In this paper, we prove a common fixed-point theorem for four self-mappings with a function family on Sb-metric spaces. In addition, we investigate some geometric properties of the fixed-point set of a given self-mapping. In this context, we obtain a fixed-disc (resp. fixed-circle), fixed-ellipse, fixed-hyperbola, fixed-Cassini curve and fixed-Apollonious circle theorems on Sb-metric spaces.
Citation: Nihal Taş, Irshad Ayoob, Nabil Mlaiki. Some common fixed-point and fixed-figure results with a function family on Sb-metric spaces[J]. AIMS Mathematics, 2023, 8(6): 13050-13065. doi: 10.3934/math.2023657
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In this paper, we prove a common fixed-point theorem for four self-mappings with a function family on Sb-metric spaces. In addition, we investigate some geometric properties of the fixed-point set of a given self-mapping. In this context, we obtain a fixed-disc (resp. fixed-circle), fixed-ellipse, fixed-hyperbola, fixed-Cassini curve and fixed-Apollonious circle theorems on Sb-metric spaces.
Fixed-point theory has been extensively researched in various areas, such as mathematics, engineering, and physics. Of particular importance is metric fixed-point theory, which is used in various branches of mathematics like topology, analysis, and applied mathematics. This theory was initiated with the famous Banach fixed-point theorem [1]. This theorem ensures that a self-mapping will have a unique fixed point. Despite this, there remain instances of self-mapping that have a fixed point but do not meet the criteria set by the Banach fixed point theorem, such as:
Let X=R, and (X,ζ) be the usual metric space. Consider a self-mapping g:R→R defined as
gμ=2μ−4, |
for all μ∈R. Then g has a unique fixed point μ=4, but g does not meet the criteria of Banach contraction principle.
There are two popular methods used by researchers to generalize the Banach contraction principle. The first entails extending the utilized contractive condition while the second centers around generalizing the underlying metric space. For example, Gb-metric spaces, G-metric spaces [2], complex valued Gb-metric spaces [3,4], S-metric spaces, A-metric spaces [5], Sb-metric spaces, fuzzy cone metric spaces [6], modular metric spaces [7,8,9] et al. were defined for this purpose (for more details, see [10,11,12,13] and the references therein). Especially, we focus on the notion of Sb-metric spaces. To do this, we recall the following basic concepts:
Definition 1.1. [14] Let X be a nonempty set and s≥1 a given real number. A function Sb:X×X×X→[0,∞) is said to be Sb -metric if and only if for all μ,τ,ℏ,ρ∈X the following conditions are satisfied:
(Sb1) Sb(μ,τ,ℏ)=0 if and only if μ=τ=ℏ,
(Sb2) Sb(μ,τ,ℏ)≤s[Sb(μ,μ,ρ)+Sb(τ,τ,ρ)+Sb(ℏ,ℏ,ρ)].
The pair (X,Sb) is called an Sb-metric space.
As every S-metric is a Sb-metric with s=1, we observe that Sb -metric spaces are extensions of S-metric spaces. However, the converse statement is not always true (see [14] and [15] for more details).
Definition 1.2. [15] Let (X,Sb) be an Sb-metric space and s>1. An Sb-metric Sb is called symmetric if
Sb(μ,μ,τ)=Sb(τ,τ,μ), |
for all μ,τ∈X.
Definition 1.3. [14] Let (X,Sb) be an Sb -metric space, and {ℏn} be a sequence in X.
1) Then the sequence {ℏn} converges to ℏ∈X if and only if Sb(ℏn,ℏn,ℏ)→0 as n→∞, that is, for each ε>0 there exists n0∈N such that for all n≥n0, Sb(ℏn,ℏn,ℏ)<ε. It is denoted by
limn→∞ℏn=ℏ. |
2) Then the sequence {ℏn} is called a Cauchy sequence if for each ε>0 there exists n0∈N such that Sb(ℏn,ℏn,ℏm)<ε for each n,m≥n0.
3) The Sb-metric space (X,Sb) is called complete if every Cauchy sequence is convergent.
On the other hand, in the literature, besides various fixed point theorems there are also common fixed point theorems on metric and some extended metric spaces (for example, see [14,16,17,18,19] and the references therein).
