In this paper, we first introduce the concepts of d- and d−1-proximal Ćirić contraction mappings. Also, we present new definitions and notations by taking into account the lack of symmetry property of quasi-metric spaces. Moreover, we give some examples to support our definitions and notations. Then, we prove some right and left best proximity point results for these mappings on best orbitally complete quasi-metric spaces. Hence, we obtain some generalizations of famous results in the literature.
Citation: Mustafa Aslantas, Hakan Sahin, Raghad Jabbar Sabir Al-Okbi. Some best proximity point results on best orbitally complete quasi metric spaces[J]. AIMS Mathematics, 2023, 8(4): 7967-7980. doi: 10.3934/math.2023401
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In this paper, we first introduce the concepts of d- and d−1-proximal Ćirić contraction mappings. Also, we present new definitions and notations by taking into account the lack of symmetry property of quasi-metric spaces. Moreover, we give some examples to support our definitions and notations. Then, we prove some right and left best proximity point results for these mappings on best orbitally complete quasi-metric spaces. Hence, we obtain some generalizations of famous results in the literature.
A real valued function d defined on ℑ×ℑ that satisfies the axioms of a metric with the exception that the distance between distinct points is nonzero is known as a pseudo metric on a non-empty set ℑ. Numerous well-known results on metric space, such as the Baire category, Cantor intersection, and Banach fixed-point theorems, are applicable in this situation and are readily extended to pseudo-metric contexts. But, these extensions are not as simple as in the pseudo metric spaces, when the symmetry condition is eliminated. Despite this, a number of authors have focused on ignoring the symmetry constraint since unsymmetric distance functions have a wide range of applications in both mathematics and many other fields [1,2,3]. Wilson [4] introduced the idea of quasi-metric in this context for the first time. Then, Kelly [5] succeeded in generalizing a number of well-known conclusions, including the Baire category theorem and the Urysohn lemma, by taking into consideration biotopological spaces, which are intimately related to quasi-metric spaces. In the same paper, the Cauchy sequence for a quasi pseudo metric space has been also defined. Reilly et al. [6] pointed out that any convergent sequence might not be Cauchy according to Kelly's definition. They suggested a variety of definitions of the Cauchy sequence in a quasi-metric space to address this drawback. Categorizing concepts of Cauchyness and completeness, Altun et al. [7] recently established a few fixed-point theorems on quasi-metric spaces. Many intriguing and notable results can be found in the literature [8,9,10,11,12,13]. Now, we recall some notations and definitons in quasi-metric space. Let d:ℑ×ℑ→[0,∞) be a function where ℑ is a nonempty set. Consider the following conditions
(i) d(ˇs,ˇs)=0,
(ii) d(ˇs,z)≤d(ˇs,˘u)+d(˘u,z),
(iii) d(ˇs,˘u)=d(˘u,ˇs)=0⟺ˇs=˘u,
(iv) d(ˇs,˘u)=0⟺ˇs=˘u,
for all ˇs,˘u,z∈ℑ.
● If it meets the requirements (ⅰ) and (ⅱ), d is referred to as quasi-pseudometric on ℑ.
● If it meets the requirements (ⅱ) and (ⅲ), then d is referred to as quasi-metric on ℑ.
● If it meets the requirements (ⅱ) and (ⅳ), then d is referred to as T1-quasi-metric on ℑ.
It is evident that every quasi-metric is a quasi-pseudo metric, and that every T1-quasi-metric is a quasi-metric. Indeed, let ℑ=[0,∞) and d:ℑ×ℑ→R be a function defined by d(ˇs,˘u)=max{˘u−ˇs,0}. Then, (ℑ,d) is a quasi-metric space, but it is not a T1-quasi metric space. If we take ˇs=2 and ˘u=1, then we have d(2,1)=0, but 2≠1.
