, y = v, we have
where
which in turn yields
so that φ(d(At, t)) = 0, i.e., At = t = St, which shows that t is a common fixed point of the pair (A, S).
Also, the pair (B, T) is weakly compatible, and Bv = Tv; therefore, Bt = BTw = TBw = Tt. Suppose that Bt ≠ t. On using inequality (2.1) with x = u, y = t, we have
where
which in turn yields
so that φ(d(t, Bt)) = 0, i.e., d(t, Bt) = 0. Therefore, Bt = t = Tt which shows that t is a common fixed point of the pair (B, T) and in all t is a common fixed point of both the pairs (A, S) and (B, T).
To prove the uniqueness of common fixed point, let on contrary that there is another common fixed point t^{′} ∈ X such that At^{′} = Bt^{′} = St^{′} = Tt^{′} = t^{′}. Using inequality (2.1) with x = t, y = t^{′}, we have
where
which in turn yields
so that φ(d(t, t^{′}) = 0, i.e., t = t^{′}. Hence, t is a unique common fixed point of the mappings A, B, S, and T. This concludes the proof. □
Remark 2.2
Theorem 2.1 improves the relevant results of Radenović et al. [22] as the requirements on the closedness and containment among the ranges of the involved mappings are not needed.
Now, we furnish an illustrative example which demonstrates the validity of the hypotheses and degree of generality of our main result over comparable ones from the existing literature.
Example 2.1
Consider X = [2, 11) equipped with the usual metric. Define the self mappings A, B, S and T by
Consider two sequences , . The pairs (A, S) and (B, T) satisfy the (CLR_{ST}) property:
Take ψ ∈ Ψ and φ ∈ Φ given by
In order to check the contractive condition (2.1), consider the following nine cases: 1° x = y = 2; 2° x = 2, y ∈ (2, 5]; 3° x = 2, y ∈ (5, 11); 4° x ∈ (2, 5], y = 2; 5° x, y ∈ (2, 5]; 6° x ∈ (2, 5], y ∈ (5, 11); 7° x ∈ (5, 11), y = 2; 8° x ∈ (5, 11), y ∈ (2, 5]; 9° x, y ∈ (5, 11).
In the cases 1°, 3°, 7°, and 9°, we get that d(Ax, By) = 0; (2.1) is trivially satisfied. In the cases 2° and 8°, it is d(Ax, By) = 2 and m(x, y) = 4; so, (2.1) reduces to
In the cases 4° and 6°, we get that d(Ax, By) = 6 and m(x, y) = 7; so, (2.1) reduces to
Finally, in the case 5°, we obtain d(Ax, By) = 4 and m(x, y) = 5, and again we have
Hence, all the conditions of Theorem 2.1 are satisfied, and 2 is a unique common fixed point of the pairs (A, S) and (B, T) which also remains a point of coincidence. Here, one may notice that all the involved mappings are discontinuous at their unique common fixed point 2.
However, notice that the subspaces S(X) and T(X) are not closed subspaces of X, and required inclusions among the ranges of the involved maps do not hold. Therefore, the results of Radenović et al. [22] cannot be used in the context of this example which establishes the genuineness of our extension.
In view of Theorem 2.1 and Lemma 2.1, the following corollary is immediate.
Corollary 2.1
Let A, B, S and T be selfmappings of a metric space (X, d) satisfying all the hypotheses of Lemma 2.1. Then A, B, S, and T have a unique common fixed point, provided that both the pairs (A, S) and (B, T) are weakly compatible.
Proof
Owing to Lemma 2.1, it follows that the pairs (A, S) and (B, T) enjoy the (CLR_{ST}) property. Hence, all the conditions of Theorem 2.1 are satisfied; A, B, S and T have a unique common fixed point provided that both the pairs (A, S) and (B, T) are weakly compatible. □
Here, it is worth noting that the conclusions in Example 2.1 cannot be obtained using Corollary 2.1 as conditions (2) and (3) of Lemma 2.1 are not fulfilled. In what follows, we present another example which creates a situation wherein a conclusion can be reached using Corollary 2.1.
Example 2.2
In the setting of Example 2.1, replace the selfmappings S and T by the following, retaining the rest:
Then, as in the earlier example, the pairs (A, S) and (B, T) satisfy the (CLR_{ST}) property. Moreover, inequality (2.1) can be be verified as earlier. Also, as earlier define,
Here, A(X) = {2, 8} ⊂ [2, 8] = T(X) and B(X) = {2, 4} ⊂ [2, 5] = S(X) holds. Thus, all the conditions of Corollary 2.1 are satisfied and 2 is a unique common fixed point of the involved mappings A, B, S, and T.