Recently, as a geometric approach to the fixed-point theory, the fixed-circle problem (see [20]) and the fixed-figure problem (see [21]) have been introduced. When there are more than one fixed points, it is interesting to investigate for the possible solutions as follows:
Let us define a self-mapping, g:R→R, where R is with the usual metric
gμ=μ2−1√2, |
for all μ∈R. Then g has two fixed points
μ1=−1 and μ2=1. We consider these fixed points as a unit circle C0,1={−1,1}.
For this reason, there exist some studies related to these recent aspects (for example, see [22,23,24,25,26,27,28] and the references therein).
By the above motivation, in this paper, we prove a common fixed-point theorem and some fixed-figure results on Sb-metric spaces.
In this section, we prove a common fixed-point result on Sb-metric spaces. To do this, we are inspired by the function family F6 introduced in [29] and the function family M defined in [30]. We modify these families as follows:
Let Ψ be the family of all lower semi-continuous functions ψ:R6+→R that satisfy the following condition:
(ψ∗). For all μ,τ,ℏ≥0 and s≥1, there exists a k∈[0,1) such that
μ≤ψ(μ,τ,τ,μ,0,ℏ) |
with ℏ≤2sμ+sτ then
μ≤kτ. |
If we define the function ψ:R6+→R such as
ψ(t1,t2,t3,t4,t5,t6)=kmax{t1,t2}, |
with k∈[0,1). Then ψ∈Ψ.
Now, we give the following theorem.
Theorem 2.1. Let (X,Sb) be a complete continuous Sb -metric space with the symmetric metric Sb. Let g,h,G,H:X→X be four self-mappings, where g,G and H are continuous and satisfying the following conditions:
(i) g(X)⊂H(X) and h(X)⊂G(X),
(ii) For all μ,τ∈X and ψ∈Ψ,
Sb(gμ,gμ,hτ)≤ψ(Sb(gμ,gμ,hτ),Sb(μ,μ,τ),Sb(μ,μ,gμ),Sb(τ,τ,hτ),Sb(τ,τ,gμ),Sb(μ,μ,hτ)), |
(iii) For all μ,τ∈X and ψ∈Ψ,
Sb(Gμ,Gμ,Hτ)≤ψ(Sb(Gμ,Gμ,Hτ),Sb(μ,μ,τ),Sb(μ,μ,Gμ),Sb(τ,τ,Hτ),Sb(τ,τ,Gμ),Sb(μ,μ,Hτ)), |
holds, then g, h, G and H have a common fixed point in X.
Proof. Let ℏ0∈X, ℏ1=gℏ0 and ℏ2=hℏ1. Using the condition (ii), we get
