Volume 2013, Issue 1 730825
Research Article
Open Access

Fixed Point Results for α-ψλ-Contractions on Gauge Spaces and Applications

Mohamed Jleli

Mohamed Jleli

Department of Mathematics, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia ksu.edu.sa

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Erdal Karapınar

Corresponding Author

Erdal Karapınar

Department of Mathematics, Atilim University, Incek, 06836 Ankara, Turkey atilim.edu.tr

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Bessem Samet

Bessem Samet

Department of Mathematics, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia ksu.edu.sa

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First published: 23 September 2013
Citations: 14
Academic Editor: Calogero Vetro

Abstract

We extend the concept of α-ψ-contractive mappings introduced recently by Samet et al. (2012) to the setting of gauge spaces. New fixed point results are established on such spaces, and some applications to nonlinear integral equations on the half-line are presented.

1. Introduction and Preliminaries

Fixed point theory plays an important role in nonlinear analysis. This is because many practical problems in applied science, economics, physics, and engineering can be reformulated as a problem of finding fixed points of nonlinear mappings. The Banach contraction principle [1] is one of the fundamental results in fixed point theory. It guarantees the existence and uniqueness of fixed points of certain self-maps of metric spaces and provides a constructive method to approximate those fixed points.

Theorem 1 (see [1].)Let (X, d) be a complete metric space. Let T : XX be a contraction self-mapping on X; that is, there exists a constant k ∈ (0,1) such that

()
for all (x, y) ∈ X × X. Then the map  T  admits a unique fixed point. Moreover, for any x0X, the sequence {Tnx0} converges to this fixed point.

During the last few decades, several extensions of this famous principle have been established. In 1961, Edelstein [2] established the following result.

Theorem 2 (see [2].)Let (X, d) be complete and ε-chainable for some ε > 0. Let T : XX be such that

()
where k ∈ (0,1) is a constant. Then  T  has a unique fixed point.

Kirk et al. [3] introduced the concept of cyclic mappings and proved the following fixed point theorem.

Theorem 3 (see [3].)Let A and B be two nonempty closed subsets of a complete metric space (X, d). Let T : XX be a self-mapping such that

()
where k ∈ (0,1) is a constant. Suppose also that T(A)⊆B and T(B) ⊂ A. Then T has a unique fixed point in AB.

Ran and Reurings [4] extended the Banach contraction principle to a metric space endowed with a partial order. They established the following result.

Theorem 4 (see [4].)Let (X, d) be a complete metric space endowed with a partial order ⪯. Let T : XX be a continuous mapping such that

()
where k ∈ (0,1) is a constant. Suppose also that there exists x0X such that x0Tx0. Then T has a fixed point.

Many extensions of the previous result exist in the literature; for more details, we refer the reader to [511] and the references therein.

Observe that all the contractive conditions (2), (3), and (4) can be written as
()
where H is a subset of X × X. In Theorem 2, we have
()
In Theorem 3, we have
()
In Theorem 4, we have
()
The contractive condition (5) is said to be a partial contraction, that is, a contraction satisfied only on a subset of HX × X.
Very recently, Samet et al. [12] observed that a partial contraction can be considered as a total contraction, that is, a contraction satisfied for every pair (x, y) ∈ X × X. More precisely, if we define the function α : X × X → [0, ) by
()
we show that (5) is equivalent to
()
In [12], the authors considered a more general inequality; that is,
()
where ψ : [0, )→[0, ) is a function satisfying some conditions. The above inequality is called an α-ψ-contraction. In [12], some fixed point results were established under this contractive condition. For other works in this direction, we refer the reader to [1316].

The aim of this work is to extend, generalize, and improve the obtained results in [12]. More precisely, the concept of α-ψ-contractive mappings is extended to the setting of gauge spaces. New fixed point results are established on such spaces, and some applications to nonlinear integral equations on the half-line are presented.

Through this paper, 𝔼 will denote a gauge space endowed with a separating gauge structure 𝒟 = {dλ} λ∈Λ, where Λ is a directed set.

A sequence {xn} ⊂ 𝔼 is said to be convergent if there exists an x𝔼 such that for every ε > 0 and λ ∈ Λ, there is an N with dλ(xn, x) < ε, for all nN.

A sequence {xn} ⊂ 𝔼 is said to be Cauchy, if for every ε > 0 and λ ∈ Λ, there is an N with dλ(xn, xn+p) < ε, for all nN and p.

A gauge space is called complete if any Cauchy sequence is convergent.

A subset of  𝔼  is said to be closed if it contains the limit of any convergent sequence of its elements.

