
We study the Euler's totient function (called also the Euler's phi function) in the framework of finite complete hypergroups. These are algebraic hypercompositional structures constructed with the help of groups, and endowed with a multivalued operation, called hyperoperation. On them the Euler's phi function is multiplicative and not injective. In the second part of the article we find a relationship between the subhypergroups of a complete hypergroup and the subgroups of the group involved in the construction of the considered complete hypergroup. As sample application of this connection, we state a formula that relates the Euler's totient function defined on a complete hypergroup to the same function applied to its subhypergroups.
Citation: Andromeda Sonea, Irina Cristea. Euler's totient function applied to complete hypergroups[J]. AIMS Mathematics, 2023, 8(4): 7731-7746. doi: 10.3934/math.2023388
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We study the Euler's totient function (called also the Euler's phi function) in the framework of finite complete hypergroups. These are algebraic hypercompositional structures constructed with the help of groups, and endowed with a multivalued operation, called hyperoperation. On them the Euler's phi function is multiplicative and not injective. In the second part of the article we find a relationship between the subhypergroups of a complete hypergroup and the subgroups of the group involved in the construction of the considered complete hypergroup. As sample application of this connection, we state a formula that relates the Euler's totient function defined on a complete hypergroup to the same function applied to its subhypergroups.
In the past few decades, fixed point theory was developed by a large number of authors, especially in metric spaces, which can be observed in [1,2,3,4,5,6]. In 1993, Czerwik [7] initiated the concept of b-metric spaces. Later, many authors proved fixed point theorems in b-metric spaces [8,9,10]. However, the general metric notion was introduced by Branciari [11] in 2000, the so-called Branciari metric. The notion of generalization of Branciari b-metric spaces was introduced by George et al. [12] in 2015. Johnsonbaugh [13] explored certain fundamental mathematical principles, including foundational topics relevant to fixed-point theory and discrete structures, which underpin many concepts in fixed-point applications. Younis et al. [14] introduced graphical rectangular b-metric space and proved fixed point theorem. Younis et al. [15] presented graphical b-metric space and proved fixed point theorem. Younis et al. [16] presented graphical extended b-metric space and proved fixed point theorem. Younis et al. [17] proved fixed points results using graphical B c-Kannan-contractions by numerical iterations within the structure of graphical extended b-metric spaces. Younis et al. [18] presented a fixed point result for Kannan type mappings, in the framework of graphical b-metric spaces. Younis et al. [19] introduced the notion of controlled graphical metric type spaces and proved the fixed point theorem. Haroon Ahmad et al. [20] developed the graphical bipolar b-metric space and proved the fixed point theorem.
The following preliminary is given for better understanding by the readers.
Let (Υ,ϱ) be a metric space. Let Δ denote the diagonal of the Cartesian product Υ×Υ. Consider a directed graph Ω such that the set V(Ω) of its vertices coincides with Υ, and the set E(Ω) of its edges contains all loops, i.e., E(Ω)⊇Δ. We assume Ω has no parallel edges, so we can identify Ω with the pair (V(Ω),E(Ω)). Moreover, we may treat Ω as a weighted graph (see [13], p.309) by assigning to each edge the distance between its vertices. By Ω−1, we denote the conversion of a graph Ω, i.e., the graph obtained from Ω by reversing the direction of edges. Thus, we have
E(Ω−1)={(ϑ,σ)|(σ,ϑ)∈Ω}. |
The letter ˜Ω denotes the undirected graph obtained from Ω by ignoring the direction of edges. Actually, it will be more convenient for us to treat ˜Ω as a directed graph for which the set of its edges is symmetric. Under this convention,
E(˜Ω)=E(Ω)∪E(Ω−1). | (1.1) |
We call (V′,E′) a subgraph of Ω if V′⊆V(Ω),E′⊆E(Ω) and, for any edge (ϑ,σ)∈E′,ϑ,σ∈V′.
If ϑ and σ are vertices in a graph Ω, then a path in Ω from ϑ to σ of length r(r∈N) is a sequence (ϑi)ri=0 of r+1 vertices such that ϑ0=ϑ,ϑr=σ and (ϑζ−1,ϑζ)∈E(Ω) for i=1,.....,r. A graph Ω is connected if there is a path between any two vertices. Ω is weakly connected if, treating all of its edges as being undirected, there is a path from every vertex to every other vertex. More precisely, Ω is weakly connected if ˜Ω is connected.
We define a relation P on Υ by: (ϑPσ)Ω if and only if there is a directed path from ϑ to σ in Ω. We write η,κ∈(ϑPσ)Ω if η,κ is contained in some directed path from ϑ to σ in Ω. For l∈N, we denote
[ϑ]lΩ={σ∈Υ:there is a directed path from ϑtoσof lengthl}. |
A sequence {ϑζ} in Υ is said to be Ω-term wise connected if (ϑζPσζ) for all ζ∈N. Further details one can see [21,22,23,24,25].
Definition 1.1. Let Υ be a nonempty set endowed with a graph Ω, ℵ≥1 and ϱ:Υ×Υ⟶[0,+∞) satisfy the assumptions below for every ϑ,σ∈Υ:
(T1) ϱ(ϑ,σ)=0 if and only if ϑ=σ;
(T2) ϱ(ϑ,σ)=ϱ(σ,ϑ);
(T3) (ϑPσ)Ω, η,φ∈(ϑPσ)Ω implies ϱ(ϑ,σ)≤ℵ[ϱ(ϑ,φ)+ϱ(φ,ω)+ϱ(ω,σ)] for all distinct points φ,ω∈Υ/{ϑ,σ}.