Sb(gℏ0,gℏ0,hℏ1)=Sb(ℏ1,ℏ1,ℏ2)≤ψ(Sb(gℏ0,gℏ0,hℏ1),Sb(ℏ0,ℏ0,ℏ1),Sb(ℏ0,ℏ0,gℏ0),Sb(ℏ1,ℏ1,hℏ1),Sb(ℏ1,ℏ1,gℏ0),Sb(ℏ0,ℏ0,hℏ1))=ψ(Sb(ℏ1,ℏ1,ℏ2),Sb(ℏ0,ℏ0,ℏ1),Sb(ℏ0,ℏ0,ℏ1),Sb(ℏ1,ℏ1,ℏ2),Sb(ℏ1,ℏ1,ℏ1),Sb(ℏ0,ℏ0,ℏ2))=(Sb(ℏ1,ℏ1,ℏ2),Sb(ℏ0,ℏ0,ℏ1),Sb(ℏ0,ℏ0,ℏ1),Sb(ℏ1,ℏ1,ℏ2),0,Sb(ℏ0,ℏ0,ℏ2)). | (2.1) |
By (Sb2) and the symmetry property of Sb, we have
Sb(ℏ0,ℏ0,ℏ2)=Sb(ℏ2,ℏ2,ℏ0)≤s[2Sb(ℏ2,ℏ2,ℏ1)+Sb(ℏ0,ℏ0,ℏ1)]=2sSb(ℏ1,ℏ1,ℏ2)+sSb(ℏ0,ℏ0,ℏ1). | (2.2) |
Using (2.1), (2.2) and (ψ∗), there exists a k∈[0,1) such that
Sb(ℏ1,ℏ1,ℏ2)≤kSb(ℏ0,ℏ0,ℏ1). |
Continuing this process with induction with the condition (i), we can define the sequence {ℏn} as follows:
ℏ2n+1=gℏ2n=Hℏ2n |
and
ℏ2n=hℏ2n−1=Gℏ2n−1. |
Using (ii), for μ=ℏ2n and τ=ℏ2n+1, we find
Sb(gℏ2n,gℏ2n,hℏ2n+1)=Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2)≤ψ(Sb(gℏ2n,gℏ2n,hℏ2n+1),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n,ℏ2n,gℏ2n),Sb(ℏ2n+1,ℏ2n+1,hℏ2n+1),Sb(ℏ2n+1,ℏ2n+1,gℏ2n),Sb(ℏ2n,ℏ2n,hℏ2n+1))=ψ(Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2),Sb(ℏ2n+1,ℏ2n+1,ℏ2n+1),Sb(ℏ2n,ℏ2n,ℏ2n+2))=ψ(Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2),0,Sb(ℏ2n,ℏ2n,ℏ2n+2)) | (2.3) |
By (Sb2) and the symmetry property of Sb, we have
Sb(ℏ2n,ℏ2n,ℏ2n+2)=Sb(ℏ2n+2,ℏ2n+2,ℏ2n)≤s[2Sb(ℏ2n+2,ℏ2n+2,ℏ2n+1)+Sb(ℏ2n,ℏ2n,ℏ2n+1)]=2sSb(ℏ2n+1,ℏ2n+1,ℏ2n+2)+sSb(ℏ2n,ℏ2n,ℏ2n+1). | (2.4) |
Using (2.3), (2.4) and (ψ∗), there exists a k∈[0,1) such that
Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2)≤kSb(ℏ2n,ℏ2n,ℏ2n+1). | (2.5) |
Using (iii), for μ=ℏ2n−1 and τ=ℏ2n, we get
Sb(Gℏ2n−1,Gℏ2n−1,Hℏ2n)=Sb(ℏ2n,ℏ2n,ℏ2n+1)≤ψ(Sb(Gℏ2n−1,Gℏ2n−1,Hℏ2n),Sb(ℏ2n−1,ℏ2n−1,ℏ2n),Sb(ℏ2n−1,ℏ2n−1,Gℏ2n−1),Sb(ℏ2n,ℏ2n,Hℏ2n),Sb(ℏ2n,ℏ2n,Gℏ2n−1),Sb(ℏ2n−1,ℏ2n−1,Hℏ2n))=ψ(Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n−1,ℏ2n−1,ℏ2n),Sb(ℏ2n−1,ℏ2n−1,ℏ2n),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n,ℏ2n,ℏ2n),Sb(ℏ2n−1,ℏ2n−1,ℏ2n+1))=ψ(Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ2n−1,ℏ2n−1,ℏ2n),Sb(ℏ2n−1,ℏ2n−1,ℏ2n),Sb(ℏ2n,ℏ2n,ℏ2n+1),0,Sb(ℏ2n−1,ℏ2n−1,ℏ2n+1)) | (2.6) |
By (Sb2) and the symmetry property of Sb, we find
Sb(ℏ2n−1,ℏ2n−1,ℏ2n+1)=Sb(ℏ2n+1,ℏ2n+1,ℏ2n−1)≤s[2Sb(ℏ2n+1,ℏ2n+1,ℏ2n)+Sb(ℏ2n−1,ℏ2n−1,ℏ2n)]=2sSb(ℏ2n,ℏ2n,ℏ2n+1)+sSb(ℏ2n−1,ℏ2n−1,ℏ2n). | (2.7) |