For more details on guage spaces, we refer the reader to Dugundji [17].

We denote by Ψ the set of functions ψ : [0, )→[0, ) satisfying the following conditions:
  • (C1)

    ψ is nondecreasing;

  • (C2)

    , for all t > 0, where ψn is the nth iterate of ψ;

  • (C3)

    ψ(a) + ψ(b) ≤ ψ(a + b), for all a, b ≥ 0.

It is easy to show that under the conditions (C1) and (C2), we have ψ(t) < t, for all t > 0. Moreover, under the conditions (C1) and (C3), we have
()
for all n and a, b ≥ 0.

Example 5. Let ψ : [0, )→[0, ) be the function defined by

()
Then ψ ∈ Ψ.

Definition 6. Let α : 𝔼 × 𝔼 → [0, ) be a given function. Let N and x, y𝔼. We say that is an N-α-path from  x  to  y  if

()

We denote
()
Let N and x𝔼. For λ ∈ Λ and yx[N, α], and let
()

2. Fixed Point Results for α-ψλ-Contractions

Definition 7. Let T : 𝔼𝔼 be a given self-mapping. Let α : 𝔼 × 𝔼 → [0, ) be a given function, and let {ψλ} λ∈Λ ⊂ Ψ. We say that T is an α-ψλ-contraction if

()
for all λ ∈ Λ and x, y𝔼.

Definition 8. Let T : 𝔼𝔼 be a given self-mapping. Let α : 𝔼 × 𝔼 → [0, ) be a given function. We say that T is α-admissible if

()

The following lemma will be useful to establish our fixed point results.

Lemma 9. Let T : 𝔼𝔼 be a self-mapping. Suppose that there exist α : 𝔼 × 𝔼 → [0, ) and {ψλ} λ∈Λ ⊂ Ψ such that the following conditions hold:

  • (i)

    T is an α-ψλ-contraction;

  • (ii)

    T is α-admissible.

Let ε > 0 and N. Then, for every λ ∈ Λ, xX, and yx[N, α], one has
  • (I)

    TyTx[N, α] with

    ()

  • (II)

    for all k ∪ {0}, one has TkyTkx[N, α] with

    ()

Proof. Let λ ∈ Λ, xX, and yx[N, α]. Let be an N-α-path from x to  y  such that

()
Since α(x, x1) ≥ 1, we have
()
Since α(x1, x2) ≥ 1, we have
()
Recursively from i = 3 to N, since α(xi−1, xi) ≥ 1, we have
()
On the other hand, since  T  is  α-admissible, is an N-α-path from Tx to Ty. So, we have
()
Thus, we proved (I).

Again, since α(Tx, Tx1) ≥ 1, we have

()
Since α(Tx1, Tx2) ≥ 1, we have
()
Recursively from i = 3 to N, since α(Txi−1, Txi) ≥ 1, we have
()
On the other hand, since T is α-admissible, is an N-α-path from T2x to T2y. So, we have
()
Continuing this process, by induction, we get (II).

Definition 10. Let T : 𝔼𝔼 be a self-mapping, and let α : 𝔼 × 𝔼 → [0, ) be a given function. For N, we say that a sequence {xn} ⊂ 𝔼 is an N-α-Picard trajectory from x0 if xn = Txn−1xn−1[N, α] for all n. We denote by 𝒯N(T, α, x0), the set of all N-α-Picard trajectories from  x0.

Definition 11. Let T : 𝔼𝔼 be a self-mapping, and let α : 𝔼 × 𝔼 → [0, ) be a given function. For N, we say that T is N-α-Picard continuous from x0𝔼 if the limit of any convergent sequence {xn} ∈ 𝒯N(T, α, x0) is a fixed point of T.

We have the following fixed point result.

Theorem 12. Let T : 𝔼𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ and α : 𝔼 × 𝔼 → [0, ) be a given function. Suppose that the following conditions hold:

  • (i)

    T is an α-ψλ-contraction;

  • (ii)

    T is α-admissible;

  • (iii)

    there exist N and x0𝔼 such that Tx0x0[N, α];

  • (iv)

    T is N-α-Picard continuous from x0.

Then T has a fixed point.

Proof. Let λ ∈ Λ and ε > 0. From condition (iii) and Lemma 9, we have T2x0Tx0[N, α] and

()
Again, from Lemma 9, we have T3x0T2x0[N, α] and
()
Continuing this process, by induction, for n ≥ 2, we have Tn+1x0Tnx0[N, α] and
()
Thus, {Tnx0} ∈ 𝒯N(T, α, x0) and, for m ≥ 1,
()
From condition (C2), we have
()
which implies that {Tnx0} is a Cauchy sequence in the complete gauge space 𝔼. Since T is N-α-Picard continuous from x0, the limit of {Tnx0} is a fixed point of T.