In this case, the pair (Υ,ϱ) is called a graphical Branciari ℵ-metric space with constant ℵ≥1.
Example 1.1. Let Υ=B∪U, where B={0,12,13,14} and U=[1,2]. Define the graphical Branciari ℵ-metric space ϱ:Υ×Υ⟶[0,+∞) as follows:
{ϱ(ϑ,σ)=ϱ(σ,ϑ)for allϑ,σ∈Υ,ϱ(ϑ,σ)=0⟺ϑ=σ. |
and
{ϱ(0,12)=ϱ(12,13)=0.2,ϱ(0,13)=ϱ(13,14)=0.02,ϱ(0,14)=ϱ(12,14)=0.5,ϱ(ϑ,σ)=|ϑ−σ|2,otherwise. |
equipped with the graph Ω=(V(Ω),E(Ω)) so that Υ=V(Ω) with E(Ω)).
It can be seen that the above Figure 1 depicts the graph given by Ω=(V(Ω),E(Ω)).
Definition 1.2. Let {ϑζ} be a sequence in a graphical Branciari ℵ-metric space (Υ,ϱ). Then,
(S1) {ϑζ} converges to ϑ∈Υ if, given ϵ>0, there is ζ0∈N so that ϱ(ϑζ,ϑ)<ϵ for each ζ>ζ0. That is, limζ→∞ϱ(ϑζ,ϑ)=0.
(S2) {ϑζ} is a Cauchy sequence if, for ϵ>0, there is ζ0∈N so that ϱ(ϑζ,ϑm)<ϵ for all ζ,m>ζ0. That is, limζ,m→∞ϱ(ϑζ,ϑm)=0.
(S3) (Υ,ϱ) is complete if every Cauchy sequence in Υ is convergent in Υ.
Definition 1.3. (see [8]) A function Q:(0,+∞)⟶R belongs to F if it satisfies the following condition:
(F1) Q is strictly increasing;
(F2) There exists k∈(0,1) such that limϑ→0+ϑkQ(ϑ)=0.
In [8], the authors omitted Wardowski's (F2) condition from the above definition. Explicitly, (F2) is not required, if {αζ}ζ∈N is a sequence of positive real numbers, then limζ→+∞αζ=0 if and only if limζ→+∞Q(αζ)=−∞. The reason for this is the following lemma.
Lemma 1.1. If Q:(0,+∞)⟶R is an increasing function and {αζ}ζ∈N⊂(0,+∞) is a decreasing sequence such that limζ→+∞Q(αζ)=−∞, then limζ→+∞αζ=0.
We can also see some properties concerning Qℵ,ℓ and Q′ℵ,ℓ.
Definition 1.4. (see [9]) Let ℵ≥1 and ℓ>0. We say that Q∈F belongs to Fℵ,ℓ if it also satisfies (Qℵℓ) if infQ=−∞ and ϑ,σ∈(0,∞) are such that ℓ+Q(ℵϑ)≤Q(σ) and ℓ+Q(ℵσ)≤Q(η), then
ℓ+Q(ℵ2ϑ)≤Q(ℵσ). |
In [10], the authors introduced the following condition (F4).
(Q′ℵℓ) if {αζ}ζ∈N⊂(0,+∞) is a sequence such that ℓ+Q(ℵαζ)≤Q(αn−1), for all ζ∈N and for some ℓ≥0, then ℓ+Q(ℵζαζ)≤Q(ℵn−1αn−1), for all ζ∈N∗.
Proposition 1.1. (see [8]) If Q is increasing, then (Fℵℓ) is equivalent to (F′ℵℓ).
Definition 4.5. Let (Υ,ϱ) be a graphical Branciari ℵ-metric space. We say that a mapping Π:Υ→Υ is a Ω-Q-contraction if
(A1) Π preserves edges of Ω, that is, (ϑ,σ)∈E(Ω) implies (Πϑ,Πσ)∈E(Ω);
(A2) There exists ℓ>0 and Q∈Fℵ,ℓ, such that
∀ϑ,σ∈Υ,(ϑ,σ)∈E(Ω),ϱ(Πϑ,Πσ)>0⇒ℓ+Q(ℵϱ(Πϑ,Πσ))≤Q(ϱ(ϑ,σ)). |
Chen, Huang, Li, and Zhao [24], proved fixed point theorems for Q-contractions in complete Branciari b-metric spaces. The aim of this paper is to study the existence of fixed point theorems for Q-contractions in complete Branciari b-metric spaces endowed with a graph Ω by introducing the concept of Ω-Q-contraction.
Theorem 2.1. Let (Υ,ϱ) be a complete graphical Branciari ℵ-metric space and Q∈Fℵ,ℓ. Let Π:Υ⟶Υ be a self mapping such that
(C1) there exists ϑ0∈Υ such that Πϑ0∈[ϑ0]lΩ, for some l∈N;
(C2) Π is a Ω-Q-contraction.
Then Π has a unique fixed point.