Using (2.6), (2.7) and (ψ∗), there exists a k∈[0,1) such that
Sb(ℏ2n,ℏ2n,ℏ2n+1)≤kSb(ℏ2n−1,ℏ2n−1,ℏ2n). | (2.8) |
Using the inequalities (2.5) and (2.8), we get
Sb(ℏ2n+1,ℏ2n+1,ℏ2n+2)≤kSb(ℏ2n,ℏ2n,ℏ2n+1)≤k2Sb(ℏ2n−1,ℏ2n−1,ℏ2n) |
and so, using similar arguments, we have
Sb(ℏn,ℏn,ℏn+1)≤knSb(ℏ0,ℏ0,ℏ1). | (2.9) |
Now we show that the sequence {ℏn} is Cauchy. Using (Sb2), (2.9) and the symmetry property of Sb, for all n,m∈N with m>n, we get
Sb(ℏn,ℏn,ℏm)≤s[2Sb(ℏn,ℏn,ℏn+1)+Sb(ℏm,ℏm,ℏn+1)]=s[2Sb(ℏn,ℏn,ℏn+1)+Sb(ℏn+1,ℏn+1,ℏm)]≤2sSb(ℏn,ℏn,ℏn+1)+s2[2Sb(ℏn+1,ℏn+1,ℏn+2)+Sb(ℏm,ℏm,ℏn+2)]=2sSb(ℏn,ℏn,ℏn+1)+s2[2Sb(ℏn+1,ℏn+1,ℏn+2)+Sb(ℏn+2,ℏn+2,ℏm)]≤2sSb(ℏn,ℏn,ℏn+1)+2s2Sb(ℏn+1,ℏn+1,ℏn+2)+⋯≤2sknSb(ℏ0,ℏ0,ℏ1)+2s2kn+1Sb(ℏ0,ℏ0,ℏ1)+⋯≤2skn1−skSb(ℏ0,ℏ0,ℏ1). |
Since s≥1 and k∈[0,1), taking n,m→∞, we get
Sb(ℏn,ℏn,ℏm)→0 |
and so {ℏn} is Cauchy. Since (X,Sb) is a complete Sb-metric space, {ℏn} is convergent to a point ℏ∈X, that is,
limn→∞Sb(ℏn,ℏn,ℏ)=0. |
Next, we establish that ℏ is a common fixed point of g, h, G and H. Using (ii), for μ=ℏ2n and τ=ℏ, we get
Sb(gℏ2n,gℏ2n,hℏ)=Sb(ℏ2n+1,ℏ2n+1,hℏ)≤ψ(Sb(gℏ2n,gℏ2n,hℏ),Sb(ℏ2n,ℏ2n,ℏ),Sb(ℏ2n,ℏ2n,gℏ2n),Sb(ℏ,ℏ,hℏ),Sb(ℏ,ℏ,gℏ2n),Sb(ℏ2n,ℏ2n,hℏ))=ψ(Sb(ℏ2n+1,ℏ2n+1,hℏ),Sb(ℏ2n,ℏ2n,ℏ),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ,ℏ,hℏ),Sb(ℏ,ℏ,ℏ2n+1),Sb(ℏ2n,ℏ2n,hℏ)) |
and taking n→∞, we obtain
Sb(ℏ,ℏ,hℏ)≤ψ(Sb(ℏ,ℏ,hℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,hℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,hℏ))=ψ(Sb(ℏ,ℏ,hℏ),0,0,Sb(ℏ,ℏ,hℏ),0,Sb(ℏ,ℏ,hℏ)) | (2.10) |
and
Sb(ℏ,ℏ,hℏ)≤2sSb(ℏ,ℏ,hℏ)+s.0. | (2.11) |
Using (2.10), (2.11) and (ψ∗), there exists a k∈[0,1) such that
Sb(ℏ,ℏ,hℏ)≤k.0=0, |
that is,
hℏ=ℏ. |
Using the continuity hypothesis of g, we have
limn→∞Sb(ℏ2n,ℏ2n,ℏ)=0⟹limn→∞Sb(gℏ2n,gℏ2n,gℏ)=0⟹limn→∞Sb(ℏ2n+1,ℏ2n+1,gℏ)=0⟹Sb(ℏ,ℏ,gℏ)=0⟹gℏ=ℏ. |
Hence ℏ is a common fixed point g and h. Using (iii), for μ=ℏ2n and τ=ℏ, we get
Sb(Gℏ2n,Gℏ2n,Hℏ)=Sb(ℏ2n+1,ℏ2n+1,Hℏ)≤ψ(Sb(Gℏ2n,Gℏ2n,Hℏ),Sb(ℏ2n,ℏ2n,ℏ),Sb(ℏ2n,ℏ2n,Gℏ2n),Sb(ℏ,ℏ,Hℏ),Sb(ℏ,ℏ,Gℏ2n),Sb(ℏ2n,ℏ2n,Hℏ))=ψ(Sb(ℏ2n+1,ℏ2n+1,Hℏ),Sb(ℏ2n,ℏ2n,ℏ),Sb(ℏ2n,ℏ2n,ℏ2n+1),Sb(ℏ,ℏ,Hℏ),Sb(ℏ,ℏ,ℏ2n+1),Sb(ℏ2n,ℏ2n,Hℏ)) |