Let (ℑ,d) be a quasi-metric space and d−1:ℑ×ℑ→[0,∞) and ds:ℑ×ℑ→[0,∞) be mappings defined by
d−1(ˇs,˘u)=d(˘u,ˇs) |
and
ds(ˇs,˘u)=max{d(ˇs,˘u),d−1(ˇs,˘u)} |
for all ˇs,˘u∈ℑ. Then, d−1 is a quasi-metric (called a conjugate of d) and ds is an ordinary metric on ℑ. The subset Γ of ℑ is said to be d-open if for all ˇs∈Γ there exists r>0 such that
Bd(ˇs,r)={˘u∈ℑ:d(ˇs,˘u)<r}⊆Γ; |
the subset Γ of ℑ is said to be d−1-open if for all ˇs∈Γ there exists r>0 such that
Bd−1(ˇs,r)={˘u∈ℑ:d(˘u,ˇs)<r}⊆Γ. |
If τd (τd−1) denotes the family of all d-open subsets of ℑ (the family of all d−1-open subsets of ℑ), then τd (τd−1) is a T0-topology on ℑ. If d is a T1 -quasi-metric, then τd is a T1-topology on ℑ.
Let {ˇst} be sequence in ℑ. It is clear that the sequence {ˇst} converges to ˇs∈ℑ with respect to τd if and only if d(ˇs,ˇst)→0 as t→∞.
Now, we provide several definitions of Cauchyness and completeness in quasi metric spaces.
Definition 1 ([7]). Let {ˇst} be a sequence in a quasi-metric space (ℑ,d). A sequence {ˇst} is called
(i) a right K-Cauchy sequence if for every ε>0there exists t0∈N such that d(ˇsr,ˇst)<ε for all r≥t≥t0,
(ii) a left K-Cauchy sequence if for every ε>0there exists t0∈N such that d(ˇst,ˇsr)<ε for all r≥t≥t0.
Definition 2 ([14]). A quasi-metric space (ℑ,d) is called
(i) a right (left) d-complete if every right (left) K-Cauchy sequence is convergent to a point in ℑ with respect to d,
(ii) a right (left) d−1-complete if every right (left) K-Cauchy sequence is convergent to a point in ℑ with respect to d−1.
On the other hand, using the nonself mappings H:Γ→Λ where Γ and Λ are nonempty subsets of a metric space, different from the literature, the metric fixed-point theory has been developed. There is no fixed-point of the mapping H in the situation of Γ∩Λ=∅. In this situation, it makes sense to check to see if there is a point ˇs in Γ such that d(ˇs,Hˇs)=d(Γ,Λ), known as the best proximity point of H. Thus, Basha and Veeramani [15] demonstrated various best proximity point results for multivalued mappings and derived an optimal solution to the minimization problem minˇs∈Γd(ˇs,Hˇs). Many authors have recently explored this subject because the best proximity point theory incorporates the fixed-point theory in a particular situation Γ=Λ=ℑ [16,17,18,19,20,21,22,23,24]. Now, we present some notations and definitions about the best proximity point theory. Let Γ and Λ be subsets of a metric space (ℑ,d). Then, consider the following sets:
Γ0={ˇs∈Γ:d(ˇs,˘u)=d(Γ,Λ) for some ˘u∈Λ}, |
and
Λ0={˘u∈Λ:d(ˇs,˘u)=d(Γ,Λ) for some ˇs∈Γ}. |
Definition 3. [25] Let (ℑ,d) be a metric space, H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ and ˇs∈Γ. Then, the set of iterative sequences
OH(ˇs)={{ˇst}⊆Γ:ˇs0=ˇs and d(ˇst+1,Hˇst)=d(Γ,Λ) for all t∈N} |
is called the orbit of ˇs.
Note that, when Γ=Λ=ℑ in Definition 3, it becomes
OH(ˇs)={{Htˇs}:t∈N}. |
Definition 4. [25] Let (ℑ,d) be a metric space and H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ. If for each ˇs∈Γ and {ˇst}∈OH(ˇs) the implication
ˇsti→ˇs∗⇒Hˇsti→Hˇs∗ as i→∞ |
holds for any subsequence {ˇsti} of {ˇst}, then H is called best orbitally continuous at a point ˇs∗∈Γ.
