Suzuki’s type fixed point theorems for generalized mappings in partial cone metric spaces over a solid cone
 Wenqing Xu^{1, 2},
 Chuanxi Zhu^{1}Email author and
 Chunfang Chen^{1}
https://doi.org/10.1186/s136630160538z
© Xu et al. 2016
Received: 14 July 2015
Accepted: 31 March 2016
Published: 6 April 2016
Abstract
In this paper, we obtain some Suzukitype fixed point theorems for generalized mappings in partial cone metric spaces over a solid cone. Our results unify and generalize various known comparable results in the literature. We also provide illustrative examples in support of our new results.
Keywords
partial cone metric spaces solid cone Suzuki type fixed pointMSC
47H10 54H251 Introduction and preliminaries
In 1994, Matthews [1] introduced the notion of a partial metric space as a part of the study of denotational semantics of data for networks, showing that the Banach contraction mapping principle can be generalized to the partial metric context for applications in program verification. After that, many fixed point theorems for mappings satisfying different contractive conditions in (ordered) partial metric spaces have been proved (see [2–4]).
In 2007, Huang and Zhang [5] introduced the concept of cone metric spaces and extended the Banach contraction principle to cone spaces over a normal solid cone. Moreover, they defined the convergence via interior points of the cone. Such an approach allows the investigation of the case that the cone is not necessarily normal. Since then, there were many references concerned with fixed point results in (ordered) cone spaces (see [6–15]). In 2012, based on the definition of cone metric spaces and partial metric spaces, Sonmez [16, 17] defined a partial cone metric space and proved some fixed point theorems for contractive type mappings in complete partial cone metric spaces.
Recently, without using the normality of the cone, Malhotra et al. [18] and Jiang and Li [19] extended the results of [16, 17] to θcomplete partial cone metric spaces.
First, we recall the definition of partial metric spaces (see [1]).
Definition 1.1
([1])
 \((p1)\) :

\(p(x,x)=p(x,y)=p(y,y)\) if and only if \(x=y\);
 \((p2)\) :

\(p(x,x)\leq p(x,y)\);
 \((p3)\) :

\(p(x,y)=p(y,x)\);
 \((p4)\) :

\(p(x,y)\leq p(x,z)+p(z,y)p(z,z)\).
The pair \((X,p)\) is called a partial metric space. If \(p(x,y)=0\), then the \((p1)\) and \((p2)\) imply that \(x=y\), but the converse does not hold in general. A trivial example of a partial metric space is the pair \((\mathbb{R}^{+},p)\), where \(p:\mathbb{R}^{+}\times\mathbb{R}^{+}\to\mathbb {R}^{+}\) is defined as \(p(x,y)=\max\{x,y\}\); see also [1].
 (i)
\(ax+by \in P\) for all \(x,y \in P\) and \(a,b\geq0\), and
 (ii)
\(P \cap(P) = \{\theta\}\), where θ is the zero element of E.
Each cone P of E determines a partial order ⪯ on E by \(x \preceq y \) if and only if \(y x \in P\) for all \(x,y \in E\). We shall write \(x\prec y\) if \(x\preceq y\) and \(x\neq y\).
A cone P of a topological vector space E is solid if \(\operatorname {int}P \neq\varnothing\), where intP is the interior of P. For all \(x,y \in E\) with \(yx \in \operatorname {int}P\), we write \(x\ll y\). Let P be a solid cone of a topological vector space E. A sequence \(\{u_{n}\}\) of E weakly converges [18] to \(u\in E\) (denoted \(u_{n}\stackrel{w}{\to}u\)) if for each \(c\in \operatorname {int}P\), there exists a positive integer \(n_{0}\) such that \(uc\ll u_{n} \ll u+c\) for all \(n\geq n_{0}\). A cone P of a normed vector space \((E,\Vert \cdot \Vert )\) is normal if there exists \(K > 0\) such that \(\theta \preceq x\preceq y\) implies that \(\Vert x\Vert \leq K\Vert y\Vert \) for all \(x,y \in P\), and the minimal K is called a normal constant of P. Next, we state the definitions of cone metric and partial cone metric spaces and some of their properties (see [5, 16–19]).
Definition 1.2
([5])
 \((d1)\) :

\(d(x,y)=\theta\) if and only if \(x=y\);
 \((d2)\) :

