- Research
- Open Access
Fixed point of α-ψ-contractive type mappings in uniform spaces
- Muhammad Usman Ali^{1},
- Tayyab Kamran^{2} and
- Erdal Karapınar^{3, 4}Email author
https://doi.org/10.1186/1687-1812-2014-150
© Ali et al.; licensee Springer. 2014
- Received: 1 April 2014
- Accepted: 13 June 2014
- Published: 22 July 2014
Abstract
In this paper, we shall investigate the existence and uniqueness of a fixed point of α-ψ-contractive mappings in the context of uniform spaces. We shall also prove some common fixed point theorems by introducing the notion of α-admissible pairs. We shall construct some examples to support our novel results.
MSC:46T99, 47H10, 54H25.
Keywords
- α-ψ-contractive
- α-admissible maps
1 Introduction
One of the interesting metric fixed point results was given by Samet et al. [1] by introducing the notions of α-admissible and α-ψ-contractive type mappings. They reported results via these new notions, and they extended and unified most of the related existing metric fixed point results in the literature. In particular, the authors [2] showed that fixed point results via cyclic contractions are consequences of their related results. Naturally, many authors have started to investigate the existence and uniqueness of a fixed point theorem via admissible mappings and variations of the concept of α-ψ-contractive type mappings, for reference see [1–19]. The notion of cyclic contraction was introduced by Kirk et al. [20]. The main advantage of the cyclic contraction is that the given mapping does not need to be continuous. It has been appreciated by several authors; see e.g. [21–26] and related references therein.
In this paper, we shall consider the characterization of the notions of α-ψ-contractive and α-admissible mappings in the context uniform spaces. Further, we shall prove some fixed point theorems by using these concepts. We shall also use α-admissible pairs to investigate the existence and uniqueness of a common fixed point in the setting of uniform spaces. We shall also establish some examples to illustrate the main results.
- (i)
if G is in ϑ, then G contains the diagonal $\{(x,x)|x\in X\}$;
- (ii)
if G is in ϑ and H is a subset of $X\times X$ which contains G, then H is in ϑ;
- (iii)
if G and H are in ϑ, then $G\cap H$ is in ϑ;
- (iv)
if G is in ϑ, then there exists H in ϑ, such that, whenever $(x,y)$ and $(y,z)$ are in H, then $(x,z)$ is in G;
- (v)
if G is in ϑ, then $\{(y,x)|(x,y)\in G\}$ is also in ϑ.
The pair $(X,\vartheta )$ is called a uniform space and the element of ϑ is called entourage or neighborhood or surrounding. The pair $(X,\vartheta )$ is called a quasiuniform space (see e.g. [27, 28]) if property (v) is omitted.
For a subset $V\in \vartheta $, a pair of points x and y are said to be V-close if $(x,y)\in V$ and $(y,x)\in V$. Moreover, a sequence $\{{x}_{n}\}$ in X is called a Cauchy sequence for ϑ, if, for any $V\in \vartheta $, there exists $N\ge 1$ such that ${x}_{n}$ and ${x}_{m}$ are V-close for $n,m\ge N$. For $(X,\vartheta )$, there is a unique topology $\tau (\vartheta )$ on X generated by $V(x)=\{y\in X|(x,y)\in V\}$ where $V\in \vartheta $.
A sequence $\{{x}_{n}\}$ in X is convergent to x for ϑ, denoted by ${lim}_{n\to \mathrm{\infty}}{x}_{n}=x$, if, for any $V\in \vartheta $, there exists ${n}_{0}\in \mathbb{N}$ such that ${x}_{n}\in V(x)$ for every $n\ge {n}_{0}$. A uniform space $(X,\vartheta )$ is called Hausdorff if the intersection of all the $V\in \vartheta $ is equal to Δ of X, that is, if $(x,y)\in V$ for all $V\in \vartheta $ implies $x=y$. If $V={V}^{-1}$ then we shall say that a subset $V\in \vartheta $ is symmetrical. Throughout the paper, we shall assume that each $V\in \vartheta $ is symmetrical. For more details, see e.g. [27, 29–32].