Definition 5. [25] Let (ℑ,d) be a metric space and ∅≠Γ,Λ⊆ℑ. Assume that H:Γ→Λ and g:Γ→R are two mappings. We say that g is best orbitally lower semicontinuous at ˇs∗ in Γ if for each ˇs∈Γ and {ˇst}∈OH(ˇs) the implication
ˇsti→ˇs∗ as i→∞⇒g(ˇs∗)≤liminfi→∞g(ˇsti) |
holds for any subsequence {ˇsti} of {ˇst}.
Definition 6. [25] Let (ℑ,d) be a metric space, ∅≠Γ,Λ⊆ℑ and H:Γ→Λ be a mapping. If for all ˇs∈Γ and {ˇst} in OH(ˇs), every Cauchy subsequence {ˇsti} of {ˇst} converges to a point in Γ0, then Γ is said to be H-best orbitally complete.
In this paper, we first introduce the concepts of d- and d−1-proximal Ćirić contraction mappings. Then, we obtain some right and left best proximity point results for these mappings in best orbitally complete quasi metric spaces. Also, we present new definitions and notations by taking into account the lack of symmetry property of quasi-metric spaces. Moreover, we give some examples to support our definitions and notations.
First, we recall some notations and definitions about best proximity points in a quasi-metric space. Let (ℑ,d) be a quasi-metric space and Γ,Λ⊆ℑ. Then, consider the following sets:
Γℓ0={ˇs∈Γ:d(ˇs,˘u)=d(Γ,Λ) for some ˘u∈Λ}, |
Γr0={ˇs∈Γ:d(˘u,ˇs)=d(Λ,Γ) for some ˘u∈Λ}, |
and
Λℓ0={˘u∈Λ:d(ˇs,˘u)=d(Γ,Λ) for some ˇs∈Γ}, |
Λr0={˘u∈Λ:d(˘u,ˇs)=d(Λ,Γ) for some ˇs∈Γ}. |
Definition 7. [25] Let (ℑ,d) be a quasi-metric space, ∅≠Γ,Λ⊆ℑ and H:Γ→Λ be a mapping. A point ˇs is called a right (left) best proximity point of H if d(Hˇs,ˇs)=d(Λ,Γ) (d(ˇs,Hˇs)=d(Γ,Λ)).
Now, we present some definitions.
Definition 8. Let (ℑ,d) be a quasi-metric space, H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ and ˇs∈Γ. Then, the set of iterative sequences
OρH(ˇs)={{ˇst}⊆Γ:ˇs0=ˇs and ρ(ˇst+1,Hˇst)=ρ(Γ,Λ) for all t∈N} |
is called the ρ(d or d−1)-orbit of ˇs.
Definition 9. Let (ℑ,d) be a quasi-metric space and H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ. We say that Γ is
i) H-best ρ-orbitally right d-complete if for all ˇs∈Γ and {ˇst} in OρH(ˇs), every right K-Cauchy subsequence {ˇsti} of {ˇst} converges to a point in Γr0 with respect to d.
iii) H-best ρ-orbitally left d-complete if for all ˇs∈Γ and {ˇst} in OρH(ˇs), every left K-Cauchy subsequence {ˇsti} of {ˇst} converges to a point in Γℓ0 with respect to d.
Definition 10. Let (ℑ,d) be a quasi-metric space and H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ. Then, a function g:Γ→R is called
i) d-best ρ-orbitally lower semicontinuous at a point ˇs∗∈Γ if for each ˇs∈Γ and {ˇst} in OρH(ˇs), the implication
ˇstid⟶ˇs∗ as i→∞⇒g(ˇs∗)≤limi→∞infg(ˇsti) |
holds for any subsequence {ˇsti} of {ˇst}.
ii) d−1-best ρ-orbitally lower semicontinuous at a point ˇs∗∈Γ if for each ˇs∈Γ and {ˇst} in OρH(ˇs), the implication
ˇstid−1⟶ˇs∗ as i→∞⇒g(ˇs∗)≤limi→∞infg(ˇsti) |
holds for any subsequence {ˇsti} of {ˇst}.