Proof. Let ϑ0∈Υ be such that Πϑ0∈[ϑ0]lΩ, for some l∈N, and {ϑζ} be the Π-Picard sequence with initial value ϑ0. Then, there exists a path {σi}li=0 such that ϑ0=σ0, Πϑ0=σl and (σi−1,σi)∈E(Ω) for i=1,2,3....l. Since Π is a Ω-Q-contraction, by (A1), (Πσi−1,Πσi)∈E(Ω) for i=1,2,3...l. Therefore, {Πσi}li=0 is a path from Πσ0=Πϑ0=ϑ1 to Πσ1=ρ2ϑ0=ϑ2 of length l, and so ϑ2∈[ϑ1]lΩ. Continuing this process, we obtain that Πζσi}li=0, is a path from Πζσ0=Πζϑ0=ϑζ to Πζσl=ΠζΠϑ0=ϑζ+1 of length l, and so, ϑζ+1∈[ϑζ]lΩ, for all ζ∈N. Thus {ϑζ} is a Ω-term wise connected sequence. For any ϑ0∈Υ, set ϑζ=Πϑζ−1,γζ=ϱ(ϑζ+1,ϑζ), and βζ=ϱ(ϑζ+2,ϑζ) with γ0=ϱ(ϑ1,ϑ0) and β0=ϱ(ϑ2,ϑ0). Now, we consider the following two cases:
(E1) If there exists ζ0∈N∪{0} such that ϑζ0=ϑζ0+1, then we have Πϑζ0=ϑζ0. It is clear that ϑζ0 is a fixed point of Π. Therefore, the proof is finished.
(E2) If ϑζ≠ϑζ+1, for any ζ∈N∪{0}, then we have γζ>0, for each ζ∈N.
ℓ+Q(ℵϱ(Πϑζ,Πϑζ+1))≤Q(ϱ(ϑζ,ϑζ+1)),Q(ℵγζ+1)≤Q(γζ)−ℓ, for every ζ∈N. |
By proposition 1.1, we obtain
Q(ℵζ+1γζ+1)≤Q(ℵζγζ)−ℓ, ∀ζ∈N. | (2.1) |
Furthermore, for any ζ∈N, we have
Q(ℵζγζ)≤Q(ℵζ−1γζ−1)−ℓ≤Q(ℵζ−2γζ−2)−2ℓ≤⋯≤Q(γ0)−ζℓ. | (2.2) |
Since limζ→∞(Q(γ0)−ζℓ)=−∞, then
limζ→∞Q(ℵζγζ)=−∞. |
From (2.1) and ((F1)), we derive that the sequence {ℵζγζ}∞ζ=1 is decreasing. By Lemma1.1, we derive that
limζ→∞(ℵζγζ)=0. |
By (F2), there exists k∈(0,1) such that
limζ→∞(ℵζγζ)kQ(ℵζγζ)=0. |
Multiplying (2.2) by (ℵζγζ)k results
0≤ζ(ℵζγζ)kℓ+(ℵζγζ)kQ(ℵζγζ)≤(ℵζγζ)kQ(γ0), ∀ζ∈N, |
which implies limζ→∞ζ(ℵζγζ)k=0. Then, there exists ζ1∈N such that ζ(ℵζγζ)k≤1, ∀ ζ≥ζ1. Thus,
ℵζγζ≤1ζ1k, ∀ζ≥ζ1. | (2.3) |
Therefore, the series ∑∞i=1ℵiγi is convergent. For all ζ,ω∈N, we drive the proof into two cases.
(a) If ω>2 is odd, we obtain
ϱ(ϑζ+3,ϑζ)≤ℵϱ(ϑζ+3,ϑζ+2)+ℵϱ(ϑζ+2,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ), |
ϱ(ϑζ+5,ϑζ)≤ℵϱ(ϑζ+5,ϑζ+2)+ℵϱ(ϑζ+2,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ)≤ℵ2ϱ(ϑζ+5,ϑζ+4)+ℵ2ϱ(ϑζ+4,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+2)+ℵγζ+1+ℵγζ. |
Consequently,
ϱ(ϑζ+ω,ϑζ)≤ℵϱ(ϑζ+ω,ϑζ+2)+ℵϱ(ϑζ+2,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ)≤ℵ2ϱ(ϑζ+ω,ϑζ+4)+ℵ2ϱ(ϑζ+4,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+2)+ℵγζ+1+ℵγζ≤ℵ3ϱ(ϑζ+ω,ϑζ+6)+ℵ3γζ+5+ℵ3γζ+4+ℵ2γζ+3+ℵ2γζ+2+ℵγζ+1+ℵγζ⋮≤ℵω−12γn+p−1+ℵω−12γζ+ω−2+ℵω−12γζ+ω−3+ℵω−32γζ+ω−4+ℵω−32γζ+ω−5+⋯+ℵ2γζ+2+ℵγζ+1+ℵγζ≤ℵω+12γζ+ω−1+ℵω2γζ+ω−2+ℵω−12γζ+ω−3+ℵω−22γζ+ω−4+ℵω−32γζ+ω−5+⋯+ℵ32γζ+1+ℵ22γζ≤ℵω+1γζ+ω−1+ℵωγζ+ω−2+ℵω−1γζ+ω−3+ℵω−2γζ+ω−4+ℵω−3γζ+ω−5+⋯+ℵ3γζ+1+ℵ2γζ≤1ℵζ−2(ℵζ+ω−1γζ+ω−1+ℵζ+ω−2γζ+ω−2+ℵζ+ω−3γζ+ω−3+⋯+ℵζ+1γζ+1+ℵζγζ)=1ℵζ−2ζ+ω−1∑i=ζℵiγi≤1ℵζ−2∞∑i=ζℵiγi. |
