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Multipled fixed point theorems in cone metric spaces
© Olaoluwa and Olaleru; licensee Springer. 2014
Received: 31 July 2013
Accepted: 21 November 2013
Published: 18 February 2014
In this paper, the concept of common multipled fixed points of w-compatible nonlinear contractive maps in cone metric spaces, which are generalizations of common coupled fixed points, is introduced. A new concept of product cone metric spaces is used to establish the existence and uniqueness of their multipled fixed points. Our results generalize several results in the literature including those of Olaleru (Fixed Point Theory Appl. 2009:657914, 2009), Sabetghadam et al. (Fixed Point Theory Appl. 2009:125426, 2009) and Abbas et al. (Appl. Math. Comput. 217:195-202, 2010; Appl. Math. Comput. 216:80-86, 2010). In addition, the methodology of proof in this manuscript shows that some fixed point results in cone metric spaces are equivalent to multipled fixed point results and vice versa.
MSC: 47H10, 47H09, 54H25.
Over the years, fixed point theory has evolved from one-dimensional fixed point in metric spaces to multi-dimensional fixed points in metric-type spaces. Among such spaces are cone metric spaces and partially ordered metric spaces whose structures are induced implicitly or explicitly by partial orderings.
Huang and Zhang  top the chronological list of authors who have established fixed point results in cone metric spaces. They proved some fixed point theorems for single maps in cone metric spaces. Their results were followed by several other fixed point results on existence and uniqueness of fixed points for single contractive maps and common fixed point results of pairs of maps, sets of three self-maps and quartets of maps (see [2–8]). Theorems with nonuniqueness of fixed points of Ciric-type maps were later proved by Karapinar  in cone metric spaces.
Sabetghadam et al. in  introduced the concept of coupled fixed points in cone metric spaces and established some coupled fixed point theorems in cone metric spaces. Lakshmikantham and Ciric  proved coupled coincidence and coupled common fixed point theorems for nonlinear contractive mappings in partially ordered complete metric spaces. Later, Abbas et al.  extended their results to cone metric spaces. Karapinar  proved some coupled fixed point theorems for nonlinear contractions in cone metric spaces, Shatanawi et al.  proved some coupled fixed point theorems in ordered cone metric spaces with a c-distance, and Kadelburg and Radenović  established coupled fixed point results under TVS-cone metric and w-cone-distance.
Berinde in  and , and Amini-Harandi  discussed coupled and tripled fixed points of mixed monotone maps in partially ordered metric spaces, while Karapinar in  studied quadruple fixed point theorems for weak Φ-contractions.
Recently, Samet and Vetro  generalized the notion of coupled fixed points to fixed points of N-order and proved existence results for single maps in complete metric spaces. Their results came as an extension of the theory of fixed points to finite dimensions. Roldán et al.  then added more variation to the definition of multi-dimensional fixed points in partially ordered complete metric spaces by exploiting the notions of permutations and partitions. Their definition, though more general than that of Samet and Vetro , can hardly be used to establish fixed point theorems beyond the Banach contraction principle for two maps.
The main purpose of this article is to prove results on common fixed points of N-order of some contractive w-compatible mappings in cone metric spaces for as many as four maps, with generalized inequalities of Ciric type. Furthermore, we prove that multipled fixed point results can, in fact, generate fixed point results of lower dimensions.
We recall some notions in the concept of cone metric spaces.
Definition 1.1 
P is closed, nonempty and ;
, , imply that ;
Given a cone P, a partial ordering ⪯ with respect to P is defined by if and only if . We shall write for , where intP stands for the interior of P. Also, we will use to indicate that and .
The cone P in a normed space E is called normal whenever there is a real number such that for all , implies . The least positive number satisfying this norm inequality is called the normal constant of P. Janković et al. in  proved that only fixed point results in non-normal cones improve the existing fixed point results in metric spaces.
Definition 1.2 
Let X be a nonempty set and let E be a real Banach space equipped with the partial ordering ⪯ with respect to the cone . Suppose that the mapping satisfies:
(d1) for all and if and only if ;
(d2) for all ;
(d3) for all .
Then d is called a cone metric on X and is called a cone metric space.