Now, we present the definition of a d-proximal Ćirić-type contraction.
Definition 11. Let (ℑ,d) be a quasi-metric space and ∅≠Γ,Λ⊆ℑ. A mapping H:Γ→Λ is said to be a d -proximal Ćirić-type contraction if there exists k∈[0,1) such that
d(u1,Hˇs1)=d(Γ,Λ)d(u2,Hˇs2)=d(Γ,Λ)} |
implies
d(u1,u2)≤kMd(ˇs1,ˇs2,u1,u2) | (2.1) |
for all u1,u2,ˇs1,ˇs2∈Γ where
Md(ˇs1,ˇs2,u1,u2)=max{d(ˇs1,ˇs2),d(ˇs1,u1),d(ˇs2,u2),12(d(ˇs1,u2)+d(ˇs2,u1))}. |
Now, we present the following theorem which is our first main result.
Theorem 1. Let (ℑ,d) be a quasi-metric space, ∅≠Γ,Λ⊆ℑ, Γr0≠∅ and H:Γ→Λ be a d−1-proximal Ćirić-type contraction satisfying H(Γr0)⊆Λr0. If Γ is H-best d−1-orbitally right d−1-complete and a function g:Γ→R given as g(ˇs)=d(Hˇs,ˇs) is d−1-best d−1 -orbitally lower semicontinuous on Γ, then H has a right best proximity point in Γ.
Proof. Let ˇs0∈Γr0 be any given point. Since Hˇs0∈H(Γr0)⊆Λr0, there exists ˇs1∈Γr0 such that
d(Hˇs0,ˇs1)=d(Λ,Γ). |
Similarly, there exists ˇs2∈Γr0 such that
d(Hˇs1,ˇs2)=d(Λ,Γ). |
Continuing this process, we can construct a sequence {ˇst} such that
d(Hˇst,ˇst+1)=d(Λ,Γ) | (2.2) |
for all t∈N, that is, {ˇst}∈Od−1H(ˇs0). If there exists t0∈N such that ˇst0=ˇst0+1, then the proof is complete. Then, we suppose that ˇst≠ˇst+1 for all t≥1. Due to the fact that H is a d−1-proximal Ćirić-type contraction, we have
d(ˇst+1,ˇst)≤kMd−1(ˇst−1,ˇst,ˇst,ˇst+1) |
where
Md−1(ˇst−1,ˇst,ˇst,ˇst+1)=max{d(ˇst,ˇst−1),d(ˇst,ˇst−1),d(ˇst+1,ˇst),12(d(ˇst+1,ˇst−1)+d(ˇst,ˇst))}=max{d(ˇst,ˇst−1),d(ˇst+1,ˇst),12d(ˇst+1,ˇst−1)}≤max{d(ˇst,ˇst−1),d(ˇst+1,ˇst),12(d(ˇst+1,ˇst)+d(ˇst,ˇst−1)}=max{d(ˇst,ˇst−1),d(ˇst+1,ˇst)} |
for all t≥1. If Md−1(ˇst0−1,ˇst0,ˇst0,ˇst0+1)=d(ˇst0+1,ˇst0) for some t0∈N, then we have
d(ˇst0+1,ˇst0)≤kd(ˇst0+1,ˇst0)<d(ˇst0+1,ˇst0) |
which is a contradiction. Then, we get Md−1(ˇst−1,ˇst,ˇst,ˇst+1)=d(ˇst,ˇst−1) for all t≥1.