Corollary 13. Let T : 𝔼𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ and α : 𝔼 × 𝔼 → [0, ) be a given function. Suppose that the following conditions hold:

  • (i)

    T is an α-ψλ-contraction;

  • (ii)

    T is α-admissible;

  • (iii)

    there exist N and x0𝔼 such that Tx0x0[N, α];

  • (iv)

    T is continuous.

Then T has a fixed point.

Proof. Let {xn} ∈ 𝒯N(T, α, x0) be such that xnx𝔼. Since T is continuous, we have xn+1 = TxnTx. Since 𝔼 is endowed with a separating gauge structure, we have x = Tx. The conclusion follows from Theorem 12.

Corollary 14. Let T : 𝔼𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ and α : 𝔼 × 𝔼 → [0, ) be a given function. Suppose that the following conditions hold:

  • (i)

    T is an α-ψλ-contraction;

  • (ii)

    T is α-admissible;

  • (iii)

    there exist N and x0𝔼 such that Tx0x0[N, α];

  • (iv)

    for every {xn} ∈ 𝒯N(T, α, x0) such that xnx𝔼, there exist a subsequence {xn(k)}  of  {xn}  and k0  such that α(xn(k), x) ≥ 1 for kk0.

Then T has a fixed point.

Proof. Let {xn} ∈ 𝒯N(T, α, x0) be such that xnx𝔼. From condition (iv), there exist a subsequence {xn(k)} of {xn} and k0 such that α(xn(k), x) ≥ 1 for kk0. Since T is an α-ψλ-contraction, for all λ ∈ Λ and kk0, we have

()
Letting k in the above inequality, we obtain that
()
Since 𝔼 is endowed with a separating gauge structure, we have x = Tx. The conclusion follows from Theorem 12.

For T : 𝔼𝔼, we denote by Fix (T) the set of fixed points of T; that is,
()
The next result gives us a sufficient condition that ensures the uniqueness of the fixed point.

Theorem 15. Suppose that all the conditions of Theorem 12 are satisfied. Moreover, suppose that

  • (v)

    for every (x, y) ∈   Fix (T) ×   Fix (T) with xy, there exists N(x, y) ∈ such that yx[N(x, y), α].

Then T has a unique fixed point.

Proof. From Theorem 12, The mapping T has at least one fixed point. Suppose that u, v𝔼 are two fixed points of T with uv. From the condition (v), there exists N(u, v) ∈ such that vu[N(u, v), α]. Let λ ∈ Λ and ε > 0. From Lemma 9, we have

()
for all n. Letting n in the above inequality, we obtain that dλ(u, v) = 0 for all λ ∈ Λ, which is a contradiction with uv (since we have a separating gauge structure). We deduce that u = v.

The following result follows immediately from Theorems 12 and 15 with N = 1 and α(x, y) = 1 for every x, y𝔼.

Corollary 16. Let T : 𝔼𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ. Suppose that for all λ ∈ Λ, for all x, y𝔼, one has

()
Then T has a unique fixed point.

Corollary 17. Let T : 𝔼𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ and α : 𝔼 × 𝔼 → [0, ) be a given function. Suppose that the following conditions hold:

  • (i)

    T is an α-ψλ-contraction;

  • (ii)

    T is α-admissible;

  • (iii)

    there exists x0𝔼   such that α(x0, Tx0) ≥ 1;

  • (iv)

    T is continuous

(or) for any sequence {xn} ⊂ 𝔼 such that xn = Txn−1, xnx𝔼 and α(xn−1, Txn−1) ≥ 1 for n, there exist a subsequence {xn(k)} of {xn} and k0 such that α(xn(k), x) ≥ 1 for kk0.

Then T has a fixed point. Moreover, if

  • (v)

    for every (x, y) ∈ Fix (T) × Fix (T) with xy, there exists z𝔼 such that α(x, z) ≥ 1 and α(z, y) ≥ 1,

one has uniqueness of the fixed point.

Proof. The existence follows from Theorem 12 with N = 1. The uniqueness follows from Theorem 15 with N(x, y) = 2.