(b) If ω>2 is even, we can obtain
ϱ(ϑζ+4,ϑζ)≤ℵϱ(ϑζ+4,ϑζ+2)+ℵϱ(ϑζ+2,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ). |
Furthermore, we conclude that
ϱ(ϑζ+ω,ϑζ)≤ℵϱ(ϑζ+ω,ϑζ+2)+ℵϱ(ϑζ+2,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ)≤ℵ2ϱ(ϑζ+ω,ϑζ+4)+ℵ2ϱ(ϑζ+4,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+2)≤ℵ3ϱ(ϑζ+ω,ϑζ+6)+ℵ3γζ+5+ℵ3γζ+4+ℵ2γζ+3+ℵ2γζ+2⋮≤ℵω−22ϱ(ϑζ+ω,ϑζ+ω−2)+ℵω−22γζ+ω−3+ℵω−22γζ+ω−4+ℵω−42γζ+ω−5+ℵω−42γζ+ω−6+⋯+ℵγζ+1+ℵγζ≤ℵω−22ϱ(ϑζ+ω,ϑζ+ω−2)+ℵω−12γζ+ω−3+ℵω−22γζ+ω−4+ℵω−32γζ+ω−5+ℵω−42γζ+ω−6+⋯+ℵ32γζ+1+ℵ22γζ≤ℵω−22ϱ(ϑζ+ω,ϑζ+ω−2)+1ℵζ−2(ℵζ+ω−3γζ+ω−3+ℵζ+ω−4γζ+ω−4+⋯+ℵζ+1γζ+1+ℵζγζ)≤ℵω−22ϱ(ϑζ+ω,ϑζ+ω−2)+1ℵζ−2ζ+ω−1∑i=ζℵiγi≤ℵω−22βζ+ω−2+1ℵζ−2∞∑i=ζℵiγi. |
Since βζ≥0, we can assume that βζ>0, ∀ζ∈N. By a similar method, replacing γζ with βζ in (2.3), there exists ζ2∈N such that
ℵζβζ≤1ζ1k, ∀ζ≥ζ2, |
which implies limζ→∞ℵζβζ=0 and limζ→∞βζ=0. Together (a) with (b), for every ω∈N, letting ζ→∞,
ϱ(ϑn+p,ϑζ)→0. |
Thus, {ϑζ}ζ∈N is a Cauchy sequence. Since (Υ,ϱ) is complete, there exists ϑ∗∈Υ such that limζ→∞ϑζ=ϑ∗. Now
Q(ϱ(Πϑ,Πσ))≤ℓ+Q(ϱ(Πϑ,Πσ))≤ℓ+Q(ℵϱ(Πϑ,Πσ))≤Q(ϱ(ϑ,σ)) |
holds for all ϑ,σ∈Υ with ϱ(Πϑ,Πσ)>0. Since Q is increasing, then
ϱ(Πϑ,Πσ)≤ϱ(ϑ,σ). | (2.4) |
It follows that
0≤ϱ(ϑζ+1,Πϑ∗)≤ϱ(ϑζ,ϑ∗)→0 as ζ→∞. |
Hence, ϑ∗=Πϑ∗. Suppose that ϑ∗ and σ∗ are two different fixed points of Π. Suppose that, Πϑ∗=ϑ∗≠σ∗=Πσ∗ and (ϑ∗,σ∗)∈E(Ω). Then
ℓ+Q(ℵϱ(Πϑ∗,Πσ∗))≤Q(ϱ(ϑ∗,σ∗))≤Q(ℵϱ(ϑ∗,σ∗))=Q(ℵϱ(Πϑ∗,Πσ∗)), |
As ζ→∞, which implies ℓ≤0, a contradiction. Therefore ϑ∗=σ∗. Hence, Π has a unique fixed point in Υ.
Next, we prove common fixed point theorems on complete graphical Branciari ℵ-metric space.
Theorem 2.2. Let (Υ,ϱ) be a complete graphical Branciari ℵ-metric space with constant ℵ>1. If there exist ℓ>0 and Q∈Fℵ,ℓ, such that Λ,Π:Υ⟶Υ are two self mappings on Υ and satisfy
(H1) for every ϑ∈Υ, (ϑ,Λϑ)∈E(G) and (ϑ,Πϑ)∈E(G);
(H2) Π and Λ are generalized G-Q contraction
ℓ+Q(ϱ(Λϑ,Πσ))≤Q(ξ1ϱ(ϑ,σ)+ℵϱ(ϑ,Λϑ)+cϱ(σ,Πσ)), | (2.5) |
for any ξ1,ℵ,c∈[0,1) with ξ1+ℵ+c<1,ℵℵ<1, and min{ϱ(Λϑ,Πσ),ϱ(ϑ,σ),ϱ(ϑ,Λϑ),ϱ(σ,Πσ)}>0 for any (ϑ,σ)∈E(G). Then Λ and Π have a unique common fixed point.