d(ˇst+1,ˇst)≤kd(ˇst,ˇst−1) |
for all t≥1. Therefore, we have
d(ˇst+1,ˇst)≤kd(ˇst,ˇst−1)≤k2d(ˇst−1,ˇst−2)⋮≤ktd(ˇs1,ˇs0) |
for all t∈N. Then, we have
limt→∞d(ˇst+1,ˇst)=0. | (2.3) |
Then, we get, for all r>t,
d(ˇsr,ˇst)=d(ˇsr,ˇsr−1)+d(ˇsr−1,ˇsr−2)+⋯+d(ˇst+1,ˇst)≤kr−1d(ˇs1,ˇs0)+kr−2d(ˇs1,ˇs0)+⋯+ktd(ˇs1,ˇs0)=ktd(ˇs1,ˇs0)(1+k+⋯+kr−t−1)=ktd(ˇs1,ˇs0)1−kr−t1−k≤ktd(ˇs1,ˇs0)1−k, |
and so {ˇst} is a right K-Cauchy sequence in Γ. Then, there exists ˇs∗∈Γr0 such that d(ˇst,ˇs∗)⟶0 as t→∞, since Γ is H-best d−1-orbitally right d−1-complete. On the other hand, from (2.2) and (2.3) we have
d(Λ,Γ)≤d(Hˇst,ˇst)≤d(Hˇst,ˇst+1)+d(ˇst+1,ˇst)=d(Λ,Γ)+d(ˇst+1,ˇst) |
for all t∈N. For the limit as t→∞, we have
limt→∞d(Hˇst,ˇst)=d(Λ,Γ). | (2.4) |
Then, since g is d−1-best d−1-orbitally lower semicontinuous on Γ, we have
d(Λ,Γ)≤d(Hˇs∗,ˇs∗)=g(ˇs∗)≤limt→∞infg(ˇst)=limt→∞infd(Hˇst,ˇst)=d(Λ,Γ), |
and so we get d(Hˇs∗,ˇs∗)=d(Λ,Γ). Hence, ˇs∗ is a right best proximity point of H.
The following example is given to show the effectiveness of Theorem 1.
Example 1. Let ℑ=[0,∞)×[0,∞) and d:ℑ×ℑ→R be a function defined by
d(ˇs,˘u)={0,ˇs=˘u2ˇs1+˘u1+|ˇs2−˘u2|,ˇs≠˘u |
for all ˇs=(ˇs1,ˇs2),˘u=(˘u1,˘u2)∈ℑ. Hence, (ℑ,d) is a quasi-metric space. Consider the sets Γ={0}×[0,∞), Λ={1}×[0,∞) and a mapping H:Γ→Λ given as H(0,ˇs)=(1,ˇs3). Then, we have d(Λ,Γ)=2, and so we get
Od−1H((0,ˇs))={{(0,ˇst)}⊆Γ:(0,ˇs)=(0,ˇs0) andd(H(0,ˇst),(0,ˇst+1))=d(Λ,Γ) for all t∈N}={{(0,ˇst)}⊆Γ:ˇs=ˇs0 andd((1,ˇst3),(0,ˇst+1))=2 for all t∈N}={{(0,ˇst)}⊆Γ:ˇs=ˇs0 and ˇst+1=ˇst3 for all t∈N}={{(0,ˇs3t)}⊆Γ:t∈N}. |
Also, we obtain Γr0=Γ, Λr0=Λ and H(Γr0)⊆Λr0. Let for all ˇs∈Γ and {ˇst} in Od−1H((0,ˇs)), {ˇsti} be a right K-Cauchy subsequence of {ˇst}. Since every sequence in Od−1H((0,ˇs)) is convergent with respect to d−1, the subsequence {ˇsti} is convergent to (0,0) with respect to d−1. Hence, we have that ˇstid−1→(0,0)∈Γ. Then, Γ is H-best d−1 -orbitally right d−1complete. It is clear that a mapping g:Γ→R given as g(ˇs)=d(Hˇs,ˇs) is d−1-best d−1 -orbitally lower semicontinuous on Γ. Now, we will show that H is a d−1-proximal Ćirić-type contraction for k=13. Let ˇs1,ˇs2,u1,u2∈Γ satisfying
d(Hˇs1,u1)=d(Λ,Γ)d(Hˇs2,u2)=d(Λ,Γ). |
Hence, we have that ˇs1=(0,a),ˇs2=(0,b),u1=(0,a3) and u2=(0,b3) where a,b∈[0,∞). Then,
d(u1,u2)=|a3−b3|=13d(ˇs1,ˇs2)≤13Md(ˇs1,ˇs2,u1,u2). |
Therefore, all conditions of Theorem 1 hold. Hence, H is a right best proximity point ˇs∗ in Γ which is ˇs∗=(0,0).