Now, we shall recall the notions of A-distance and E-distance.
Let $(X,\vartheta )$ be a uniform space. A function $p:X\times X\to [0,\mathrm{\infty})$ is said to be an A-distance if, for any $V\in \vartheta $, there exists $\delta >0$ such that if $p(z,x)\le \delta $ and $p(z,y)\le \delta $ for some $z\in X$, then $(x,y)\in V$.
- (i)
p is an A-distance,
- (ii)
$p(x,y)\le p(x,z)+p(z,y)$, $\mathrm{\forall}x,y,z\in X$.
Let $(X,\vartheta )$ be a uniform space and let d be a metric on X. It is evident that $(X,{\vartheta}_{d})$ is a uniform space where ${\vartheta}_{d}$ is a set of all subsets of $X\times X$ containing a ‘band’ ${U}_{\u03f5}=\{(x,y)\in {X}^{2}|d(x,y)<\u03f5\}$ for some $\u03f5>0$. Moreover, if $\vartheta \subseteq {\vartheta}_{d}$, then d is an E-distance on $(X,\vartheta )$.
- (a)
If $p({x}_{n},y)\le {\alpha}_{n}$ and $p({x}_{n},z)\le {\beta}_{n}$ for all $n\in \mathbb{N}$, then $y=z$. In particular, if $p(x,y)=0$ and $p(x,z)=0$, then $y=z$.
- (b)
If $p({x}_{n},{y}_{n})\le {\alpha}_{n}$ and $p({x}_{n},z)\le {\beta}_{n}$ for all $n\in \mathbb{N}$, then $\{{y}_{n}\}$ converges to z.
- (c)
If $p({x}_{n},{x}_{m})\le {\alpha}_{n}$ for all $n,m\in \mathbb{N}$ with $m>n$, then $\{{x}_{n}\}$ is a Cauchy sequence in $(X,\vartheta )$.
Let p be an A-distance. A sequence in a uniform space $(X,\vartheta )$ with an A-distance is said to be a p-Cauchy if, for every $\u03f5>0$, there exists ${n}_{0}\in \mathbb{N}$ such that $p({x}_{n},{x}_{m})<\u03f5$ for all $n,m\ge {n}_{0}$.
- (i)
X is S-complete if, for every p-Cauchy sequence $\{{x}_{n}\}$, there exists x in X with ${lim}_{n\to \mathrm{\infty}}p({x}_{n},x)=0$.
- (ii)
X is p-Cauchy complete if, for every p-Cauchy sequence $\{{x}_{n}\}$, there exists x in X with ${lim}_{n\to \mathrm{\infty}}{x}_{n}=x$ with respect to $\tau (\vartheta )$.
- (iii)
$T:X\to X$ is p-continuous if ${lim}_{n\to \mathrm{\infty}}p({x}_{n},x)=0$ implies ${lim}_{n\to \mathrm{\infty}}p(T({x}_{n}),T(x))=0$.
Remark 1.6 Let $(X,\vartheta )$ be a Hausdorff uniform space which is S-complete. If a sequence $\{{x}_{n}\}$ be a p-Cauchy sequence, then we have ${lim}_{n\to \mathrm{\infty}}p({x}_{n},x)=0$. Regarding Lemma 1.4(b), we derive ${lim}_{n\to \mathrm{\infty}}{x}_{n}=x$ with respect to the topology $\tau (\vartheta )$, and hence S-completeness implies p-Cauchy completeness.
Definition 1.7 [20]
- (i)
${A}_{i}$, $i=1,2,\dots ,m$ are nonempty sets;
- (ii)
$T({A}_{1})\subset {A}_{2},\dots ,T({A}_{m-1})\subset {A}_{m},T({A}_{m})\subset {A}_{1}$.
2 Main results
Let Ψ be the family of functions $\psi :[0,\mathrm{\infty})\to [0,\mathrm{\infty})$ satisfying the following conditions:
(${\mathrm{\Psi}}_{1}$) ψ is nondecreasing;
(${\mathrm{\Psi}}_{2}$) ${\sum}_{n=1}^{+\mathrm{\infty}}{\psi}^{n}(t)<\mathrm{\infty}$ for all $t>0$, where ${\psi}^{n}$ is the n th iterate of ψ.