and taking n→∞, we obtain
Sb(ℏ,ℏ,Hℏ)≤ψ(Sb(ℏ,ℏ,Hℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,Hℏ),Sb(ℏ,ℏ,ℏ),Sb(ℏ,ℏ,Hℏ))=ψ(Sb(ℏ,ℏ,Hℏ),0,0,Sb(ℏ,ℏ,Hℏ),0,Sb(ℏ,ℏ,Hℏ)) | (2.12) |
and
Sb(ℏ,ℏ,Hℏ)≤2sSb(ℏ,ℏ,Hℏ)+s.0. | (2.13) |
Using (2.12), (2.13) and (ψ∗), there exists a k∈[0,1) such that
Sb(ℏ,ℏ,Hℏ)≤k.0=0, |
that is,
Hℏ=ℏ. |
Using the continuity hypothesis of G, we have
limn→∞Sb(ℏ2n,ℏ2n,ℏ)=0⟹limn→∞Sb(Gℏ2n,Gℏ2n,Gℏ)=0⟹limn→∞Sb(ℏ2n+1,ℏ2n+1,Gℏ)=0⟹Sb(ℏ,ℏ,Gℏ)=0⟹Gℏ=ℏ. |
Consequently, we obtain
ℏ=hℏ=gℏ=Hℏ=Gℏ, |
that is, ℏ is a common fixed point of four self-mappings g, h, G and H.
In this section, we investigate some fixed-figure results on Sb-metric spaces. At first, we recall the following notions:
Definition 3.1. [22,31] Let (X,Sb) be an Sb-metric space with s≥1 and ℏ0,ℏ1,ℏ2∈X, r∈[0,∞).
● The circle is defined by
CSbℏ0,r={μ∈X:Sb(μ,μ,ℏ0)=r}. |
● The disc is defined by
DSbℏ0,r={μ∈X:Sb(μ,μ,ℏ0)≤r}. |
● The ellipse is defined by
ESbr(ℏ1,ℏ2)={μ∈X:Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2)=r}. |
● The hyperbola is defined by
HSbr(ℏ1,ℏ2)={μ∈X:|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|=r}. |
● The Cassini curve is defined by
CSbr(ℏ1,ℏ2)={μ∈X:Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2)=r}. |
● The Apollonious circle is defined by
ASbr(ℏ1,ℏ2)={μ∈X−{ℏ2}:Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2)=r}. |
Definition 3.2. [22] Let g:X→X be a self-mapping where (X,Sb) is a Sb-metric space with s≥1. Let Fix(g) be set of all fixed points of g, then a geometric figure F is said to be a fixed figure of g if F is contained in Fix(g).
Let us define the number r as
r=inf{Sb(μ,μ,gμ):μ∉Fix(g)}. | (3.1) |
Theorem 3.1. Let (X,Sb) be an Sb-metric space with s≥1, g:X→X be a self-mapping, Sb be symmetric and r be defined as in (3.1). If there exist ℏ0∈X and ψ∈Ψ for all μ∈X−{ℏ0} such that
μ∉Fix(g)⟹Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ0),Sb(μ,μ,gℏ0),Sb(gμ,gμ,μ),Sb(gℏ0,gℏ0,ℏ0),Sb(gμ,gμ,ℏ0)) |
and gℏ0=ℏ0, then DSbℏ0,r⊂Fix(g). Especially, we have CSbℏ0,r⊂Fix(g).