Proof. Let ϑ0∈Υ. Suppose that Λϑ0=ϑ0, then the proof is finished, so we assume that Λϑ0≠ϑ0. As (ϑ0,Λϑ0)∈E(G), so (ϑ0,ϑ1)∈E(G). Also, (ϑ1,Πϑ1)∈E(G) gives (ϑ1,ϑ2)∈E(G). Continuing this way, we define a sequence {ϑj} in Υ such that (ϑj,ϑj+1)∈E(G) with
Λϑ2j=ϑ2j+1,Πϑ2j+1=ϑ2j+2,j=0,1,2,⋯. | (2.6) |
Combining with (2.5) and (2.6), we have
ℓ+Q(ϱ(ϑ2j+1,ϑ2j+2))=ℓ+Q(ϱ(Λϑ2j,Πϑ2j+1))≤Q(ξ1ϱ(ϑ2j,ϑ2j+1)+ℵϱ(ϑ2j,Λϑ2j)+cϱ((ϑ2j+1,Πϑ2j+2))=Q(ξ1ϱ(ϑ2j,ϑ2j+1)+ℵϱ(ϑ2j,ϑ2j+1)+cϱ((ϑ2j+1,ϑ2j+2)). |
Let λ=(ξ1+ℵ)/(1−c), 0<λ<1 since ξ1+ℵ+c<1. Using the strictly monotone increasing property of Q,
ϱ((ϑ2j+1,ϑ2j+2))<ξ1+ℵ1−cϱ(ϑ2j,ϑ2j+1)=λϱ(ϑ2j,ϑ2j+1). |
Similarly,
ϱ((ϑ2j+2,ϑ2j+3))<ξ1+ℵ1−cϱ(ϑ2j+1,ϑ2j+2)=λϱ(ϑ2j+1,ϑ2j+2). |
Hence,
ϱ((ϑζ,ϑζ+1))<λϱ(ϑζ−1,ϑζ),ζ∈N. |
For any ζ∈N, we obtain
ϱ((ϑζ,ϑζ+1))<λϱ(ϑζ−1,ϑζ)<λ2ϱ(ϑζ−2,ϑζ−1)<⋯<λζϱ(ϑ0,ϑ1). |
Notice that
ℓ+Q(ℵϱ(ϑ1,ϑ3))=ℓ+Q(ℵϱ(Λϑ0,Λϑ2))≤Q(ϱ(ϑ0,ϑ2)),ℓ+Q(ℵϱ(ϑ2,ϑ4))=ℓ+Q(ℵϱ(Πϑ1,Πϑ3))≤Q(ϱ(ϑ1,ϑ3)). |
Since Q is strictly monotone increasing, we have
ϱ(ϑ1,ϑ3)≤1ℵϱ(ϑ0,ϑ2),ϱ(ϑ2,ϑ4)≤1ℵϱ(ϑ1,ϑ3)≤1ℵ2ϱ(ϑ0,ϑ2). |
By induction, we obtain
ϱ(ϑζ,ϑζ+2)<1ℵζϱ(ϑ0,ϑ2),ζ∈N. |
We consider the following two cases:
(ⅰ) Let m=ζ+ω, if ω is odd and ω>2, we have
ϱ(ϑζ,ϑm)≤ℵ(ϱ(ϑζ,ϑζ+1)+ϱ(ϑζ+1,ϑζ+2)+ϱ(ϑζ+2,ϑm))≤ℵϱ(ϑζ,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ+2)+ℵ2ϱ(ϑn+2,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+4)+ℵ2ϱ(ϑζ+4,ϑm)≤ℵϱ(ϑζ,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ+2)+ℵ2ϱ(ϑn+2,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+4)+ℵ3ϱ(ϑζ+4,ϑζ+5)+ℵ3ϱ(ϑζ+5,ϑζ+6)+⋯+ℵm−ζ2ϱ(ϑm−2,ϑm−1)+ℵm−ζ2ϱ(ϑm−1,ϑm)≤ℵλζϱ(ϑ0,ϑ1)+ℵλζ+1ϱ(ϑ0,ϑ1)+ℵ2λζ+2ϱ(ϑ0,ϑ1)+ℵ2λζ+3ϱ(ϑ0,ϑ1)+ℵ3λζ+3ϱ(ϑ0,ϑ1)+ℵ3λζ+4ϱ(ϑ0,ϑ1)+⋯+ℵm−ζ2λm−2ϱ(ϑ0,ϑ1)+ℵm−ζ2λm−1ϱ(ϑ0,ϑ1)≤(ℵλζ+ℵ2λζ+2+ℵ3λζ+4+⋯+ℵm−ζ2λm−2)ϱ(ϑ0,ϑ1)+(ℵλζ+1+ℵ2λζ+3+ℵ3λζ+5+⋯+ℵm−ζ2λm−1)ϱ(ϑ0,ϑ1)≤(ℵλζ+ℵ2λζ+2+ℵ3λζ+4+⋯+ℵm−ζ2λm−2)(1+λ)ϱ(ϑ0,ϑ1)≤ℵλζ(1+ℵλζ+2+ℵ2λζ+4+⋯+ℵm−ζ−22λm−ζ−2)(1+λ)ϱ(ϑ0,ϑ1)≤ℵλζ.1−ℵm−ζ2λm−ζ1−ℵλ2(1+λ)ϱ(ϑ0,ϑ1)≤ℵλζ.1−ℵω2λω1−ℵλ2(1+λ)ϱ(ϑ0,ϑ1). |