Now, we give the following definition.
Definition 12. Let (ℑ,d) be a quasi-metric space and H:Γ→Λ be a mapping where ∅≠Γ,Λ⊆ℑ. We say that H is d-d-best ρ (d or d−1)-orbitally continuous at a point ˇs∗∈Γ if for every ˇs∈Γ and {ˇst} in OρH(ˇs), the implication
ˇstid⟶ˇs∗⇒Hˇstid⟶Hˇs∗, as i→∞ |
holds for any subsequence {ˇsti} of {ˇst}.
The following example is important to better understand Definition 12.
Example 2. Let ℑ=[0,∞)×[0,∞) and d:ℑ×ℑ→R be a function defined as
d(ˇs,˘u)=max{˘u1−ˇs1,0}+|ˇs2−˘u2| |
for all ˇs=(ˇs1,ˇs2),˘u=(˘u1,˘u2)∈ℑ. Then, (ℑ,d) is a quasi-metric space. Consider the sets Γ={(1+1t,0):t∈N}∪{(1,0)}, Λ={(1t,1):t∈N} and a mapping H:Γ→Λ given as
Hˇs={(12,1)ˇs=(1,0)(1t+1,1)ˇs=(1+1t,0). |
Then, we have that d(Γ,Λ)=1, and so we get that OdH(ˇs) is the set of all sequences in Γ. Also, it can be seen that the sequence {ˇst} in OdH(ˇs) converges to (1,0) with respect to d−1. Also, we have
limt→∞d(Hˇs,Hˇst)=limt→∞d((12,1),(1t+1,1))=limt→∞max{1t+1−12,0}=0. |
Therefore, H is d−1-d-best d-orbitally continuous on Γ. But, we have
limt→∞d(Hˇst,Hˇs)=limt→∞d((1t+1,1),(12,1))=limt→∞max{12−1t+1,0}=12. |
Hence, H is not d−1-d−1-best d-orbitally continuous on Γ.
Proposition 1. Let (ℑ,d) be a quasi-metric space and ∅≠Γ,Λ⊆ℑ. Assume that H:Γ→Λ is a mapping and g:Γ→R is a function given as g(ˇs)=d(ˇs,Hˇs). Then, the following statements are true.
i) If H is d-d−1-best ρ-orbitally continuous at a point ˇs∗∈Γ, then g is d-best ρ -orbitally lower semicontinuous at a point ˇs∗∈Γ.
ii) If H is d−1-d-best ρ-orbitally continuous at a point ˇs∗∈Γ, then g is d−1-best ρ -orbitally lower semicontinuous at a point ˇs∗∈Γ.
Proof. Let ˇs∈Γ, {ˇst} be a sequence in OρH(ˇs) and {ˇsti} be a subsequence of {ˇst} such that d(ˇs∗,ˇsti)→0 as i→∞. Since H is d-d−1-best ρ-orbitally continuous on Γ, we have that d(Hˇsti,Hˇs∗)→0 as i→∞. Hence, we get
g(ˇs∗)=d(ˇs∗,Hˇs∗)≤d(ˇs∗,ˇsti)+d(ˇsti,Hˇsti)+d(Hˇsti,Hˇs∗). |
Taking the limit inferior as i→∞, we obtain
g(ˇs∗)≤limi→∞infd(ˇsti,Hˇsti)=limi→∞infg(ˇsti). |
The proof of (ⅰ) is complete. We can prove ⅱ) in a way that is similar to the above one.
The converse of Proposition 1 may not be true. We give an example to demonstrate this fact.
