1. Introduction
Pursuant upon the reporting of the famous Contraction Mapping Theorem (CMT) by S.Banach [
1] in 1922, the study of the existence and uniqueness of fixed points and common fixed points in metric and metric-like spaces and their applications has become a subject of great interest. In 1979, Itoh [
2] presented the application of fixed point results to differential equations in Banach spaces. Many authors proved the Banach contraction principle in various generalized metric spaces. In the sequel, the notion of rectangular metric space was introduced by Branciari [
3] in 2000. He replaced the right-hand side of the triangular inequality of the metric space with a three-term expression and established an analogous proof of the CMT. Since then, many fixed point theorems for various contractions on rectangular metric spaces have appeared in the literature [
4,
5,
6,
7,
8,
9,
10].
In 1965, Zadeh [
11] introduced the concept of fuzzy sets, which has varied applications in logical semantics. The concept of the continuous t-norm was introduced by Schweizer et al. [
12]. Kramosil and Michlek [
13] were the first to introduce the notion of fuzzy metric space, using continuous t-norms as an analog to metric spaces, and analyzed the notions with the probabilistic/statistical extension of metric spaces. The concept of fuzzy sets and fuzzy metric space has varied applications in applied sciences, such as fixed point theorems, signal and image processing including medical imaging, decision making, etc. Garbiec [
14] reported the fuzzy extension of the Banach contraction mapping theorem. Since then, many fixed point results have been reported by researchers using different types of contractive conditions in the setting of fuzzy metric space, dislocated fuzzy metric space, intuitionistic fuzzy metric space, etc.; see [
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29]. More recently, Ali et al. [
30] have presented some applications of the best proximity points of non-self maps in the setting of non-Archimedian fuzzy metric spaces.
In the recent past, many researchers have reported fixed point results in the setting of fuzzy metric spaces and the like. For instance, in 2018, Mlaiki [
31] presented fixed point results by defining the concept of controlled metric spaces. Konwar [
32], in 2020, defined intutionistic fuzzy b-metric space and established fixed point results under various contractive conditions. Saif Ur Rehman et al. [
33] proved some
fuzzy cone contraction results with integral-type application.
In the sequel, the concept of neutrosophic metric spaces was introduced by Kirisci and Simsek and various fixed point results were established by them in the setting of these spaces [
34,
35,
36]. Subsequently, Sowndrarajan et al. [
37] reported several fixed point results in neutrosophic metric spaces. Sezen [
38] presented the concept of controlled fuzzy metric spaces and derived various fixed point results. The concept of fuzzy double-controlled metric space was given by Saleem et al. [
39] in 2021. More recently, Uddin et al. [
40] established the fixed point theorem on neutrosophic double-controlled metric space and presented an application to the derived result thereon.
Inspired by the above, in the present work, we define the notion of neutrosophic rectangular triple-controlled metric space and establish fixed point theorems. Accordingly, we have organized the rest of the manuscript as follows. Some preliminaries and a monograph are presented in
Section 2. In
Section 3, we define the neutrosophic metric space and define the Cauchy sequence and its convergence and establish fixed point results. We support the derived results with non-trivial examples. In
Section 4, we establish the existence of a unique analytical solution to the Fredholm integral equation. We have also supplemented the derived results by finding the closed form of the unique solution to the intergral equation.
3. Main Results
We commence this section by defining neutrosophic rectangular triple-controlled metric space.
Definition 7. Let and be given non-comparable functions, ⋆ be a continuous t-norm, and ⋄ be a continuous t-co-norm. The neutrosophic set on is said to be a neutrosophic rectangular triple-controlled metric on ℑ, if, for any and all distinct , the following conditions are satisfied:
- (i)
;
- (ii)
;
- (iii)
for all , if and only if ;
- (iv)
;
- (v)
;
- (vi)
is continuous and ;
- (vii)
;
- (viii)
for all , if and only if ;
- (ix)
;
- (x)
;
- (xi)
is continuous and ;
- (xii)
;
- (xiii)
for all , if and only if ;
- (xiv)
;
- (xv)
;
- (xvi)
is continuous and ;
- (xvii)
If , then and .
Then, is called a neutrosophic rectangular triple-controlled metric space.
Example 1. Let and be three non-comparable functions given by , , and . Define as
Then, is a neutrosophic rectangular triple-controlled metric space with continuous t-norm and continuous t-co-norm, .
Proof. We need to prove only (v), (x) and (xv).