Remark 2.3
The conclusions of Lemma 2.1, Theorem 2.1 and Corollary 2.1 remain true if we choose or .
By setting A, B, S, and T suitably, we can deduce corollaries involving two as well as three selfmappings. As a sample, we can deduce the following corollary involving two selfmappings:
Corollary 2.2
Let A and S be selfmappings of a metric space (X, d). Suppose that
1. the pair (A, S) satisfies the (CLR_{S}) property,
2. there exist φ ∈ Φ and ψ ∈ Ψ such that
for all x, y ∈ X, where and k ∈ {3, 4, 5}.
Then, (A, S) has a coincidence point. Moreover, if the pair (A, S) is weakly compatible, then the pair has a unique common fixed point in X.
As an application of Theorem 2.1, we have the following result involving four finite families of selfmappings.
Theorem 2.2
Let , , , and be four finite families of selfmappings of a metric space (X, d) with A = A_{1}A_{2} ⋯ A_{m}, B = B_{1}B_{2} ⋯ B_{n}, S = S_{1}S_{2} ⋯ S_{p}, and T = T_{1}T_{2} ⋯ T_{q} satisfying the condition (2.1). Suppose that the pairs (A, S) and (B, T) satisfy the (CLR_{ST}) property, then (A, S) and (B, T) have a point of coincidence each.
Moreover , , , and have a unique common fixed point if the families ({A_{i}}, {S_{k}}) and ({B_{r}}, {T_{h}}) commute pairwise where i ∈ {1, 2, …, m}, k ∈ {1, 2, …, p}, j ∈ {1, 2, …, n}, and l ∈ {1, 2, …, q}.
Proof
The proof of this theorem can be completed on the lines of Theorem 2.2 of Imdad et al. [31]. □
Remark 2.4
A result similar to Theorem 2.2 can be outlined using Corollary 2.1.
Remark 2.5
Theorem 2.2 extends the results of Radenović et al. [22] and Abbas and Dorić [12].
Now, we indicate that Theorem 2.2 can be utilized to derive common fixed point theorems for any finite number of mappings. As a sample, we can derive a common fixed point theorem for six mappings by setting two families of two members, while the rest by two of single members.
Corollary 2.3
Let A, B, H, R, S, and T be selfmappings of a metric space (X, d). Suppose that
1. the pairs (A, SR) and (B, TH) share the (CLR_{(SR)(TH)}) property,
2. there exist φ ∈ Φ and ψ ∈ Ψ such that
for all x, y ∈ X, where , and k ∈ {3, 4, 5}.
Then, (A, SR) and (B, TH) have a coincidence point each. Moreover, A, B, H, R, S, and T have a unique common fixed point provided AS = SA, AR = RA, SR = RS, BT = TB, BH = HB, and TH = HT.
By choosing A_{1} = A_{2} = ⋯ = A_{m} = A, B_{1} = B_{2} = ⋯ = B_{n} = B, S_{1} = S_{2} = ⋯ = S_{p} = S, and T_{1} = T_{2} = ⋯ = T_{q}=T in Theorem 2.2, we get the following corollary:
Corollary 2.4
Let A, B, S, and T be selfmappings of a metric space (X, d). Suppose that
1. the pairs (A^{m}, S^{p}) and (B^{n}, T^{q}) share the property, where m, n, p, and q are fixed positive integers;
2. there exist φ ∈ Φ and ψ ∈ Ψ such that
for all x, y ∈ X, where , and k ∈ {3, 4, 5}.
Then, A, B, S, and T have a unique common fixed point provided that AS = SA and BT = TB.
Remark 2.6
Notice that Corollary 2.4 is a slight but partial generalization of Theorem 2.1 as the commutativity requirements (that is, AS = SA and BT = TB) in this corollary are relatively stronger as compared to weak compatibility in Theorem 2.1.
Remark 2.7
Results similar to Corollary 2.4 can be derived from Corollary 2.1.
Remark 2.8
It may be pointed out that the earlier proved results, namely Theorems 2.1 and 2.2 (also Corollaries 2.1– 2.4) remain valid in symmetric space (X,d) whenever d is continuous.
Competing interest
The authors declare that they have no competing interests.
Authors’ contributions
MI, SC, and ZK contributed equally. All authors read and approved the final manuscript.
Acknowledgements
We express our gratitude to all the learned referees for their careful reading of our manuscript especially for the remarks which brought several improvements. The third author is grateful to the Ministry of Education, Science and Technological Development of Serbia.
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