(ⅱ) Let m=ζ+ω, if ω is even and ω>2, we have
ϱ(ϑζ,ϑm)≤ℵ(ϱ(ϑζ,ϑζ+1)+ϱ(ϑζ+1,ϑζ+2)+ϱ(ϑζ+2,ϑm))≤ℵϱ(ϑζ,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ+2)+ℵ2ϱ(ϑn+2,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+4)+ℵ2ϱ(ϑζ+4,ϑm)≤ℵϱ(ϑζ,ϑζ+1)+ℵϱ(ϑζ+1,ϑζ+2)+ℵ2ϱ(ϑn+2,ϑζ+3)+ℵ2ϱ(ϑζ+3,ϑζ+4)+ℵ3ϱ(ϑζ+4,ϑζ+5)+ℵ3ϱ(ϑζ+5,ϑζ+6)+⋯+ℵm−ζ−22ϱ(ϑm−4,ϑm−3)+ℵm−ζ−22ϱ(ϑm−3,ϑm−2)+ℵm−ζ−22ϱ(ϑm−2,ϑm)≤ℵλζϱ(ϑ0,ϑ1)+ℵλζ+1ϱ(ϑ0,ϑ1)+ℵ2λζ+2ϱ(ϑ0,ϑ1)+ℵ2λζ+3ϱ(ϑ0,ϑ1)+ℵ3λζ+3ϱ(ϑ0,ϑ1)+ℵ3λζ+4ϱ(ϑ0,ϑ1)+⋯+ℵm−ζ−22λm−4ϱ(ϑ0,ϑ1)+ℵm−ζ−22λm−3ϱ(ϑ0,ϑ1)+ℵm−ζ−22ϱ(ϑm−2,ϑm)≤(ℵλζ+ℵ2λζ+2+ℵ3λζ+4+⋯+ℵm−ζ−22λm−4)ϱ(ϑ0,ϑ1)+(ℵλζ+1+ℵ2λζ+3+ℵ3λζ+5+⋯+ℵm−ζ−22λm−3)ϱ(ϑ0,ϑ1)+ℵm−ζ−22ϱ(ϑm−2,ϑm)≤(ℵλζ+ℵ2λζ+2+ℵ3λζ+4+⋯+ℵm−ζ−22λm−4)(1+λ)ϱ(ϑ0,ϑ1)+ℵm−ζ−22ϱ(ϑm−2,ϑm)≤ℵλζ(1+ℵλζ+2+ℵ2λζ+4+⋯+ℵm−ζ−42λm−ζ−4)(1+λ)ϱ(ϑ0,ϑ1)+ℵm−ζ−22ϱ(ϑm−2,ϑm)≤ℵλζ.1−ℵm−ζ−22λm−ζ−21−ℵλ2(1+λ)ϱ(ϑ0,ϑ1)+ℵ−ζϱ(ϑ0,ϑ2)≤ℵλζ.1−ℵω−22λω−21−ℵλ2(1+λ)ϱ(ϑ0,ϑ1)+ℵ−ζϱ(ϑ0,ϑ2). |
As m,ζ⟶∞, ϱ(ϑζ,ϑm)⟶0 for all ω>2. Hence, {ϑζ} is a Cauchy sequence in Υ. Since (Υ,ϱ) is complete, there exists z∗∈Υ such that
limζ→∞ϱ(ϑζ,z∗)=0. |
Suppose that ϱ(Λz∗,z∗)>0, then
ℓ+Q(ϱ(Λz∗,ϑ2j+2))≤Q(ξ1ϱ(z∗,ϑ2j+1)+ℵϱ(z∗,Λz∗)+cϱ(ϑ2j+1,ϑ2j+2)). |
Using the strictly monotone increasing property of Q, we get
ϱ(Λz∗,ϑ2j+2)<ξ1ϱ(z∗,ϑ2j+1)+ℵϱ(z∗,Λz∗)+cϱ(ϑ2j+1,ϑ2j+2)). |
We can also see that
ϱ(Λz∗,z∗)<ℵ[ϱ(Λz∗,ϑ2j+2)+ϱ(ϑ2j+2,ϑ2j+1)+ϱ(ϑ2j+1,z∗)]. |
It follows that
1ℵϱ(Λz∗,z∗)≤limj→∞ infϱ(Λz∗,ϑ2j+2)≤limj→∞ supϱ(Λz∗,ϑ2j+2)≤ℵϱ(z∗,Λz∗). |
Hence, 1ℵ≤ℵ which is an absurdity. Therefore, ϱ(Λz∗,z∗)=0. Similarly, we can obtain Πz∗=z∗. Therefore, we have
Πz∗=Λz∗=z∗. |
Suppose that ϑ∗ and σ∗ are two different common fixed points of Λ and Π. Suppose that, Λϑ∗=ϑ∗≠σ∗=Πσ∗ and (ϑ∗,σ∗)∈E(G). Then,
ℓ+Q(ϱ(ϑ∗,σ∗))=ℓ+Q(ϱ(Λϑ∗,Πσ∗))≤Q(ξ1ϱ(ϑ∗,σ∗)+ℵϱ(ϑ∗,Λϑ∗)+cϱ(σ∗,Πσ∗))=Q(ξ1ϱ(ϑ∗,σ∗)+ℵϱ(ϑ∗,ϑ∗)+cϱ(σ∗,σ∗)). |
Using the strictly monotone increasing property of Q, (1−ξ1)ϱ(ϑ∗,σ∗)<0, which is an absurdity. Hence ϑ∗=σ∗.