Proof. Let r=0. Then we have DSbℏ0,r={ℏ0}. By the hypothesis gℏ0=ℏ0, we obtain
DSbℏ0,r⊂Fix(g). |
Let r>0 and μ∈DSbℏ0,r such that μ∉Fix(g). Using the hypothesis, we get
Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ0),Sb(μ,μ,gℏ0),Sb(gμ,gμ,μ),Sb(gℏ0,gℏ0,ℏ0),Sb(gμ,gμ,ℏ0))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ0),Sb(μ,μ,ℏ0),Sb(gμ,gμ,μ),Sb(ℏ0,ℏ0,ℏ0),Sb(gμ,gμ,ℏ0))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ0),Sb(μ,μ,ℏ0),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ0)). | (3.2) |
By (Sb2) and the symmetry property of Sb, we have
Sb(gμ,gμ,ℏ0)≤s[2Sb(gμ,gμ,μ)+Sb(ℏ0,ℏ0,μ)]=2sSb(gμ,gμ,μ)+sSb(μ,μ,ℏ0). | (3.3) |
Using (3.2), (3.3) and (ψ∗), there exists a k∈[0,1) such that
Sb(gμ,gμ,μ)≤kSb(μ,μ,ℏ0)≤kr≤kSb(gμ,gμ,μ)<Sb(gμ,gμ,μ), |
a contradiction. Hence it should be μ∈Fix(g). Consequently, we get
DSbℏ0,r⊂Fix(g). |
Using the similar arguments, it can be easily see that
CSbℏ0,r⊂Fix(g). |
Theorem 3.2. Let (X,Sb) be an Sb-metric space with s≥1, g:X→X be self-mapping, Sb be a symmetric and r be defined as in (3.1). If there exist ℏ1,ℏ2∈X and ψ∈Ψ for all μ∈X−{ℏ1,ℏ2} such that
μ∉Fix(g)⟹Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)+Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),Sb(gℏ1,gℏ1,ℏ1)+Sb(gℏ2,gℏ2,ℏ2),Sb(gμ,gμ,ℏ1)+Sb(gμ,gμ,ℏ2)) |
and gℏ1=ℏ1, gℏ2=ℏ2 with g(μ)∈ESbr(ℏ1,ℏ2), then ESbr(ℏ1,ℏ2)⊂Fix(g).
Proof. Let r=0. Then we have ESbr(ℏ1,ℏ2)={ℏ1}={ℏ2}. By the hypothesis gℏ1=ℏ1 and gℏ2=ℏ2, we obtain
ESbr(ℏ1,ℏ2)⊂Fix(g). |
Let r>0 and μ∈ESbr(ℏ1,ℏ2) such that μ∉Fix(g). Using the hypothesis, we get
Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)+Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),Sb(gℏ1,gℏ1,ℏ1)+Sb(gℏ2,gℏ2,ℏ2),Sb(gμ,gμ,ℏ1)+Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(gμ,gμ,μ),Sb(ℏ1,ℏ1,ℏ1)+Sb(ℏ2,ℏ2,ℏ2),Sb(gμ,gμ,ℏ1)+Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(μ,μ,ℏ1)+Sb(μ,μ,ℏ2),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ1)+Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),r,r,Sb(gμ,gμ,μ),0,r). |
Since
r≤2sSb(gμ,gμ,μ)+sr, |
using (ψ∗), there exists a k∈[0,1) such that
Sb(gμ,gμ,μ)≤kr≤kSb(gμ,gμ,μ)<Sb(gμ,gμ,μ), |
a contradiction. Hence it should be μ∈Fix(g). Consequently, we get
ESbr(ℏ1,ℏ2)⊂Fix(g). |
Theorem 3.3. Let (X,Sb) be an Sb-metric space with s≥1, g:X→X be self-mapping, Sb be a symmetric and r be defined as in (3.1). If r>0 and there exist ℏ1,ℏ2∈X, ψ∈Ψ for all x∈X−{ℏ1,ℏ2} such that
μ∉Fix(g)⟹Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,|Sb(μ,μ,gℏ1)−Sb(μ,μ,gℏ2)|,Sb(gμ,gμ,μ),|Sb(gℏ1,gℏ1,ℏ1)−Sb(gℏ2,gℏ2,ℏ2)|,|Sb(gμ,gμ,ℏ1)−Sb(gμ,gμ,ℏ2)|) |
and gℏ1=ℏ1, gℏ2=ℏ2 with g(μ)∈HSbr(ℏ1,ℏ2), then HSbr(ℏ1,ℏ2)⊂Fix(g).