These functions are known in the literature as (c)-comparison functions. It is easily proved that if ψ is a (c)-comparison function, then $\psi (t)<t$ for any $t>0$.
Definition 2.1 [1]
We shall characterize the notion of α-ψ-contractive mapping, introduced by Samet et al. [1], in the context of uniform space as follows.
- (i)
T is α-admissible;
- (ii)
there exists ${x}_{0}\in X$ such that $\alpha ({x}_{0},T{x}_{0})\ge 1$ and $\alpha (T{x}_{0},{x}_{0})\ge 1$;
- (iii)
T is p-continuous.
Then T has a fixed point $u\in X$.
Hence the sequence $\{{x}_{n}\}$ is a p-Cauchy in the S-complete space X. Thus, there exists $u\in X$ such that ${lim}_{n\to \mathrm{\infty}}p({x}_{n},u)=0$, which implies ${lim}_{n\to \mathrm{\infty}}{x}_{n}=u$. Since T is p-continuous, we have ${lim}_{n\to \mathrm{\infty}}p(T{x}_{n},Tu)=0$, which implies that ${lim}_{n\to \mathrm{\infty}}({x}_{n+1},Tu)=0$. Hence we have ${lim}_{n\to \mathrm{\infty}}p({x}_{n},u)=0$ and ${lim}_{n\to \mathrm{\infty}}({x}_{n},Tu)=0$. Thus by Lemma 1.4(a) we have $u=Tu$. □
In the following theorem, we omit the p-continuity by replacing a suitable condition on the obtained iterative sequence.
- (i)
T is α-admissible;
- (ii)
there exists ${x}_{0}\in X$ such that $\alpha ({x}_{0},T{x}_{0})\ge 1$ and $\alpha (T{x}_{0},{x}_{0})\ge 1$;
- (iii)
for any sequence $\{{x}_{n}\}$ in X with ${x}_{n}\to x$ as $n\to \mathrm{\infty}$ and $\alpha ({x}_{n},{x}_{n+1})\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$, then $\alpha ({x}_{n},x)\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$.
Then T has a fixed point $u\in X$.
Letting $n\to \mathrm{\infty}$ in above inequality, we shall have ${lim}_{n\to \mathrm{\infty}}p({x}_{n},Tu)=0$. Hence we have ${lim}_{n\to \mathrm{\infty}}p({x}_{n},u)=0$ and ${lim}_{n\to \mathrm{\infty}}p({x}_{n},Tu)=0$. Thus by Lemma 1.4(a) we have $u=Tu$. □
and $\psi (t)=\frac{t}{3}$ for all $t\ge 0$. One can easily see that T is α-ψ-contractive and α-admissible mapping. Also for ${x}_{0}=\frac{1}{2}$ we have $\alpha ({x}_{0},T{x}_{0})=\alpha (T{x}_{0},{x}_{0})=1$. Moreover, for any sequence $\{{x}_{n}\}$ in X with ${x}_{n}\to x$ as $n\to \mathrm{\infty}$ and $\alpha ({x}_{n-1},{x}_{n})=1$ for each $n\in \mathbb{N}$ we have $\alpha ({x}_{n},x)=1$ for each $n\in \mathbb{N}$. Therefore by Theorem 2.4, T has a fixed point.
- (H)
For all $x,y\in Fix(T)$, there exists $z\in X$ such that $\alpha (z,x)\ge 1$ and $\alpha (z,y)\ge 1$.
Here, $Fix(T)$ denotes the set of fixed points of T.
The following theorem guarantees the uniqueness of a fixed point.
Theorem 2.6 Adding the condition (H) to the hypothesis of Theorem 2.3 (respectively, Theorem 2.4), we obtain the uniqueness of fixed point of T.
From (2.13) and (2.14) together with Lemma 1.4(a), it follows that $u=v$. Thus we have proved that u is the unique fixed point of T. □
Definition 2.7 [9]
A pair of two self-mappings $T,S:X\to X$ is said to be α-admissible, if, for any $x,y\in X$ with $\alpha (x,y)\ge 1$, we have $\alpha (Tx,Sy)\ge 1$ and $\alpha (Sx,Ty)\ge 1$.
for any $x,y\in X$, where $\psi \in \mathrm{\Psi}$.
