Suzuki-type fixed point results in metric type spaces

Suzuki’s fixed point results from (Suzuki, Proc. Am. Math. Soc. 136:1861-1869, 2008) and (Suzuki, Nonlinear Anal. 71:5313-5317, 2009) are extended to the case of metric type spaces and cone metric type spaces. Examples are given to distinguish our results from the known ones.

MSC: 47H10, 54H25.

In 2008 Suzuki proved the following refinement of Banach’s fixed point principle.

Theorem 1 ([1], Theorem 2])

Letbe a complete metric space. Letbe a selfmap andbe defined by

If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z.

There were various extensions of Suzuki’s result, such as Kikkawa-Suzuki’s version of Kannan’s theorem [2] and Popescu’s version of Ćirić’s theorem [3].

Suzuki proved also the following version of Edelstein’s fixed point theorem.

Theorem 2 ([4], Theorem 3])

Letbe a compact metric space. Letbe a selfmap, satisfying for all, the condition

ThenThas a unique fixed point inX.

This theorem was generalized in [5].

Let E be a real Banach space with the zero vector θ. A subset P of E is called a cone if: (a) P is closed, non-empty and ; (b) , , imply that ; (c) . Given a cone P, we define the partial ordering ⪯ with respect to P by if and only if . We shall write for , where intP stands for the interior of P and use for and . If , then P is called a solid cone. It is said to be normal if there is a number such that for all , implies . Such a minimal constant K is called the normal constant of P.

Huang and Zhang re-introduced cone metric spaces in [6] (this notion was known under various names since the mid of the 20th century, see a survey in [7]), replacing the set of real numbers by an ordered Banach space as the codomain for a metric. Cone metric spaces over normal cones inspired another generalization of metric spaces that were called metric type spaces by Khamsi [8] (see also [9-12]; note that, in fact, spaces of this kind were used earlier under the name of b-spaces by Czerwik [13]). Cvetković et al.[14] and Shah et al.[15] extended Khamsi’s definition and defined cone metric type spaces as follows:

Definition 1 ([14,15])

Let X be a nonempty set, E a Banach space with the solid cone P and let be a real number. If the function satisfies the following properties:

(a) if and only if ;

(b) for all ;

(c) for all ,

then D is called a cone metric type function and is called a cone metric type space (CMTS).

In particular, when and , CMTS reduces to a metric type space (MTS) of [8,9,12].

Of course, for we get the cone metric space (CMS) of [6], resp. the usual metric space.

Example 1 ([14])

Let be an orthonormal basis of with inner product and let . Define

where is the class of functions being equal to the function f a.e. Further, let

and let be defined by

It was shown in [14] that is a solid cone in and that is a CMTS. In particular, for we get an MTS and for a CMS.

Example 2 ([8,10])

Let be any CMS over a normal cone with normal constant . Then is an MTS, where . In this case the spaces and have the same topologies (see [10], Theorem 2.7]).

If is a CMTS over a normal cone with a normal constant , then is an MTS, where . Similarly as above, the spaces and have the same topologies.

Notions such as convergent and Cauchy sequences, as well as completeness, are introduced in (cone) metric type spaces in the standard way. The following obviously holds in an arbitrary (cone) metric type space:

We will sometimes need the continuity of metric-type function D in one variable:

or in two variables:

The last property always holds in the case of an MTS generated by a CMS over a normal cone, see Example 2, but not in general, as the following example shows.

Example 3 Let and let be defined by

Then it is easy to see that for all , we have

Thus, is a metric-type space. Let for each . Then

that is, , but as .

Recall that a selfmap is said to have the property (P) [16] if for each , where is the set of fixed points of T.

In this paper, we extend Suzuki’s Theorems 1 and 2, as well as Popescu’s results from [3] to the case of metric type spaces and cone metric type spaces. Examples are given to distinguish our results from the known ones.

2.1 Results in metric type spaces

Theorem 3Letbe a complete MTS whereDis continuous in each variable. Letbe a selfmap andbe defined by

whereis the positive solution of. If there existssuch that for each,

where

thenThas a unique fixed pointand for each, the sequenceconverges to z. Moreover, Thas the property (P).

Note that for , Theorem 3 reduces to a special case of Theorem 2.1 by Popescu [3].

Proof First note that implies that and it follows by (2.2) that

wherefrom

for each .

Let be arbitrary and form the sequence by and for . It follows from (2.3) that

and, by induction,

Using [12], Lemma 3.1] we conclude that is a Cauchy sequence, tending to some z in the complete space X. Obviously, also .

Let us prove now that

holds for each . Since and (and hence ) and, by continuity of D, , it follows that there exists such that

holds for each . Assumption (2.2) implies that for such n

Passing to the limit when (and using continuity of D), we get that

It is easy to see that (2.6) follows from the previous relation.

Putting in (2.3), we get that

holds for each (where ). It follows by induction that

We will prove now that

for each . For this relation is obvious. Suppose that it holds for some . If , then and . If , then we can apply (2.6) to obtain that

Using (2.8) and the induction hypothesis, we get that

and (2.9) is proved by induction.

In order to prove that , we suppose that and consider the two possible cases.

Case I. (and hence ). We will prove first that

for . For this is obvious and for it follows from (2.8). Suppose that (2.10) holds for some . Then

wherefrom . It follows (using (2.8)) that

Assumption (2.2) implies that

It is easy to see (using (2.8), (2.9) and the inductive hypothesis) that the last maximum is equal to , i.e., and relation (2.10) is proved by induction.