Corollary 18. Let ⪯ be a partial order on the complete gauge space 𝔼. Let T : 𝔼𝔼 be a self-mapping and {ψλ} λ∈Λ ⊂ Ψ. Suppose that the following conditions hold:

  • (i)

    for all λ ∈ Λ, for all x, y𝔼 such that x and y are comparable, one has

    ()

  • (ii)

    x, y𝔼, x and y are comparable ⇒Tx and Ty are comparable;

  • (iii)

    there exists x0𝔼 such that x0 and Tx0 are comparable;

  • (iv)

    T is continuous,

(or) for any sequence {xn} ⊂ 𝔼 such that xn = Txn−1, xnx𝔼, xn−1 and xn = Txn−1 are comparable for n, there exist a subsequence {xn(k)} of {xn} and k0 such that xn(k) and x are comparable for kk0.

Then T has a fixed point. Moreover, if

  • (v)

    for every (x, y) ∈ Fix (T) × Fix (T) with xy, there exists z𝔼 such that x and z are comparable, z and y are comparable,

one has uniqueness of the fixed point.

Proof. It follows from Corollary 17 with

()

3. Applications

In this section, we are interested in the study of the existence of solutions to the nonlinear integral equation on the real axis
()
where hC([0, ), ) and FC([0, )×[0, ) × , ). Here, is a Banach space with respect to a given norm ∥·∥.
Let 𝔼 : = C([0, ), ) and the family of pseudonorms {∥·∥n} n defined by
()
For every n, define now
()
Then 𝒟≔{dn} n is a separating gauge structure on  𝔼.

We have the following existence result.

Theorem 19. Suppose that the following conditions hold:

  • (i)

    there exist a nonempty set Γ⊆ × and a constant k < τ such that

    ()

for all t, s ≥ 0, (u, v) ∈ Γ;
  • (ii)

    there exists a nonempty set Γ𝔼𝔼 × 𝔼 such that

    ()

  • (iii)

    for all (x, y) ∈ Γ𝔼, one has

    ()

for all t ≥ 0;
  • (iv)

    there exists x0𝔼 such that

    ()

for all t ≥ 0;
  • (v)

    if {xp} ⊂ 𝔼 is a sequence such that (xp, xp+1) ∈ Γ𝔼  for p and xpx𝔼 (with respect to 𝒟), then there exist a subsequence {xp(k)} of {xp} and k0 such that

    ()

for all kk0, t ≥ 0.

Then (42) has at least one solution in 𝔼.

Proof. Consider the mapping T : 𝔼𝔼 defined by

()
for all x𝔼. We have to prove that T has at least a fixed point.

Define the function α : 𝔼 × 𝔼 → [0, ) by

()
We claim that for all n, for all x, y𝔼,
()
where ψn(t) = (k/τ)t for all n, for all t ≥ 0. Clearly, since k < τ, {ψn} n ⊂ Ψ. If α(x, y) = 0, (52) holds immediately. So, suppose that α(x, y) ≠ 0; that is, (x, y) ∈ Γ𝔼. Let n. From conditions (i) and (ii), for all t ∈ [0, n], we have
()
which implies that
()
Thus, we proved (52).

We will prove that  T  is  α-admissible. Let (x, y) ∈ 𝔼 × 𝔼 such that α(x, y) ≥ 1; that is, (x, y) ∈ Γ𝔼. From condition (iii), we have (Tx(t), Ty(t)) ∈ Γ for all t ≥ 0, which implies from condition (ii) that (Tx, Ty) ∈ ΓE; that is, α(Tx, Ty) ≥ 1. So, T is α-admissible.

From conditions (iv) and (ii), we have (x0, Tx0) ∈ Γ𝔼, which is equivalent to say that α(x0, Tx0) ≥ 1.

Finally, condition (v) implies that for every {xp} ∈ 𝒯1(T, α, x0) such that xpx𝔼, there exist a subsequence {xp(k)} of {xp} and k0 such that α(xp(k), x) ≥ 1 for kk0.

Now, All the hypotheses of Corollary 14 are satisfied; we deduce that T has at least a fixed point, which is a solution to (52).

Theorem 20. In addition to the assumptions of Theorem 19, suppose that

  • (vi)

    for all (x, y) ∈ 𝔼 × 𝔼, there exists z𝔼 such that (x, z) ∈ Γ𝔼 and (z, y) ∈ Γ𝔼.

Then (42) has one and only one solution in 𝔼.

Proof. It follows immediately from Theorem 15.

Conflict of Interests

The authors declare that there is no competing/conflict of interests regarding the publication of this paper.

    Authors’ Contribution

    All authors contributed equally and significantly in writing this paper. All authors read and approved the final paper.

      Acknowledgment

      This work is supported by the Research Center, College of Science, King Saud University.

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