Fixed Point Results for α-ψλ-Contractions on Gauge Spaces and Applications
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 : X → X be a contraction self-mapping on X; that is, there exists a constant k ∈ (0,1) such that
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 : X → X be such that
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 : X → X be a self-mapping such that
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 : X → X be a continuous mapping such that
Many extensions of the previous result exist in the literature; for more details, we refer the reader to [5–11] and the references therein.
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 x ∈ 𝔼 such that for every ε > 0 and λ ∈ Λ, there is an N ∈ ℕ with dλ(xn, x) < ε, for all n ≥ N.
A sequence {xn} ⊂ 𝔼 is said to be Cauchy, if for every ε > 0 and λ ∈ Λ, there is an N ∈ ℕ with dλ(xn, xn+p) < ε, for all n ≥ N and p ∈ ℕ.
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].
- (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.
Example 5. Let ψ : [0, ∞)→[0, ∞) be the function defined by
Definition 6. Let α : 𝔼 × 𝔼 → [0, ∞) be a given function. Let N ∈ ℕ and x, y ∈ 𝔼. We say that is an N-α-path from x to y if
2. Fixed Point Results for α-ψλ-Contractions
Definition 7. Let T : 𝔼 → 𝔼 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
Definition 8. Let T : 𝔼 → 𝔼 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 : 𝔼 → 𝔼 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.
- (I)
Ty ∈ Tx[N, α] with
() - (II)
for all k ∈ ℕ ∪ {0}, one has Tky ∈ Tkx[N, α] with
()
Proof. Let λ ∈ Λ, x ∈ X, and y ∈ x[N, α]. Let be an N-α-path from x to y such that
Again, since α(Tx, Tx1) ≥ 1, we have
Definition 10. Let T : 𝔼 → 𝔼 be a self-mapping, and let α : 𝔼 × 𝔼 → [0, ∞) be a given function. For N ∈ ℕ, we say that a sequence {xn} ⊂ 𝔼 is an N-α-Picard trajectory from x0 if xn = Txn−1 ∈ xn−1[N, α] for all n ∈ ℕ. We denote by 𝒯N(T, α, x0), the set of all N-α-Picard trajectories from x0.
Definition 11. Let T : 𝔼 → 𝔼 be a self-mapping, and let α : 𝔼 × 𝔼 → [0, ∞) be a given function. For N ∈ ℕ, we say that T is N-α-Picard continuous from x0 ∈ 𝔼 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 : 𝔼 → 𝔼 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 ∈ ℕ and x0 ∈ 𝔼 such that Tx0 ∈ x0[N, α];
- (iv)
T is N-α-Picard continuous from x0.
Proof. Let λ ∈ Λ and ε > 0. From condition (iii) and Lemma 9, we have T2x0 ∈ Tx0[N, α] and
Corollary 13. Let T : 𝔼 → 𝔼 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 ∈ ℕ and x0 ∈ 𝔼 such that Tx0 ∈ x0[N, α];
- (iv)
T is continuous.
Proof. Let {xn} ∈ 𝒯N(T, α, x0) be such that xn → x ∈ 𝔼. Since T is continuous, we have xn+1 = Txn → Tx. Since 𝔼 is endowed with a separating gauge structure, we have x = Tx. The conclusion follows from Theorem 12.
Corollary 14. Let T : 𝔼 → 𝔼 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 ∈ ℕ and x0 ∈ 𝔼 such that Tx0 ∈ x0[N, α];
- (iv)
for every {xn} ∈ 𝒯N(T, α, x0) such that xn → x ∈ 𝔼, there exist a subsequence {xn(k)} of {xn} and k0 ∈ ℕ such that α(xn(k), x) ≥ 1 for k ≥ k0.
Proof. Let {xn} ∈ 𝒯N(T, α, x0) be such that xn → x ∈ 𝔼. From condition (iv), there exist a subsequence {xn(k)} of {xn} and k0 ∈ ℕ such that α(xn(k), x) ≥ 1 for k ≥ k0. Since T is an α-ψλ-contraction, for all λ ∈ Λ and k ≥ k0, we have
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 x ≠ y, there exists N(x, y) ∈ ℕ such that y ∈ x[N(x, y), α].
Proof. From Theorem 12, The mapping T has at least one fixed point. Suppose that u, v ∈ 𝔼 are two fixed points of T with u ≠ v. From the condition (v), there exists N(u, v) ∈ ℕ such that v ∈ u[N(u, v), α]. Let λ ∈ Λ and ε > 0. From Lemma 9, we have
The following result follows immediately from Theorems 12 and 15 with N = 1 and α(x, y) = 1 for every x, y ∈ 𝔼.
Corollary 16. Let T : 𝔼 → 𝔼 be a self-mapping on the complete gauge space 𝔼. Let {ψλ} λ∈Λ ⊂ Ψ. Suppose that for all λ ∈ Λ, for all x, y ∈ 𝔼, one has
Corollary 17. Let T : 𝔼 → 𝔼 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 x0 ∈ 𝔼 such that α(x0, Tx0) ≥ 1;
- (iv)
T is continuous
Then T has a fixed point. Moreover, if
- (v)
for every (x, y) ∈ Fix (T) × Fix (T) with x ≠ y, there exists z ∈ 𝔼 such that α(x, z) ≥ 1 and α(z, y) ≥ 1,
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 : 𝔼 → 𝔼 be a self-mapping and {ψλ} λ∈Λ ⊂ Ψ. Suppose that the following conditions hold:
- (i)
for all λ ∈ Λ, for all x, y ∈ 𝔼 such that x and y are comparable, one has
() - (ii)
x, y ∈ 𝔼, x and y are comparable ⇒Tx and Ty are comparable;
- (iii)
there exists x0 ∈ 𝔼 such that x0 and Tx0 are comparable;
- (iv)
T is continuous,
Then T has a fixed point. Moreover, if
- (v)
for every (x, y) ∈ Fix (T) × Fix (T) with x ≠ y, there exists z ∈ 𝔼 such that x and z are comparable, z and y are comparable,
Proof. It follows from Corollary 17 with
3. Applications
We have the following existence result.
Theorem 19. Suppose that the following conditions hold:
- (i)
there exist a nonempty set Γ⊆ℰ × ℰ and a constant k < τ such that
()
- (ii)
there exists a nonempty set Γ𝔼⊆𝔼 × 𝔼 such that
() - (iii)
for all (x, y) ∈ Γ𝔼, one has
()
- (iv)
there exists x0 ∈ 𝔼 such that
()
- (v)
if {xp} ⊂ 𝔼 is a sequence such that (xp, xp+1) ∈ Γ𝔼 for p ∈ ℕ and xp → x ∈ 𝔼 (with respect to 𝒟), then there exist a subsequence {xp(k)} of {xp} and k0 ∈ ℕ such that
()
Then (42) has at least one solution in 𝔼.
Proof. Consider the mapping T : 𝔼 → 𝔼 defined by
Define the function α : 𝔼 × 𝔼 → [0, ∞) by
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 xp → x ∈ 𝔼, there exist a subsequence {xp(k)} of {xp} and k0 ∈ ℕ such that α(xp(k), x) ≥ 1 for k ≥ k0.
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 z ∈ 𝔼 such that (x, z) ∈ Γ𝔼 and (z, y) ∈ Γ𝔼.
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.