\(d(x,y)=d(y,x)\);
 \((d3)\) :

\(d(x,y)\preceq d(x,z)+d(z,y)\).
Definition 1.3
 \((p1)\) :

\(p(x,x)=p(x,y)=p(y,y)\) if and only if \(x=y\);
 \((p2)\) :

\(p(x,x)\preceq p(x,y)\);
 \((p3)\) :

\(p(x,y)=p(y,x)\);
 \((p4)\) :

\(p(x,y)\preceq p(x,z)+p(z,y)p(z,z)\).
Note that each cone metric is certainly a partial cone metric. The following example shows that there do exist partial cone metrics that are not cone metrics.
Example 1.1
([19])
A partial cone metric p on X over a solid cone P generates a topology \(\tau_{p}\) on X, which has a base of the family of open pballs \(\{B_{p}(x,c):x\in X,\theta\ll c\}\), where \(B_{p}(x,c)=\{y\in X:p(x,y)\ll p(x,x)+c\}\) for \(x\in X\) and \(c\in \operatorname {int}P\).
Definition 1.4
([19])
 (i)
A sequence \(\{x_{n}\}\) in X converges to \(x \in X\) (denoted by \(x_{n} \stackrel{\tau_{p}}{\to}x\)) if for each \(c\in \operatorname {int}P\), there exists a positive integer \(n_{0}\) such that \(p(x_{n},x) \ll p(x,x)+c\) for each \(n \geq n_{0}\) (that is, \(p(x_{n},x)\stackrel{w}{\to}p(x,x)\)).
 (ii)
A sequence \(\{x_{n}\}\) in X is θCauchy if for each \(c \in \operatorname {int}P\), there exists a positive integer \(n_{0}\) such that \(p(x_{n},x_{m})\ll c\) for all \(m,n\geq n_{0}\). The partial cone metric space \((X,p)\) is θcomplete if each θCauchy sequence \(\{x_{n}\}\) of X converges to a point \(x \in X\) such that \(p(x,x) = \theta\).
Definition 1.5
 (i)A sequence \(\{x_{n}\}\) in X strongly converges to \(x \in X\) (denoted by \(x_{n} \stackrel{s\tau_{p}}{\to}x\)) if$$\lim_{n\to\infty}p(x_{n},x)=\lim_{n\to\infty}p(x_{n},x_{n})= p(x,x). $$
 (ii)
A sequence \(\{x_{n}\}\) in X is Cauchy if there exists \(u\in P\) with \(\Vert u\Vert <\infty\) such that \(\lim_{m,n\to\infty}p(x_{m},x_{n})=u\). The partial cone metric space \((X,p)\) is complete if each Cauchy sequence \(\{x_{n}\}\) of X strongly converges to a point \(x \in X\) such that \(p(x,x) = u\).
Note that if P is a normal solid cone of a normed vector space \((E,\Vert \cdot \Vert )\), then each complete partial cone metric space is θcomplete. But the converse is not true. The following example ([16], Example 4) shows that a θcomplete partial cone metric space is not necessarily complete.
Example 1.2
([16])
Let X be a nonempty set, and \(S,T:X\to X\) be two mappings. A point \(x\in X\) is said to be a coincidence point of S and T if \(Sx=Tx\). A point \(y\in X\) is called point of coincidence of S and T if there exists a point \(x\in X\) such that \(y=Sx=Tx\). The mappings S, T are said to be weakly compatible if they commute at their coincidence points (that is, \(TSz=STz\) whenever \(Sz=Tz\)).
Let \((X,\sqsubseteq)\) be a partially ordered set; \(x,y\in X\) are called comparable if \(x\sqsubseteq y\) or \(y \sqsubseteq x\). A mapping \(T:X\to X\) is said to be nondecreasing if for \(x,y\in X\), \(x\sqsubseteq y\) implies \(Tx\sqsubseteq Ty\). Let \(S,T:X\to X\) be two mappings; T is said to be Snondecreasing if \(Sx\sqsubseteq Sy\) implies \(Tx\sqsubseteq Ty\) for all \(x,y\in X\).
Bhasker and Lakshmikantham [20] introduced the concepts of mixed monotone mappings and coupled fixed point.
Definition 1.6
([20])
Definition 1.7
([20])
An element \((x,y) \in X^{2}\) is said to be a coupled fixed point of the mapping \(A : X^{2}\to X\) if \(A(x,y)= x\) and \(A(y,x)=y\).
Lemma 1.1
([21])
Let X be a nonempty set, and \(S:X\to X\) a mapping. Then there exists a subset \(Y\subseteq X\) such that \(SY=SX\) and \(S:Y\to X\) is onetoone.
Paesano and Vetro [22] proved Suzukitype characterizations of completeness for partial metric spaces and fixed points for partially ordered metric spaces. Note that if in Theorem 2 of [22], we assume that p is a metric, then we obtain Theorem 2 of [23]. Recently, also, some Suzukitype fixed point and coupled fixed point results for mappings or generalized multivalued mappings in different metric spaces were investigated (see [24–29]). The aim of the paper is to give a generalized version of Theorems 2 and 3 of [22] in partially ordered partial cone metric spaces over a solid cone. Meantime, we also establish the corresponding Suzukitype coupled fixed point results for generalized mappings in partially ordered partial cone metric spaces. It is worth pointing out that some examples are presented to verify the effectiveness and applicability of our results.
2 Fixed point theorems in partial cone metric spaces
In this section, we first give some properties of partial cone metric spaces. The following properties are used (particularly when dealing with cone metric spaces in which the cone need not be normal).
Remark 2.1
 (1)
If \(a\preceq b\) and \(b\preceq c\), then \(a\preceq c\).
 (2)
If \(a\ll b\) and \(b\ll c\), then \(a\ll c\).
 (3)
If \(\theta\preceq u\ll c\) for each \(c\in \operatorname {int}P\), then \(u=\theta\).
 (4)
If \(a\preceq\lambda a\), where \(0\leq\lambda<1\), then \(a=\theta\).
 (5)
If \(a\preceq b +c\) for each \(c\in \operatorname {int}P\), then \(a\preceq b\).
Now, we establish some Suzukitype fixed point theorems for generalized mappings in partially ordered partial cone metric spaces over a solid cone.
Theorem 2.1
 (i)
there exists \(x_{0}\in X\) such that \(x_{0}\sqsubseteq Tx_{0}\);
 (ii)
for a nondecreasing sequence \(x_{n}\stackrel{\tau_{p}}{\to} x\), we have \(x_{n}\sqsubseteq x\) for all \(n\in N\);
 (iii)
for two nondecreasing sequences \(\{x_{n}\}, \{y_{n}\}\subseteq X\) such that \(x_{n}\sqsubseteq y_{n}\), \(x_{n}\stackrel{\tau_{p}}{\to} x\), and \(y_{n} \stackrel{\tau_{p}}{\to} y\) as \(n\to\infty\), we have \(x\sqsubseteq y\).
 (iv)
for all \(x,y\in X\) that are not comparable, there exists \(u\in X\) comparable with x and y.
Proof