Let
,
and
. Then
Then, this satisfies all
. Hence,
The above expression is true for all .
Then, it satisfies all
. Hence,
Hence, is a neutrosophic rectangular triple-controlled metric space. □
Remark 1. It can be seen that the preceding example satisfies both the continuous t-norm and continuous t-co-norm .
Example 2. Let , where and and be given by , and . Define as follows:
Define as Then, is a neutrosophic rectangular triple-controlled metric space with continuous t-norm and continuous t-co-norm .
Definition 8. Let be a neutrosophic rectangular triple-controlled metric space, an open ball with center ϑ, radius and on is defined as below:
Definition 9. Let be a neutrosophic rectangular triple-controlled metric space and be a sequence in ℑ. Then, is said to be
- (a)
convergent if there exists such that - (b)
Cauchy, if and only if, for each , there exists such that is called a complete neutrosophic rectangular triple-controlled metric space if every Cauchy seqeunce is convergent in ℑ.
Lemma 1. Let be a neutrosophic rectangular triple-controlled metric space. If, for some and for any ,
Moreover, from (iii), (viii), (xiii), we have . □
Theorem 1. Suppose is a complete neutrosophic rectangular triple-controlled metric space in the company of with . Let be a mapping satisfyingfor all and . Then, has a unique fixed point. Proof. Let ∈ℑ. Define the sequence by .
Using (v), (x) and (xv), we have the following cases:
Case 1. When
, i.e.,
is odd, then
and
Using (
3) in the above inequalities, we have
Case 2. When
, i.e.,
is even, then
and
Using (
3) in the above inequalities, we have
and,
Therefore,
is a Cauchy sequence. Since
is a complete neutrosophic rectangular triple-controlled metric space, we have
Now, due to the fact that ϑ is a fixed point of
, utilizing e, j, and o, we obtain
Hence, .
Now, we examine the uniqueness. Let
for some
, then
by using (iii), (viii) and (xiii), . □
Definition 10. Let be a neutrosophic rectangular triple-controlled metric space. A map is an NRT(neutrosophic rectangular triple)-controlled contraction if there exists , such that
andfor all and . We now present the fixed point result for an NRT (neutrosophic rectangular triple)-controlled contraction.
Theorem 2. Let be a complete neutrosophic rectangular triple-controlled metric space with . Let be an ND-controlled contraction. Further, suppose that for an arbitrary , and , where . Then, has a unique fixed point.
Proof. Let
∈ℑ. Define
by
. From (
4)–(
6), for all
, we deduce
Proceeding in this way, we have
Using (v), (x) and (xv), we have the following cases:
Case 1. When
, i.e.,
is odd, then
and
Using (
3) in the above inequalities, we deduce
Case 2. When
, i.e.,
is even, then
and
Using (
3) in the above inequalities, we deduce
Therefore,
is a Cauchy sequence. Since
is a complete neutrosophic rectangular triple-controlled metric space, we have
Now, we examine that ϑ is a fixed point of
. Utilizing (v), (x) and (xv), we obtain
Using the above inequality, we obtain
Hence,
. For uniqueness, consider that
for some
. Then,
which are contradictions.
Thus, we have and , and accordingly, . □
Example 3. Let and be three non-comparable functions given by
Define as Then, is a complete neutrosophic rectangular triple-controlled metric space with continuous t-norm and continuous t-co-norm .
Define by and take , thenand Thus, all conditions of Theorem 1 are satisfied with 0 as the unique fixed point for .
Application
Let , the set of real-valued continuous functions defined on .
Consider the integral equation
where
is a fuzzy function of
and
. Define
and
by means of
and
with the continuous t-norm and continuous t-co-norm defined by
and
, respectively. Define
as
and
Then,
is a complete neutrosophic rectangular triple-controlled metric space. Let
for
and for all
. Moreover, let
. Then, the integral Equation (
10) has a unique solution.
Proof. Now, for all
, we deduce
and
Thus, all the conditions of Theorem (1) are satisfied and operator
has a unique fixed point. This proves that (
10) has a unique solution. □
Example 4. Consider the integral equation
Then, it has a solution in ℑ.
Proof. Let be defined by
and set
and
, where
, and for all
. Then, we have
Furthermore, see that , where . It is easy to verify all other conditions of the preceding application and hence a solution exists in ℑ. □
Indeed, the closed form of the unique solution for the integral equation of Example 4 using the software is found to be
and the graph of the solution is shown in
Figure 1.