Now and (2.10) implies that for each . Hence, (2.6) and (2.8) imply that

Since , it follows from (2.10) that

There exists such that for and . For such n, we have that

It follows from (2.11) that

Thus, and, again from (2.10), we get that and , a contradiction.

Case II. (and so ). We will prove that there exists a subsequence of such that

holds for each . From (2.4) we know that holds for each . Suppose that

both hold for some . Then

which is impossible. Hence one of the following holds for each n:

In particular,

holds for each . In other words, there is a subsequence of such that (2.12) holds for each . But then assumption (2.2) implies that

Passing to the limit when we get that , which is possible only if , a contradiction.

Thus, we have proved that z is a fixed point of T. The uniqueness of the fixed point follows easily from (2.6). Indeed, if yz are two fixed points of T, then (2.6) implies that

wherefrom . The property (P) follows from (2.3) (see [16]). □

Suzuki-Banach-type and Suzuki-Kannan-type fixed point results in metric type spaces (versions of [1], Theorem 2] and [2], Theorem 2.2]) are special cases of Theorem 3.

Corollary 1Letbe a complete MTS whereDis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z. Moreover, Thas the property (P).

Corollary 2Letbe a complete MTS whereDis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z. Moreover, Thas the property (P).

Corollary 3Letbe a complete MTS whereDis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z. Moreover, Thas the property (P).

Adapting [1], Example 1] we give now an example of a mapping satisfying the conditions of Theorem 3 (and having a unique fixed point) but not satisfying the respective classical (non-Suzuki-type) condition in metric type spaces (see, e.g., [14], Theorem 3.4]).

Example 4 Let , and let be given by . Then is a metric type space (see Example 1). Let be given as

We will check that condition (2.2) holds true for and all . If or if , it is trivially satisfied. Let and . Then and for and for or . Hence, in any case,

Let now , . Then and and so , and (2.2) is trivially satisfied. Note that in the classical variant, in this case and , so the inequality does not hold for any .

The following is a metric-type version of Theorem 2.

Theorem 4Letbe a compact MTS, where the functionDis continuous. Letbe a selfmap, satisfying for all, the condition

ThenThas a unique fixed point inX.

Proof Denote and choose a sequence in X such that (). Since the space X is (sequentially) compact, we can suppose that there exist such that and (). We will prove that .

Suppose that and note that continuity of D implies that . Choose such that for all

holds true. Then and assumption (2.13) implies that for . Passing to the limit, we obtain that . If , the last inequality is impossible by the definition of β. If , it is possible only if (recall that we have supposed that ). But in this case and (2.13) implies that , which is again impossible by the definition of β. Hence, in all cases we obtain a contradiction and it follows that and so .

In order to prove that T has a fixed point, suppose that for all . Then, in particular, and (2.13) implies that

It follows that

when . Hence, ().

Suppose now that

both hold for some . Then

which is impossible. Thus, for each , either

holds true. Assumption (2.13) implies that for each either

holds. In other words, there exists a sequence such that holds for each , or there exists a sequence such that holds for each . In both cases, passing to the limit, we obtain that , i.e., , a contradiction with the assumption that T has no fixed points.

It follows that there exists such that . Uniqueness follows easily. □

2.2 Results in cone metric type spaces

In this subsection, we formulate cone-metric-type versions of the results from the previous subsection.

Theorem 5Letbe a complete CMTS with the normal underlying coneP, whereis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

for some

thenThas a unique fixed pointand for each, the sequenceconverges to z.

Proof Since the cone P is normal, without loss of generality, we can assume that the normal constant of P is and that the given norm in E is monotone, i.e. (see [17], Lemma 2.1]). Denote . Then D is a (real-valued) metric-type function and the space is compact (together with , see [10], Theorem 2.7]). Let us prove that the mapping T satisfies for some the condition

of Theorem 3. Suppose that . Then (indeed, if, to the contrary, i.e., it would follow that , a contradiction with the assumption). Assumption (2.14) implies that for some

Again by the monotonicity of the norm, this means that , where

Hence, condition (2.15) is satisfied, and the conclusion follows. □

In a similar way, the following corollaries and the theorem can be proved.

Corollary 4Letbe a complete CMTS whereis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z.

Corollary 5Letbe a complete CMTS whereis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

where, thenThas a unique fixed pointand for each, the sequenceconverges toz.

Corollary 6Letbe a complete CMTS whereis continuous in each variable. Letbe a selfmap andbe defined by (2.1). If there existssuch that for each,

thenThas a unique fixed pointand for each, the sequenceconverges to z.

Example 4 can be easily adapted to a CMTS.

Theorem 6Letbe a compact CMTS, where the functionis continuous. Letbe a selfmap satisfying, for all, the condition

ThenThas a unique fixed point inX.

Note that for the above theorem reduces to [5], Theorem 3.8].

The authors declare that they have no competing interests.

All authors contributed equally and significantly in writing this paper. All authors read and approved the final manuscript.

The first author gratefully acknowledges the support provided by the Deanship of Scientific Research (DSR), King Abdulaziz University during this research. The second, third and fourth authors are thankful to the Ministry of Science and Technological Development of Serbia.

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