Case 1. \(p(Tx,T^{2}x) \preceq r p(x,Tx)\).

Case 2. \(p(Tx,T^{2}x) \preceq r p(Tx,T^{2}x)\), which implies that \(p(Tx,T^{2}x)=\theta\).

Case 3. \(p(Tx,T^{2}x) \preceq r \frac{p(x,T^{2}x)+p(Tx,Tx)}{2}\preceq r \frac{p(x,Tx)+p(Tx,T^{2}x)}{2}\), which implies that \(p(Tx,T^{2}x) \preceq r p(x,Tx)\).
Define the sequence \(\{x_{n}\}\) by \(x_{n}=T^{n}x_{0}\), so that \(x_{n+1}=Tx_{n}\). If \(x_{n}=x_{n+1}=Tx_{n}\) for some n, then \(x_{n}\) becomes a fixed point of T. Now, suppose that \(x_{n}\neq x_{n+1}\) for all \(n\in N\). Then \(p(x_{n},x_{n+1})\succ\theta\).

Case 1. If \(U(x_{n1},Tx_{n1})=p(x_{n1},x_{n})\), then \(p(x_{n},x_{n+1})\preceq r p(x_{n1},x_{n})\).

Case 2. \(p(x_{n},x_{n+1})\preceq r p(x_{n},x_{n+1}) \), which implies that \(p(x_{n},x_{n+1})=\theta\).