The concept of a cone metric space is obviously more general than that of a metric space.
Definition 1.3 
Let be a cone metric space, be a sequence in X and . We say that is
(c1) a Cauchy sequence if for every with , there is some such that, for all , ;
(c2) a convergent sequence if for every with , there is some such that, for all , . Such x is called limit of the sequence .
Note that every convergent sequence in a cone metric space X is a Cauchy sequence. A cone metric space X is said to be complete if every Cauchy sequence in X is convergent in X.
Example 1.4 Let , , , such that , where is a constant. Then is a cone metric space.
Example 1.5 
Let (), , be a metric space, and be defined by . Then is a cone metric space.
Example 1.6 
Let , and . The mapping defined by , where , gives the structure of a complete cone metric space. The cone P is non-normal.
Olaleru in  and later Abbas et al. in  proved the existence of the fixed points of single maps and the common fixed points of pairs and quartets of w-compatible maps satisfying some general contractive conditions. Below are some of their main results.
Theorem 1.7 
for all , where and . Then the mapping f has a unique fixed point. Moreover, for any , the sequence converges to the fixed point.
Theorem 1.8 
for all , where and .
Suppose that f and g are weakly compatible and such that or is a complete subspace of X. Then the mappings f and g have a unique common fixed point. Moreover, for any , the sequence defined by for all n, converges to the fixed point.
Theorem 1.9 
for all , where satisfy .
We now show that results on common multipled fixed points (and of course common coupled fixed points) for single maps, pairs of maps and four maps in cone metric spaces can be derived from known results (e.g., Theorems 1.7-1.9) on fixed points of contractive maps in cone metric spaces. In order to show this, we make use of the concept of product cone metric spaces.
2 Multipled fixed points and w-compatibility
Definition 2.1 (see )
x is called a fixed point, a coupled fixed point, a tripled fixed point, and a quadruple fixed point of F if , respectively. All through this paper, we will refer to ‘fixed points of m-order’ simply as ‘multipled fixed points’; the integer m is defined as the multiplicity or the order of the multipled fixed point x.
Now, we introduce the following definitions of multipled coincidence points, common multipled fixed points and w-compatibility for maps defined in finite-dimensional product spaces.
where is the i th line of the circular matrix of x, , as defined previously in (2.3). The element is called the multipled point of coincidence of F and g.
i.e., for all , with , the circular matrix of x as previously defined.
When , x is a common fixed point, a common coupled fixed point, a common tripled fixed point and a common quadruple fixed point, respectively.
where is the i th line of the circular matrix of x as defined in (2.3). It is obvious that a point is a multipled fixed point of F if and only if it is a fixed point of .
is a fixed point of , a coincidence point of and , or a common fixed point of and if , or , respectively;
and are w-compatible if whenever .
We also state the following proposition.
An element is a multipled coincidence point (or a common multipled fixed point) of and if and only if is a coincidence point (or a common fixed point) of the associate mappings and defined in (2.7).
The maps F and g are w-compatible if and only if and are w-compatible in .
It is interesting to note the form of multipled fixed points, multipled coincidence points or common multipled fixed points, when they are unique. Rearranging equalities (2.1), (2.4) or (2.6), we can state the following.
Proposition 2.6 If is a multipled coincidence (or common multipled fixed) point of F and g, then the elements , (where is the circular matrix of x) are also multipled coincidence (or common multipled fixed) points of F and g. Hence, if x is a unique multipled coincidence (or common multipled fixed) point of F and g, then for all . Thus .
Example 2.7 Let and let be defined by .
hence the system has a unique solution, , which is the unique multipled fixed point of F.
3 Multipled fixed point theorems in cone metric spaces
We start the section by introducing and defining some notions on fixed point results in product cone metric spaces.
Definition 3.1 Let , , be m cone metric spaces with respect to cones in a Banach space E such that for all i, j. The set , together with defined by , where , , is a cone metric space with respect to the cone . Z is called a product cone metric space.
where , .
The proof of the following follows immediately.
Proposition 3.2 Let be a cone metric space with respect to d and the product cone metric space.
(p1) A sequence converges to if and only if the sequences converge to for all .