Example 3. Let ℑ=[0,∞)×[0,∞) and d:ℑ×ℑ→R be a function defined by
d(ˇs,˘u)={0,ˇs=˘uˇs1+|ˇs2−˘u2|,ˇs≠˘u |
for all ˇs=(ˇs1,ˇs2),˘u=(˘u1,˘u2)∈ℑ. Then, (ℑ,d) is a quasi-metric space. Consider the sets Γ={0}×[0,∞), Λ={1}×[0,∞) and a mapping H:Γ→Λ given as H(0,ˇs)=(1,ˇs2). Then, we have that d(Γ,Λ)=0, and so we get
OdH((0,ˇs))={{(0,ˇst)} in Γ:(0,ˇs)=(0,ˇs0) andd((0,ˇst+1),H(0,ˇst))=d(Γ,Λ) for all t∈N}={{(0,ˇst)} in Γ:ˇs=ˇs0 andd((0,ˇst+1),(1,ˇst2))=0 for all t∈N}={{(0,ˇst)} in Γ:ˇs=ˇs0 and ˇst+1=ˇst2 for all t≥1}={{(0,ˇs2t)} in Γ:t≥1}. |
Now, we will show that a mapping g:Γ→R given as g(ˇs)=d(ˇs,Hˇs) is d-best d -orbitally lower semicontinuous. Let {ˇsti} be a convergent subsequence of {ˇst} in OdH((0,ˇs)) with respect to d. From the definition of d, it can be seen that every sequence in OdH((0,ˇs)) converges to (0,0) with respect to d. Thus, we have that ˇstid→(0,0). Hence, we have
g((0,0))=d((0,0),H(0,0))=0=limi→∞infg(ˇsti), |
and so g is d-best d-orbitally lower semicontinuous. Also, we can show that g is d−1-best d-orbitally lower semicontinuous. However, H is not d-d−1-best d-orbitally continuous. Indeed, for ˇs=(0,1)∈Γ, we have
OdH((0,ˇs))={(0,12t)} :t∈N)}. |
Hence, if we take {ˇst}={(0,12t)} in OdH((0,ˇs)), then although we have ˇstd→0, we get
d(Hˇst,Hˇs)=d((1,ˇs2t+1),(1,0))=1+ˇs2t+1 |
which implies that
limt→∞d(Hˇst,Hˇs)=1. |
Hence, the sequence {Hˇst} is not convergent to Hˇs with respect to d−1; so, H is not d-d−1-best d-orbitally continuous on Γ. It can be seen that H is not d-d-best d -orbitally continuous, d−1-d-best d-orbitally continuous or d−1-d−1-best d-orbitally continuous.
Now, we present the following result by using Proposition 1.
Corollary 1. Let (ℑ,d) be a quasi-metric space, ∅≠Γ,Λ⊆ℑ, Γr0≠∅ and H:Γ→Λ be a d−1-proximal Ćirić-type contraction satisfying H(Γr0)⊆Λr0. Then, H has a right best proximity point in Γ provided that Γ is H-best d−1-orbitally right d−1-complete and H is d−1-d-best d−1-orbitally continuous on Γ.
If we take Γ=Λ=ℑ in Theorem 1 and Corollary 1, then we present the following fixed-point results which are generalizations of [26] in T1-quasi-metric spaces.
Corollary 2. Let (ℑ,d) be a T1-quasi-metric space and H:ℑ→ℑ be a d−1-proximal Ćirić-type contraction. If ℑ is H -best d−1-orbitally right d−1-complete and a function g:ℑ→R given as g(ˇs)=d(Hˇs,ˇs) is d−1-best d−1 -orbitally lower semicontinuous on ℑ, then H has a fixed-point in ℑ.
Corollary 3. Let (ℑ,d) be a T1-quasi-metric space and H:ℑ→ℑ be a d−1-proximal Ćirić-type contraction. If ℑ is H -best d−1-orbitally right d−1-complete and H is d−1-d -best d−1-orbitally continuous on ℑ, then H has a fixed-point in ℑ.
Now, we present some left best proximity point results.
Theorem 2. Let (ℑ,d) be a quasi-metric space, ∅≠Γ,Λ⊆ℑ, Γℓ0≠∅ and H:Γ→Λ be a d-proximal Ćirić-type contraction mapping satisfying H(Γℓ0)⊆Λℓ0. If Γ is H-best d-orbitally left d−1-complete and a function g:Γ→R given as g(ˇs)=d(ˇs,Hˇs) is d−1-best d -orbitally lower semicontinuous on Γ, then H has a left best proximity point in Γ.