Case 3. \(p(x_{n},x_{n+1})\preceq r \cdot\frac {p(x_{n1},x_{n+1})+p(x_{n},x_{n})}{2}\preceq r(\frac {p(x_{n1},x_{n})+p(x_{n},x_{n+1})}{2})\), which implies that \(p(x_{n},x_{n+1})\preceq r p(x_{n1},x_{n})\).
First, we show that there exists \(j\in Z^{+}\) such that \(T^{j}z=z\). Arguing by contradiction, we assume that \(T^{j}z\neq z\) for all \(j\in Z^{+}\).
Note that, by condition (ii), if \(\{x_{n}\}\) is nondecreasing, then \(x_{n}\sqsubseteq z\). Since T is nondecreasing, we get \(x_{n+1}=Tx_{n}\sqsubseteq Tz\) for all \(n\in N\). Taking the limit as \(n\to\infty\), by (iii) we obtain that \(z\sqsubseteq Tz\), which implies that \(\{T^{n}z\}\) is a nondecreasing sequence. So, we have shown that for \(\{x_{n}\}\), \(\{T^{n}z\}\) also is a θCauchy sequence. We also have \(T^{j}z\) is comparable with \(x_{n}\) for all \(j,n\in N\).
Case 2. \(p(z,T^{j+1}z)\preceq p(x_{n+1},z) +r p(x_{n},x_{n+1})\ll c\), which implies that \(p(z,T^{j+1}z)=\theta\).
Thus, in all cases, we obtain \(p(z,T^{j+1}z)\preceq r^{j}p(z,Tz)\nonumber \), that is, (2.3) holds.
 (1)
\(0\leq r<\frac{\sqrt{5}1}{2}\);
 (2)
\(\frac{\sqrt{5}1}{2} \leq r< \frac{\sqrt{2}}{2}\);
 (3)
\(\frac{\sqrt{2}}{2}\leq r< 1\).

Case 1. \(p(T^{3}z,Tz)\preceq r p(T^{2}z,z)\preceq r^{2} p(Tz,z)\preceq r p(Tz,z)\).

Case 2. \(p(T^{3}z,Tz)\preceq r p(T^{2}z,T^{3}z) \preceq r^{3}p(Tz,z)\preceq r p(Tz,z)\).

Case 3. \(p(T^{3}z,Tz)\preceq r p(Tz,z)\).

Case 4. \(p(T^{3}z,Tz)\preceq r \frac{p(T^{3}z,z)+p(Tz,T^{2}z)}{2}\preceq r[\frac{r^{2}p(Tz,z)+rp(z,Tz)}{2}]\preceq r^{2} p(Tz,z)\preceq r p(Tz,z)\).

Case 1. \(p(T^{n+1}z,Tz)\preceq r p(T^{n}z,z)\preceq r^{n} p(Tz,z)\preceq r p(Tz,z)\).

Case 2. \(p(T^{n+1}z,Tz)\preceq r p(T^{n}z,T^{n+1}z) \preceq r^{n+1}p(Tz,z)\preceq r p(Tz,z)\).

Case 3. \(p(T^{n+1}z,Tz)\preceq r p(Tz,z)\).

Case 4.$$\begin{aligned}p \bigl(T^{n+1}z,Tz \bigr)&\preceq r \frac {p(T^{n+1}z,z)+p(Tz,T^{n}z)}{2}\preceq r \biggl(\frac{r^{n} p(Tz,z)+rp(Tz,z)}{2} \biggr) \\ &\preceq r^{2} p(Tz,z)\preceq r p(Tz,z). \end{aligned} $$

Case 1. \(p(Tz,z)\preceq p(x_{2n+1},z)+r p(z,x_{2n})\ll c\) implies \(p(Tz,z)=\theta\).

Case 2. \(p(Tz,z)\preceq p(x_{2n+1},z)+rp(x_{2n},x_{2n+1})\ll c \) implies \(p(Tz,z)=\theta\).

Case 3. \(p(Tz,z)\preceq p(x_{2n+1},z)+rp(z,Tz) \ll c+ rp(z,Tz) \) implies \(p(Tz,z)=\theta\).

