1. Introduction and Preliminaries
Fixed point theory is one of the most popular tools for solving optimization and approximation problems in nonlinear analysis. For the first time, in 1922, Banach introduced a unique fixed point as follows.
Let
be a metric space. A mapping
, is said to be a contraction if there exists
such that for all
If the metric space
is complete then the mapping satisfying (
1) has a unique fixed point [
1]. In 1968, Kannan [
2] and in 1972, Chatterjea [
3] studied contractive mappings which give a unique fixed point on a complete metric space. In other words, Kannan stated the following result:
Let
be a complete metric space. If a mapping
, satisfies the inequality
where
and
, then
has a unique fixed point [
2]. The mappings satisfying (
2) are called Kannan type mappings. The significance of the Kannan fixed point theorem appeared in Subrahmanyam paper [
4]. He showed that a metric space is complete if and only if every Kannan type mapping has a fixed point. Banach contractions do not have this property; Connell in [
5] has given an example of metric space
that is not complete but every Banach contraction on
has a fixed point.
The Banach and Kannan fixed point theorems have been improved by various successful attempts. One such attempt is due to Reich, who proved that if
is a self-map on a complete metric space
that satisfies
where
and
, then
has a unique fixed point [
6]. Note that in (
3),
yields Banach’s fixed point theorem, while
yields Kannan fixed point theorem. The following example shows that this theorem is stronger than Banach and Kannan fixed point theorems.
Example 1 ([
6])
. Let , for and . It is clear that ϑ is not continuous at and so, it does not satisfy Banach fixed point condition. Now, consider that . It means that the Kannan fixed point condition cannot be satisfied. On the other hand, if we put , and , it satisfies (3), and the Reich fixed point is obtained. Definition 1 ([
7])
. Let be a metric space. We say, a mapping is sequentially convergent if for each sequence that is convergent then is also convergent. In 2011, new generalizations of Kannan fixed point theorem on a complete metric space are investigated as follows:
Theorem 1 ([
8])
. Let be a complete metric space and be mappings such that ϖ is continuous, one-to-one, and sequentially convergent. If and for all , then ϑ has a unique fixed point. Singh, Khan and, Fisher [
9] in 2012, extended Reich’s theorem. Later, In 2016, Malceski and Anevska [
10] introduced several extensions of the Kannan fixed point theorem. Hybrid contractions that merge linear and nonlinear contractions in the abstract spaces have been investigated in 2019 [
11]. Generalized
-Meir–Keeler–Khan mappings in metric spaces [
12], fixed points via simulation functions [
13], and
F-contractions [
14,
15] are new approaches to the fixed point theory. For further details on state-of-the-art fixed point theorems in metric and normed spaces, see [
16,
17,
18,
19,
20].
In this paper, Picard iteration is used proving our main results as follows:
Let
be a metric space and
be a mapping. For any
, the sequence
given by
is called the sequence of successive approximations with the initial value
[
21].
Definition 2 ([
22])
. Let be a metric space. A mapping is an interpolative Kannan type contraction if there exists a constant and such that for all with . 2. Generalized Kannan and Reich Fixed Point Theorems
In this section, using interpolative Kannan type contractions, we prove a fixed point theorem in complete metric spaces and then, applying sequentially convergent mappings, subadditive altering distance function and, constants, we generalize Kannan and Reich fixed point theorems. First, it should be noted that for ease of reading as well as for avoiding repetition, we set out here some terms that will apply to theorems and corollaries.
Remark 1. In the rest of the paper,
- 1.
let be a complete metric space,
- 2.
let be mappings such that ϖ is continuous, injective, and sequentially convergent,
- 3.
let Ψ be the class of all nondecreasing continuous functions such that ,
unless otherwise stated.
A generalization of Kannan type contraction, applying a continuous, injective, and sequentially convergent map to investigate a unique fixed point is presented in the first theorem.
Theorem 2. If , , andfor all , then ϑ has a unique fixed point. Proof. Suppose that is given and the sequence be defined as for .
By taking
and
in (
4), we get
We deduce that the sequence
is non-increasing and non-negative. Thus, there is a non-negative constant
l such that
. From (
5) we have
Then for every
we have,
Letting
in the last inequality, we conclude that
is a Cauchy sequence. As the mapping
is sequentially convergent, it implies that the sequence
is also convergent, i.e., there exists
p such that
. Since
is continuous,
. By taking
and
in (
4), we find that
Taking in the inequality above, we thus get and since is an injective, we have .