Proof. Let ˇs0∈Γℓ0 be an arbitrary point. Since Hˇs0∈H(Γℓ0)⊆Λℓ0, there exists ˇs1∈Γℓ0 such that
d(ˇs1,Hˇs0)=d(Γ,Λ). |
Similarly, there exists ˇs2∈Γl0 such that
d(ˇs2,Hˇs1)=d(Γ,Λ). |
Continuing this process, we can construct a sequence {ˇst} such that
d(ˇst+1,Hˇst)=d(Γ,Λ) | (2.5) |
for all t∈N, that is, {ˇst}∈OdH(ˇs0). If there exists t0∈N such that ˇst0=ˇst0+1, then the proof is complete. So, we assume that ˇst≠ˇst+1 for all t∈N. Similar to the proof of Theorem 1, we can obtain that
limt→∞d(ˇst,ˇst+1)=0 | (2.6) |
and the sequence {ˇst} is a left K-Cauchy sequence. There exists ˇs∗∈Γℓ0 such that d(ˇst,ˇs∗)⟶0 as t→∞, due to the fact that Γ is H-best d-orbitally left d−1-complete. On the other hand, from (2.5),
d(Γ,Λ)≤d(ˇst,Hˇst)≤d(ˇst,ˇst+1)+d(ˇst+1,Hˇst)=d(ˇst,ˇst+1)+d(Γ,Λ) |
for all t∈N. For the limit as t→∞, we have, from (2.6),
limt→∞d(ˇst,Hˇst)=d(Γ,Λ). |
Then, since g is d−1-best d-orbitally lower semicontinuous on Γ, we have
d(Γ,Λ)≤d(ˇs∗,Hˇs∗)=g(ˇs∗)≤limt→∞infg(ˇst)=limt→∞infd(ˇst,Hˇst)=d(Γ,Λ), |
and so we get that d(ˇs∗,Hˇs∗)=d(Γ,Λ). Hence, ˇs∗ is a left best proximity point of H.
Now, we present the following result by using Proposition 1.
Corollary 4. Let (ℑ,d) be a quasi-metric space, ∅≠Γ,Λ⊆ℑ, Γℓ0≠∅ and H:Γ→Λ be a d-proximal Ćirić-type contraction satisfying H(Γℓ0)⊆Λℓ0. Then, H has a right best proximity point in Γ provided that Γ is H -best d-orbitally left d−1-complete and H is d−1-d-best d -orbitally continuous on Γ.
Fixed point theory is an exciting area of research for metric spaces and generalized metric spaces. Fixed-point theorems are mainly useful when dealing with problems that arise in the theory of the existence of differential equations, integral equations, partial differential equations, dynamic programming, fractal modeling, chaos theory and various other disciplines of mathematics, statistics, engineering, economics and approximation theory. Also, best proximity point results are a generalization of the corresponding fixed-point results. Therefore, we aimed to extend some results existing in the literature with the aid of best proximity point theory for best orbitally complete quasi-metric spaces. We first presented new definitions and notations by taking into account the lack of symmetry property of quasi-metric spaces. Moreover, we gave some examples to support our definitions and notations. Hence, many researchers can extend some best proximity point and fixed-point results obtained with the help of nonlinear functions in different metric spaces to quasi-metric spaces by using these definitions in future works. We also introduced the concepts of d- and d−1-proximal Ćirić contraction mappings. Then, we proved some right and left best proximity point results for these mappings on best orbitally complete quasi-metric spaces. Therefore, we obtained some generalizations of the famous fixed-point and best proximity point results in the literature.
The authors are thankful to the referees for making valuable suggestions leading to a better presentation of the paper.
All authors declare no conflicts of interest regarding the publication of this paper.
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1. | Arshad Ali Khan, Basit Ali, Reny George, On semi best proximity points for multivalued mappings in quasi metric spaces, 2023, 8, 2473-6988, 23835, 10.3934/math.20231215 |