Case 4.which implies that \(p(Tz,z)=\theta\). Then, in all cases, we have \(p(Tz,z)=\theta\). Similarly, by (2.7) we also have \(p(Tz,z)=\theta\). Thus, \(Tz=z\). This is a contradiction.$$\begin{aligned}p(Tz,z)&\preceq p(x_{2n+1},z)+r \biggl[ \frac {p(x_{2n},Tz)+p(z,x_{2n+1})}{2} \biggr] \\ &\preceq p(x_{2n+1},z)+r \biggl[\frac {p(x_{2n},z)+p(z,Tz)+p(z,x_{2n+1})}{2} \biggr]\ll c+ rp(z,Tz), \end{aligned} $$
Therefore, in all cases, there exists \(j\in N\) such that \(T^{j}z=z\). Since \(\{T^{n}z\}\) is a θCauchy sequence, we obtain \(z=Tz\). If not, that is, if \(z\neq Tz\), from \(p(T^{nj}z,T^{nj+1}z)=p(z,Tz)\) for all \(n\in N\) it follows that \(\{T^{n}z\}\) is not a θCauchy sequence. Hence, z is a fixed point of T.
Finally, we prove the uniqueness of the fixed point. Suppose that there exist \(z_{1},z_{2}\in X\) with \(z_{1}\neq z_{2}\) such that \(Tz_{1}=z_{1}\) and \(Tz_{2}=z_{2}\). We have two possible cases:

Case 1. \(p(z_{1},z_{2})\preceq rp(z_{1},z_{2})\) implies \(p(z_{1},z_{2})=\theta\).

Case 2. \(p(z_{1},z_{2})\preceq rp(z_{1},z_{1})\preceq rp(z_{1},z_{2})\) implies \(p(z_{1},z_{2})=\theta\).

Case 3. \(p(z_{1},z_{2})\preceq rp(z_{2},z_{2})\preceq rp(z_{1},z_{2})\) implies \(p(z_{1},z_{2})=\theta\).

Case 1. \(p(z_{1},T^{n}x)\preceq rp(z_{1},T^{n1}x)\preceq r^{2}p(z_{1},T^{n2}x) \preceq\cdots\preceq r^{n} p(z_{1},x) \).

Case 2. \(p(z_{1},T^{n}x)\preceq rp(z_{1},z_{1})\preceq rp(z_{1},T^{n1}x)\preceq\cdots\preceq r^{n} p(z_{1},x) \).

Case 3. \(p(z_{1},T^{n}x)\preceq r p(T^{n1}x,T^{n}x)\preceq r^{2}p(T^{n2}x,T^{n1}x) \preceq\cdots\preceq r^{n} p(x,Tx)\).

Case 4. \(p(z_{1},T^{n}x)\preceq r[\frac{p(z_{1},T^{n1}x)+p(z_{1},T^{n}x)}{2}] \) implies \(p(z_{1},T^{n}x)\preceq rp(z_{1},T^{n1}x)\preceq r^{n} p(z_{1},x) \).
Now, in order to support the usability of Theorem 2.1, we present the following example.
Example 2.1

Case 1. If \(t\in[0,1]\) such that \(x(t)=0\), then \(Tx(t)=x(t)\).

Case 2. If \(t\in[0,1]\) such that \(2n1\leq x(t)\leq2n\), then$$Tx(t)=\frac{2n1}{2}rt+\frac{2n1}{4n}rx(t)\leq\frac{2n1}{2}r+ \frac {2n1}{2}r\leq rx(t). $$