It remains to prove the uniqueness. Let
has two distinct fixed points
. Then
This shows that . □
In the next theorem, a new generalization of Kannan fixed point theorem will be proved.
Theorem 3. If , andfor all , then ϑ has a unique fixed point. Proof. Since , for every we have . Suppose that is given and the sequence be defined as for .
By taking
and
in (
6), we get
Then, for every
with
, we have,
Letting
, we get
Since
,
. From this, we conclude that
is a Cauchy sequence and so, there exists
such that
Further, the mapping
is sequentially convergent and since the sequence
is convergent, it implies that the sequence
is also convergent, i.e., there exists
such that
. Since
is continuous,
. Thus,
Letting
in (
7), we get
Now, from and , we conclude that . Finally, is an injective map, and thus, .
To prove uniqueness, let
x be another fixed point of
. Then by (
6), we have
The last inequality implies that , i.e., . This implies that . □
By taking
and
in Theorem 3, we get the Kannan theorem [
2].
Definition 3. A function is called a subadditive altering distance function if
- 1.
,
- 2.
for all .
Example 2. It is easy to check that the functions , , , are such subadditive altering distance functions.
Considering as a subadditive altering distance function, in the following theorem, we prove another extension of Kannan theorem.
Theorem 4. Let σ be a subadditive altering distance function. If , , andfor all , then ϑ has a unique fixed point. Proof. Since
, for every
,
. Suppose that
is given and the sequence
be defined as
for
. By taking
and
in (
8) we get
Putting
, it follows that
By (
9), for all
that
, we have
Letting
in (
10), we see that
is a Cauchy sequence in
. Thus, there exists
such that
Further, the mapping
is sequentially convergent. Since the sequence
is convergent, it implies that the sequence
is also convergent, i.e., there exists
such that
. Since
is continuous,
. Thus,
Now, letting
in (
11), we get
Since and , we get and thus .
To prove uniqueness, let
x be another fixed point of
. Then by (
8), we have
Since , the last inequality implies that , i.e., . Finally, the injectivity of implies . □
By taking
in Theorem 4, we can conclude the Singh fixed point theorem [
9] and by taking
and
in Theorem 4, Reich’s fixed point theorem will be obtained [
6].
Corollary 1. Let σ be a subadditive altering distance function. If , andfor all , then ϑ has a unique fixed point. In the following example, using Theorem 4, an extension of Kannan theorem is shown.
Example 3. Let endowed with the Euclidean metric. Let defined byfor each . Obviously the inequality (2) is not held for ϑ for every and so, we cannot use the Kannan fixed point theorem in this case. Defining byfor , follows that ϖ is continuous, injective, and sequentially convergent. Therefore, for all , , we have Finally, by Corollary (1), ϑ has a unique fixed point.
Similar to Theorem 4, the following theorem could be proved. Because of the similarities, we ignore the proof.
Theorem 5. If , , andfor all , then ϑ has a unique fixed point. Theorem 6. Let σ be a subadditive altering distance function. If , , , andfor all , then ϑ has a unique fixed point. Proof. Suppose that
is given and the sequence
be defined as
for
. By taking
and
in (
13), we get
Therefore,
for each
, and
.
By (
14), for all
with
, we have
This means that
is a Cauchy sequence. There exists
such that
Further, the mapping
is sequentially convergent. Since the sequence
is convergent, it implies that the sequence
is also convergent, i.e., there exists
such that
. Since
is continuous,
. Thus,
Now, letting
in (
15), we get
But , since and . Thus, .
To prove uniqueness, let
x be another fixed point of
. Then by (
14), we have
Since , the last inequality implies that , i.e., . Finally, the injectivity of implies . □
By taking
in Theorem 6, we can conclude the new extension of Kannan fixed point theorem presented in [
10].
Corollary 2. If , andfor all , then ϑ has a unique fixed point. In the next example, we will show that for a finite set with the Euclidean metric Corollary 2 will be true.
Example 4. Let endowed with the Euclidean metric. Let defined byfor each . Obviously the inequality (2) is not held for ϑ, for every . Defining byfor , ϖ is continuous, injective, and sequentially convergent. Since , we have Therefore, for all , Thus, by (16), ϑ has a unique fixed point. Similarly, the following theorem could be solved.
Theorem 7. If , , , andfor all , then ϑ has a unique fixed point.