Proof. Let r>0 and μ∈HSbr(ℏ1,ℏ2) such that μ∉Fix(g). Using the hypothesis, we get
Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,|Sb(μ,μ,gℏ1)−Sb(μ,μ,gℏ2)|,Sb(gμ,gμ,μ),|Sb(gℏ1,gℏ1,ℏ1)−Sb(gℏ2,gℏ2,ℏ2)|,|Sb(gμ,gμ,ℏ1)−Sb(gμ,gμ,ℏ2)|)=ψ(Sb(gμ,gμ,μ),|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,Sb(gμ,gμ,μ),|Sb(ℏ1,ℏ1,ℏ1)−Sb(ℏ2,ℏ2,ℏ2)|,|Sb(gμ,gμ,ℏ1)−Sb(gμ,gμ,ℏ2)|)=ψ(Sb(gμ,gμ,μ),|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,|Sb(μ,μ,ℏ1)−Sb(μ,μ,ℏ2)|,Sb(gμ,gμ,μ),0,|Sb(gμ,gμ,ℏ1)−Sb(gμ,gμ,ℏ2)|)=ψ(Sb(gμ,gμ,μ),r,r,Sb(gμ,gμ,μ),0,r). |
Since
r≤2sSb(gμ,gμ,μ)+sr, |
using (ψ∗), there exists a k∈[0,1) such that
Sb(gμ,gμ,μ)≤kr≤kSb(gμ,gμ,μ)<Sb(gμ,gμ,μ), |
a contradiction. Hence it should be μ∈Fix(g). Consequently, we get
HSbr(ℏ1,ℏ2)⊂Fix(g). |
Theorem 3.4. Let (X,Sb) be an Sb-metric space with s≥1, g:X→X be a self-mapping, Sb be symmetric and r be defined as in (3.1). If there exist ℏ1,ℏ2∈X and ψ∈Ψ for all μ∈X−{ℏ1,ℏ2} such that
μ∉Fix(g)⟹Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),Sb(gℏ1,gℏ1,ℏ1)Sb(gℏ2,gℏ2,ℏ2),Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2)) |
and gℏ1=ℏ1, gℏ2=ℏ2 with g(μ)∈CSbr(ℏ1,ℏ2), then CSbr(ℏ1,ℏ2)⊂Fix(g).
Proof. Let r=0. Then we have CSbr(ℏ1,ℏ2)={ℏ1} or {ℏ2}. By the hypothesis gℏ1=ℏ1 and gℏ2=ℏ2, we obtain
CSbr(ℏ1,ℏ2)⊂Fix(g). |
Let r>0 and μ∈CSbr(ℏ1,ℏ2) such that μ∉Fix(g). Using the hypothesis, we get
Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),Sb(gℏ1,gℏ1,ℏ1)Sb(gℏ2,gℏ2,ℏ2),Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(gμ,gμ,μ),Sb(ℏ1,ℏ1,ℏ1)Sb(ℏ2,ℏ2,ℏ2),Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),r,r,Sb(gμ,gμ,μ),0,r). |
Since
r≤2sSb(gμ,gμ,μ)+sr, |
using (ψ∗), there exists a k∈[0,1) such that
Sb(gμ,gμ,μ)≤kr≤kSb(gμ,gμ,μ)<Sb(gμ,gμ,μ), |
a contradiction. Hence it should be μ∈Fix(g). Consequently, we get
CSbr(ℏ1,ℏ2)⊂Fix(g). |
Theorem 3.5. Let (X,Sb) be an Sb-metric space with s≥1, g:X→X be a self-mapping, Sb be symmetric and r be defined as in (3.1). If there exist ℏ1,ℏ2∈X and ψ∈Ψ for all μ∈X−{ℏ1,ℏ2} such that
μ∉Fix(g)⟹Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2)) |
and gℏ1=ℏ1, gℏ2=ℏ2 with g(μ)∈ASbr(ℏ1,ℏ2), then ASbr(ℏ1,ℏ2)⊂Fix(g).
Proof. Let r=0. Then we have ASbr(ℏ1,ℏ2)={ℏ1}. By the hypothesis gℏ1=ℏ1, we obtain
ASbr(ℏ1,ℏ2)⊂Fix(g). |
Let r>0 and μ∈ASbr(ℏ1,ℏ2) such that μ∉Fix(g). Using the hypothesis, we get
Sb(gμ,gμ,μ)<ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,gℏ1)Sb(μ,μ,gℏ2),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(μ,μ,ℏ1)Sb(μ,μ,ℏ2),Sb(gμ,gμ,μ),0,Sb(gμ,gμ,ℏ1)Sb(gμ,gμ,ℏ2))=ψ(Sb(gμ,gμ,μ),r,r,Sb(gμ,gμ,μ),0,r). |
Since
r≤2sSb(gμ,gμ,μ)+sr, |
using (ψ∗), there exists a k∈[0,1) such that
Sb(gμ,gμ,μ)≤kr≤kSb(gμ,gμ,μ)<Sb(gμ,gμ,μ), |
a contradiction. Hence it should be μ∈Fix(g). Consequently, we get
ASbr(ℏ1,ℏ2)⊂Fix(g). |
Now we give the following illustrative example of above proved geometric results.