(p2) A sequence is a Cauchy sequence in if and only if the sequences are Cauchy sequences in X for all .
(p3) is complete if and only if is complete.
is the product cone metric space of and .
It should be noted that Theorems 1.7-1.9 can be extended to product cone metric spaces endowed with metric D as defined in (3.1).
Now, we state the following theorem for a single map.
converge to for each .
for all .
Applying Theorem 1.7 to product cone metric spaces, it follows that has a unique fixed point, which is the unique multipled fixed point of F. Since, the multipled fixed point is unique, it is of the form with (from Proposition 2.6).
as defined in (3.3). □
F and g are such that ,
or is a complete subset of X, and
F and g are w-compatible,
converges to .
for all .
for all , , where (), and .
If one of , , or is a complete subspace of X, then and have a unique m-tupled point of coincidence in X.
converge both to .
for every .
The contractive condition in Theorem 1.9 is satisfied for , , and .
If one of , , or is complete, then , , or is complete in , hence by Theorem 1.9 applied to the product cone metric space , the pairs and have unique points of coincidence which are unique multiple coincidence points of and .
If, in addition, and are w-compatible, then and are w-compatible. By Theorem 1.9, , , and have a unique common fixed point which is the unique multiple common fixed point of f, g, S and T. Because of the uniqueness, it is of the form for some .
Hence the sequences and defined in (3.7) converge to . □
Remark 3.7 Theorem 3.4 extends Theorem 1.7 (Olaleru ), the results of Sabetghadam et al.  and Theorem 4.1.1 of Olaleru and Olaoluwa  to multipled fixed points. Theorem 3.5 is an extension of Theorem 2.4 of Abbas et al.  and Theorem 4.2.1 of Olaleru and Olaoluwa  to multipled fixed points. Theorem 3.6 generalizes Theorems 3.4 and 3.5 and extends the results of Abbas et al.  (Theorem 1.9) and Theorem 4.3.1 of Olaleru and Olaoluwa  to multipled fixed points. To the best of our knowledge, it is the most general theorem in the theory of multipled fixed points that deals with four maps.
Example 3.8 Let be the cone metric space defined in Example 1.6. Consider the mappings and defined by and , where .
for all , , and . Hence F and g have a unique common multipled fixed point . It is easy to notice that .
Example 3.9 Let , , and be defined by . is a cone metric space. Let and be defined by and . f, g, S, T satisfy the conditions of Theorem 3.6 with , , and . The couple is the unique common coupled fixed point of the four maps f, g, S and T.
4 Retrieving lower multiplicity fixed points
It would be interesting to show that fixed point of m-order results can be used to obtain parallel results on fixed points of a lower order k. That is, fixed point results (which are fixed point of 1-order results) can be generated from similar results on coupled, tripled or even quadruple fixed points. In the sequel, we prove Theorem 3.6 for order k, using the same theorem for a higher-order m.
Let X be a cone metric space and be a product cone metric space, . Consider the mappings and . Also consider the maps defined by and for all , where (k and m are integers such that ).
Obviously, and ; hence the conditions and imply that and , respectively. Also, if or is complete, then or is complete, respectively.
for all , where and .
for all , where , and . If f, g, S, T are such that , and either , , or is complete, then , and either , , or is complete. Under these conditions, and have unique points of coincidence of order m. From Proposition 2.6, they are of the form and .
If, in addition, and are w-compatible, they commute at their respective coincidence points and so, obviously, and also commute at their respective coincidence points . The coincidence points being unique, and are w-compatible; from Theorem 3.6, , , S and T have a unique common fixed point of order m, . It obviously follows that f, g, S, T have a unique common fixed point of k-order.
The choice of Theorem 3.6 does not lessen the generality given that the notion of w-compatibility is used. The same method can be used for Theorem 3.4 or Theorem 3.5. We summarize by stating the following theorem.
Theorem 4.1 Theorems of existence of multipled fixed points of maps satisfying some contractive conditions can be considered as applications of corresponding results of fixed points of higher multiplicity. In particular, the fixed point results are applications of coupled fixed point results or tripled fixed point results, coupled fixed point results are applications of tripled fixed point results, etc.
The authors would like to thank their referees for the helpful suggestions.
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