Case 3. If \(t\in[0,1]\) such that \(2n\leq x(t)\leq2n+1\), then$$Tx(t)=nrt+\frac{n}{2n+1}rx(t)\leq nr+ nr\leq rx(t). $$
Theorem 2.2
 (i)
there exists \(x_{0}\in X\) such that \(Sx_{0}\sqsubseteq Tx_{0}\);
 (ii)
for a nondecreasing sequence \(x_{n}\stackrel{\tau_{p}}{\to} x\), we have \(x_{n}\sqsubseteq x\) for all \(n\in N\);
 (iii)
for two nondecreasing sequences \(\{x_{n}\}, \{y_{n}\}\subseteq X\) such that \(x_{n}\sqsubseteq y_{n}\), \(x_{n}\stackrel{\tau_{p}}{\to} x\), and \(y_{n} \stackrel{\tau_{p}}{\to} y\) as \(n\to\infty\), we have \(x\sqsubseteq y\);
 (iv)
the set of points of coincidence of S and T is totally ordered, and S, T are weakly compatible.
Proof
By Lemma 1.1 there exists \(Y\subseteq X\), such that \(SY=SX\) and \(S:Y\to X\) is onetoone. Define \(f:SY\to SX\) by \(fSx=Tx\) for all \(Sx\in SY\).
Since T is Snondecreasing, we have that f is nondecreasing. In fact, \(Sx\sqsubseteq Sy\) implies \(Tx\sqsubseteq Ty\), and hence \(fSx=Tx\sqsubseteq Ty=fSy\). Since SY is θcomplete, by Theorem 2.1 we get that f has a fixed point on SY, say Sz. Then \(z= y\) is a coincidence point of S and T, that is, \(Tz=fSz=Sz\).
3 Coupled point theorems in partial cone metric spaces
In this section, we will apply the results obtained in Section 2 to establish the corresponding Suzukitype coupled fixed point theorems for generalized mappings in partially ordered partial cone metric spaces over a nonnormal cone.
For \(\tilde{a}=(x,y)\), \(\tilde{b}=(u,v)\in X^{2}\), we introduce the mapping \(\tilde{p}:X^{2}\times X^{2}\to P\) defined by \(\tilde{p}(\tilde{a},\tilde{b})=p(x,u)+p(y,v)\).
The following conclusion is valid, and for its proof, we refer to [30].
Lemma 3.1
If \((X,p )\) is a partial cone metric space over a solid cone P of a normed vector space \((E,\Vert \cdot \Vert )\), then \((X^{2},\tilde{p})\) is also a θcomplete partial cone metric space.
Proof
Suppose that the sequence \(\{\tilde{x}_{n}\}=\{(x_{n},y_{n})\}\) is a θCauchy sequence in \((X^{2},\tilde{p})\). Then, for every \(c\gg\theta\), there exists a positive integer \(n_{0}\in N\) such that \(\tilde{p}(\tilde{x}_{n},\tilde {x}_{m})=p(x_{n},x_{m})+p(y_{n},y_{m})\ll c\) for all \(n,m>n_{0}\). Then \(p(x_{n},x_{m})\ll c\) and \(p(y_{n},y_{m})\ll c\). Thus, \(\{x_{n}\}\) and \(\{y_{n}\}\) are θCauchy sequences in \((X,p)\). Since \((X,p)\) is θcomplete, there exist \(x,y\in X\) such that \(x_{n}\stackrel{\tau_{p}}{\to} x\), \(y_{n}\stackrel{\tau_{p}}{\to} y\), and \(p(x,x)=\theta\), \(p(y,y)=\theta\).
Thus, for every \(c\gg\theta\), there exists \(n_{1}\in N\) such that \(p(x_{n},x)\ll\frac{c}{2}\) and \(p(y_{n},y)\ll\frac{c}{2}\) for all \(n>n_{1}\). Then \(\tilde{p}((x_{n},y_{n}),(x,y))=p(x_{n},x)+p(y_{n},y)\ll\frac{c}{2}+\frac {c}{2}=c\), and \(\tilde{p}((x,y),(x,y))=\theta\). Thus, \(\{(x_{n},y_{n})\}\stackrel{\tau_{\tilde{p}}}{\to} (x,y) \).
Therefore, \((X^{2},\tilde{p})\) is a θcomplete partial cone metric space. □
Theorem 3.1
 (i)
there exists \(x_{0},y_{0}\in X\) such that \(x_{0}\sqsubseteq A(x_{0},y_{0})\) and \(A(y_{0},x_{0})\sqsubseteq y_{0}\);
 (ii)
for a nondecreasing sequence \(x_{n}\stackrel{\tau_{p}}{\to} x\), we have \(x_{n}\sqsubseteq x\) for all \(n\in N\);
 (iii)
for a nonincreasing sequence \(y_{n}\stackrel{\tau_{p}}{\to} x\), we have \(y\sqsubseteq y_{n}\) for all \(n\in N\);