Example 2.1. Let Υ=Γ∪Ψ, where Γ={1ζ:ζ∈{2,3,4,5}} and Ψ=[1,2]. For any ϑ,σ∈Υ, we define ϱ:Υ×Υ⟶[0,+∞) by
{ϱ(ϑ,σ)=ϱ(σ,ϑ)for allϑ,σ∈Υ,ϱ(ϑ,σ)=0⟺ϑ=σ. |
and
{ϱ(12,13)=ϱ(13,14)=ϱ(14,15)=16,ϱ(12,14)=ϱ(13,15)=17,ϱ(12,15)=ϱ(12,14)=12,ϱ(ϑ,σ)=|ϑ−σ|2,otherwise. |
Clearly, (Υ,ϱ) is a complete graphical Branciari ℵ-metric space with constant ℵ=3>1. Define the graph Ω by E(Ω)=Δ+{(13,14),(13,15),(12,2),(12,14),(15,12),(15,2),(2,1),(2,13),(1,14),(1,12)}.
Figure 2 represents the directed graph Ω. Let Π:Υ→Υ be a mapping satisfying
Πϑ={12,ϑ∈Γ,13,ϑ∈Ψ. |
Now, we verify that Π is a Ω-Q-contraction. We take ϑ=14∈Γ,σ=2∈Ψ, and ℓ=0.1. Then, ϱ(Πϑ,Πσ)=ϱ(12,13)=16>0 and
0.1+3ϱ(Πϑ,Πσ)=0.6<3.0625=ϱ(ϑ,σ). |
Let Q:(0,+∞)→R be a mapping defined by Q(ϑ)=ϑ, then it is easy to see that Q∈Fℵ,ℓ. Therefore
ℓ+Q(ℵ.ϱ(Πϑ,Πσ))≤Q(ϱ(ϑ,σ)). |
Hence, Π fulfills the conditions of Theorem 2.1 and ϑ=12 is the unique fixed point of Π.
Consider the integral equation:
ϑ(ρ)=μ(ρ)+∫ξ10m(ρ,φ)θ(φ,ϑ(φ))dφ,ρ∈[0,ξ1],ξ1>0. | (3.1) |
Let Υ=C([0,ξ1],R) be the set of real continuous functions defined on [0,ξ1] and the mapping Π:Υ→Υ defined by
Π(ϑ(ρ))=μ(ρ)+∫ξ10m(ρ,φ)θ(φ,ϑ(φ))dφ,ρ∈[0,ξ1]. | (3.2) |
Obviously, ϑ(ρ) is a solution of integral Eq (3.1) iff ϑ(ρ) is a fixed point of Π.
Theorem 3.1. Suppose that
(R1) The mappings m:[0,ξ1]×R→[0,+∞), θ:[0,ξ1]×R→R, and μ:[0,ξ1]→R are continuous functions.
(R2) ∃ ℓ>0 and ℵ>1 such that
|θ(φ,ϑ(φ))−θ(φ,σ(φ))|≤√e−ℓℵ|ϑ(φ)−σ(φ)| | (3.3) |
for each φ∈[0,ξ1] and ϑ≤σ (i.e., ϑ(φ)≤σ(φ))
(R3) ∫ξ10m(ρ,φ)dφ≤1.
(R4) ∃ ϑ0∈C([0,ξ1],R) such that ϑ0(ρ)≤μ(ρ)+∫ξ10m(ρ,φ)θ(φ,ϑ0(φ))dφ for all ρ∈[0,ξ1]
Then, the integral Eq (3.1) has a unique solution in the set {ϑ∈C([0,ξ1],R):ϑ(ρ)≤ϑ0(ρ)orϑ(ρ)≥ϑ0(ρ),for allφ∈[0,ξ1]}.
Proof. Define ϱ:Υ×Υ→[0,+∞) given by
ϱ(ϑ,σ)=supρ∈[0,ξ1]|ϑ(ρ)−σ(ρ)|2 |
for all ϑ,σ∈Υ. It is easy to see that, (Υ,ϱ) is a complete graphical Branciari ℵ-metric space with ℵ≥1. Define Π:Υ→Υ by
Π(ϑ(ρ))=μ(ρ)+∫ξ10m(ρ,φ)θ(φ,ϑ(φ))dφ,ρ∈[0,ξ1]. | (3.4) |
Consider a graph Ω consisting of V(Ω):=Υ and E(Ω)={(ϑ,σ)∈Υ×Υ:ϑ(ρ)≤σ(ρ)}. For each ϑ,σ∈Υ with (ϑ,σ)∈E(Ω), we have
|Πϑ(ρ)−Πσ(ρ)|2=|∫a0m(ρ,φ)[θ(φ,ϑ(φ))−θ(φ,σ(φ))]du|2≤(∫a0m(ρ,φ)√e−ℓℵ|ϑ(φ)−σ(φ)|du)2≤e−ℓℵ(∫a0m(ρ,φ)du)2supφ∈[0,a]|ϑ(φ)−σ(φ)|2≤e−ℓℵϱ(ϑ,σ). |
Thus,
ℵϱ(Πϑ,Πσ)≤e−ℓϱ(ϑ,σ), |
which implies that
ℓ+ln(ℵϱ(Πϑ,Πσ))≤ln(ϱ(ϑ,σ)), |
for each ϑ,σ∈Υ. By (R4), we have (ϑ0,Πϑ0)∈E(Ω), so that [ϑ0]lΩ={ϑ∈C([0,ξ1],R):ϑ(ρ)≤ϑ0(ρ)orϑ(ρ)≥ϑ0(ρ),for allφ∈[0,ξ1]}. Therefore, all the hypotheses of Theorem 2.1 are fulfilled. Hence, the integral equation has a unique solution.