Example 3.1. Let us consider Example 2.2 given in [22]. Let X=[−1,1]∪{−7,−√2,√2,73,7,8,21} and the S-metric defined as
S(μ,τ,ℏ)=|μ−ℏ|+|μ+ℏ−2τ|, |
for all μ,τ,ℏ∈X [32]. Then the function S is also an Sb-metric with s=1. Let us define the function g:X→X as
gμ={7,μ=8μ,μ∈X−{8}, |
for all μ∈X and the function ψ:R6+→R as
ψ(t1,t2,t3,t4,t5,t6)=kt2, |
with k∈[0,1). Under these assumptions, we get
r=inf{S(μ,μ,gμ):μ∉Fix(g)}=inf{S(μ,μ,gμ):μ=8}=2. |
★ If we take ℏ0=0 and k=12, then the function g satisfies the conditions of Theorem 3.1. Therefore, we obtain
DSb0,2=[−1,1]⊂Fix(g)=X−{8} |
and
CSb0,2={−1,1}⊂Fix(g)=X−{8}. |
★ If we take ℏ1=−12, ℏ2=12 and k=12, then the function g satisfies the conditions of Theorem 3.2. Therefore, we obtain
ESb2(−12,12)=[−12,12]⊂Fix(g)=X−{8}. |
★ If we take ℏ1=−1, ℏ2=1 and k=34, then the function g satisfies the conditions of Theorem 3.3. Therefore, we obtain
HSb2(−1,1)={−12,12}⊂Fix(g)=X−{8}. |
★ If we take ℏ1=−1, ℏ2=1 and k=34, then the function g satisfies the conditions of Theorem 3.4. Therefore, we obtain
CSb2(−1,1)={−√2,0,√2}⊂Fix(g)=X−{8}. |
★ If we take ℏ1=−7, ℏ2=7 and k=15, then the function g satisfies the conditions of Theorem 3.5. Therefore, we obtain
ASb2(−7,7)={73,21}⊂Fix(g)=X−{8}. |
Recently, activation functions have been used in applicable areas. Especially, these functions are used in neural network. For example, for state-of-the-art neural networks, rectified activation units are essential. Therefore, in this section, we focus on the parametric rectified linear unit (PReLU) activation functions [33]. PReLU is defined as
PReLU(μ)={αμ,μ<0μ,μ≥0, |
where α is a coefficient.
Let X=R+∪{−2,−1,0}, the S-metric defined as in Example 3.1 and the function ψ:R6+→R defined as in Example 3.1. Let us consider α=0.8, then we obtain the function PReLU as
PReLU(μ)={0.8μ,μ<0μ,μ≥0, |
for all μ∈R. If we take ℏ0=0 and k=12, then the function PReLU satisfies the conditions of Theorem 3.1. Indeed, for μ∈(−∞,0), we get
S(PReLU(μ),PReLU(μ),μ)=2|0.8μ−μ|=0.4|μ|<|μ|=2k|μ|=kS(μ,μ,ℏ0)=kψ(Sb(PReLUμ,PReLUμ,μ),Sb(μ,μ,ℏ0),Sb(μ,μ,PReLUℏ0),Sb(PReLUμ,PReLUμ,μ),Sb(PReLUℏ0,PReLUℏ0,ℏ0),Sb(PReLUμ,PReLUμ,ℏ0)). |
Also, we obtain
r=inf{S(μ,μ,PReLUμ):μ∉Fix(PReLU)}=inf{0.2|μ|:μ<0}=inf{0.2,0.4}=0.2. |
Consequently, we have
DSb0,0.2=[0,0.1]⊂Fix(PReLU)=R+∪{0} |
and similarly
CSb0,0.2={0.1}⊂Fix(PReLU)=R+∪{0}. |
Finally, we say that the parametric rectified linear unit (PReLU) activation function fixes the disc DSb0,0.2 and CSb0,0.2, that is, PReLU has at least two fixed figure. In this way, the learning capacity of the activation function PReLU increases.
The authors I. Ayoob and N. Mlaiki would like to thank the Prince Sultan University for paying the publication fees for this work through TAS LAB.
The authors declare no conflicts of interest.
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