 (iv)
for two nondecreasing sequences \(\{x_{n}\}, \{u_{n}\}\subseteq X\) such that \(x_{n}\sqsubseteq u_{n}\) for all \(n\in N\), \(x_{n}\stackrel{\tau_{p}}{\to} x\), and \(u_{n} \stackrel{\tau_{p}}{\to} u\) as \(n\to\infty\), we have \(x\sqsubseteq u\);
 (v)
for two nonincreasing sequences \(\{y_{n}\}, \{v_{n}\}\subseteq X\) such that \(v_{n}\sqsubseteq y_{n}\) for all \(n\in N\), \(v_{n}\stackrel{\tau_{p}}{\to} v\), and \(y_{n} \stackrel{\tau_{p}}{\to} y\) as \(n\to\infty\), we have \(v\sqsubseteq y\).
Proof
If \(\tilde{ a} \prec\tilde{b}\), then \(x \sqsubseteq u\) and \(v \sqsubseteq y\). Noting the mixed monotone property of A, we see that if \(A(x,y) \sqsubseteq A(u,v)\) and \(A(v,u) \sqsubseteq A(y,x)\), then \(T \tilde{a}\prec T\tilde{b}\). Thus, T is a nondecreasing mapping with respect to the order ≺ on X̃.
If a nondecreasing monotone sequence \(\{ \tilde{x}_{n}\} = \{(x_{n},y_{n})\}\) in X̃ \(\tau_{p}\)converges to \(\tilde{ x} = (x,y)\), then \(\tilde{x}_{n}= (x_{n},y_{n})\prec(x_{n+1},y_{n+1}) = \tilde{x}_{n+1}\), that is, \(x_{n}\sqsubseteq x_{n+1}\) and \(y_{n+1}\sqsubseteq y_{n}\). Thus, \(\{x_{n}\}\) is a nondecreasing sequence \(\tau_{p}\)converging to x, and \(\{y_{n}\}\) is a nonincreasing sequence \(\tau_{p}\)converging to y. Thus, \(x_{n}\sqsubseteq x\) and \(y \sqsubseteq y_{n}\) for all \(n\in N\). This implies \(\tilde{x}_{n}\prec \tilde{x}\).
If two nondecreasing sequence \(\{ \tilde{x}_{n}\} = \{(x_{n},y_{n})\}\), \(\{ \tilde{y}_{n}\} = \{(u_{n},v_{n})\}\) are such that \(\tilde{x}_{n}\prec\tilde {y}_{n}\) for all \(n\in N\), \(\tilde{x}_{n}\stackrel{\tau_{\tilde{p}}}{\to} (x,y)\), and \(\tilde {y}_{n}\stackrel{\tau_{\tilde{p}}}{\to} (u,v)\) as \(n\to\infty\), then \(x_{n}\sqsubseteq u_{n}\), \(v_{n}\sqsubseteq y_{n}\), \(x_{n}\sqsubseteq x_{n+1}\), \(y_{n+1}\sqsubseteq y_{n}\), and \(u_{n}\sqsubseteq u_{n+1}\), \(v_{n+1}\sqsubseteq v_{n}\). Thus, \(\{ x_{n}\}, \{u_{n}\}\subseteq X\) are two nondecreasing sequences, \(x_{n}\sqsubseteq u_{n}\) for all \(n\in N\), \(x_{n}\stackrel{\tau_{p}}{\to} x\), and \(u_{n} \stackrel{\tau_{p}}{\to} u\) as \(n\to\infty\), and by condition (iv) we have \(x\sqsubseteq u\). Similarly, by condition (v) we have \(v\sqsubseteq y\). Thus, \((x,y)\prec(u,v)\).
Therefore, all hypotheses of Theorem 2.1 are satisfied. Following Theorem 2.1, we deduce that A has a coupled point, that is, there exist \(z,w \in\tilde{X}\) such that \(A(z,w) = z\) and \(A(w,z) = w\). □
Now, we present the following example.
Example 3.1
Also, conditions (i)(v) of Theorem 3.1 are satisfied. From Theorem 2.1 we obtain that A has a fixed point in X; indeed, \(x=(0,0)\) is a fixed point of A.
Declarations
Acknowledgements
The authors thank the editor and the referees for their valuable comments and suggestions. The research has been supported by the National Natural Science Foundation of China (11071108, 11361042, 11326099, 11461045) and the Provincial Natural Science Foundation of Jiangxi, China (2010GZS0147, 20114BAB201007, 20142BAB211004) and supported partly by the Provincial Graduate Innovation Foundation of Jiangxi, China (YC2012B004).
Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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