We recall many important definitions from fractional calculus theory. For a function ϑ∈C[0,1], the Reiman-Liouville fractional derivative of order δ>0 is given by
1Γ(ξ−δ)dξdtξ∫t0ϑ(e)de(t−e)δ−ξ+1=Dδϑ(t), |
provided that the right hand side is pointwise defined on [0,1], where [δ] is the integer part of the number δ,Γ is the Euler gamma function. For more details, one can see [26,27,28,29].
Consider the following fractional differential equation
eDηϑ(t)+f(t,ϑ(t))=0,0≤t≤1,1<η≤2;ϑ(0)=ϑ(1)=0, | (4.1) |
where f is a continuous function from [0,1]×R to R and eDη represents the Caputo fractional derivative of order η and it is defined by
eDη=1Γ(ξ−η)∫t0ϑξ(e)de(t−e)η−ξ+1. |
Let Υ=(C[0,1],R) be the set of all continuous functions defined on [0,1]. Consider ϱ:Υ×Υ→R+ to be defined by
ϱ(ϑ,ϑ′)=supt∈[0,1]|ϑ(t)−ϑ′(t)|2 |
for all ϑ,ϑ′∈Υ. Then (Υ,ϱ) is a complete graphical Branciari ℵ-metric space with ℵ≥1. The given fractional differential equation (4.1) is equivalent to the succeeding integral equation
ϑ(t)=∫10G(t,e)f(q,ϑ(e))de, |
where
G(t,e)={[t(1−e)]η−1−(t−e)η−1Γ(η),0≤e≤t≤1,[t(1−e)]η−1Γ(η),0≤t≤e≤1. |
Define Π:Υ→Υ defined by
Πϑ(t)=∫10G(t,e)f(q,ϑ(e))de. |
It is easy to note that if ϑ∗∈Π is a fixed point of Π then ϑ∗ is a solution of the problem (4.1).
Theorem 4.1. Assume the fractional differential Eq (4.1). Suppose that the following conditions are satisfied:
(S1) there exists t∈[0,1], ℵ∈(0,1) and ϑ,ϑ′∈Υ such that
|f(t,ϑ)−f(t,ϑ′)|≤√e−ℓℵ|ϑ(t)−ϑ′(t)| |
for all ϑ≤ϑ′ (i.e., ϑ(t)≤ϑ′(t)).
(S2)
supt∈[0,1]∫10|G(t,e)|de≤1. |
(S3) ∃ ϑ0∈C([0,1],R) such that ϑ0(t)≤∫10G(t,e)f(q,ϑ(e))de for all t∈[0,1].
Then the fractional differential Eq (4.1) has a unique solution in the set {ϑ∈C([0,1],R):ϑ(t)≤ϑ0(t)orϑ(t)≥ϑ0(t),for allt∈[0,1]}.
Proof. Consider a graph Ω consisting of V(Ω):=Υ and E(Ω)={(ϑ,ϑ′)∈Υ×Υ:ϑ(ρ)≤σ(ρ)}. For each ϑ,ϑ′∈Υ with (ϑ,ϑ′)∈E(Ω), we have
|Πϑ(t)−Πϑ′(t)|2=|∫10G(t,e)f(q,ϑ(e))de−∫1oG(t,e)f(q,ϑ′(e))de|2≤(∫10|G(t,e)|de)2(∫10|f(q,ϑ(e))−f(q,ϑ′(e))|de)2≤e−ℓℵ|ϑ(t)−ϑ′(t)|2. |
Taking the supremum on both sides, we get
ℓ+ln(ℵϱ(Πϑ,Πϑ′))≤ln(ϱ(ϑ,ϑ′)), |
for each ϑ,ϑ′∈Υ. By (S3), we have (ϑ0,Πϑ0)∈E(Ω), so that [ϑ0]lΩ={ϑ∈C([0,1],R):ϑ(t)≤ϑ0(t)orϑ(t)≥ϑ0(t),for allt∈[0,1]}. Therefore, all the hypotheses of Theorem 2.1 are fulfilled. Hence, the fractional differential Eq (4.1) has a unique solution.
In this paper, we have established fixed point results for Ω-Q-contraction in the setting of complete graphical Branciari ℵ -metric spaces. The directed graphs have been supported by Figures 1 and 2. The proven results have been supplemented with a non-trivial example and also applications to solve Fredholm integral equation and fractional differential equation have also been provided.
All authors equally contributed to this work. All authors read and approved the final manuscript.
The authors extend their appreciation to Prince Sattam bin Abdulaziz University for funding this research work through the Project Number PSAU/2023/01/24098.
The authors convey their sincere appreciation to the Chief/Guest Editor and the anonymous reviewers for their suggestion and valuable comments which helped to bring the manuscript to its present form.
The authors declare no conflict of interest.
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