- (i)
$(T,S)$ is α-admissible;
- (ii)
there exists ${x}_{0}\in X$ such that $\alpha ({x}_{0},T{x}_{0})\ge 1$ and $\alpha (T{x}_{0},{x}_{0})\ge 1$;
- (iii)
for any sequence $\{{x}_{n}\}$ in X with ${x}_{n}\to x$ as $n\to \mathrm{\infty}$ and $\alpha ({x}_{n},{x}_{n+1})\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$, then $\alpha ({x}_{n},x)\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$.
Then T and S have a common fixed point.
Letting $n\to \mathrm{\infty}$ in (2.23), we have $p({x}_{n},Tu)=0$. Hence we have ${lim}_{n\to \mathrm{\infty}}p({x}_{n},u)=0$ and ${lim}_{n\to \mathrm{\infty}}p({x}_{n},Tu)=0$. Thus by Lemma 1.4(a) we have $u=Tu$. Analogously, one can derive $u=Su$. Therefore $u=Tu=Su$. □
Remark 2.10 Note that Theorem 2.9 is valid if one replaces condition (ii) with
(ii)′ there exists ${x}_{0}\in X$ such that $\alpha ({x}_{0},S{x}_{0})\ge 1$ and $\alpha (S{x}_{0},{x}_{0})\ge 1$.
We shall get the following result by letting $S=T$ in Theorem 2.9.
- (i)
T is α-admissible;
- (ii)
there exists ${x}_{0}\in X$ such that $\alpha ({x}_{0},T{x}_{0})\ge 1$ and $\alpha (T{x}_{0},{x}_{0})\ge 1$;
- (iii)
for any sequence $\{{x}_{n}\}$ in X with ${x}_{n}\to x$ as $n\to \mathrm{\infty}$ and $\alpha ({x}_{n},{x}_{n+1})\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$, then $\alpha ({x}_{n},x)\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$.
Then T has a fixed point.
and $\psi (t)=\frac{t}{2}$ for all $t\ge 0$. One can easily see that $(T,S)$ is an α-ψ-contractive and α-admissible pair. Also for ${x}_{0}=\frac{1}{2}$ we have $\alpha ({x}_{0},T{x}_{0})=\alpha (T{x}_{0},{x}_{0})=1$. Moreover, for any sequence $\{{x}_{n}\}$ in X with ${x}_{n}\to x$ as $n\to \mathrm{\infty}$ and $\alpha ({x}_{n},{x}_{n+1})\ge 1$ for each $n\in \mathbb{N}\cup \{0\}$, we have $\alpha ({x}_{n},x)\ge 1$ for each $n\in \mathbb{N}$. Therefore by Theorem 2.9, T and S have a common fixed point.
- (I)
For each $x,y\in CFix(T,S)$, we have $\alpha (x,y)\ge 1$, where $CFix(T,S)$ is the set of all common fixed points of T and S.
Theorem 2.13 Adding the condition (I) to the hypothesis of Theorem 2.9, we obtain the uniqueness of the common fixed point of T and S.
which is impossible for $p(u,v)>0$. Consequently, we have $p(u,v)=0$. Analogously, one can show that $p(u,u)=0$. Thus we have $u=v$, which is a contradiction to our assumption. Hence T and S have a unique common fixed point. □
3 Consequences
3.1 Standard contractions on uniform space
Taking in Theorem 2.6, $\alpha (x,y)=1$ for all $x,y\in X$, we shall obtain immediately the following fixed point theorems.
for all $x,y\in X$. Then T has a unique fixed point.
By substituting $\psi (t)=kt$, where $k\in [0,1)$, in Corollary 3.1, we shall get the following.
for all $x,y\in X$. Then T has a unique fixed point.
Taking in Theorem 2.13 $\alpha (x,y)=1$ for all $x,y\in X$, we shall obtain immediately the following common fixed point theorem.
for all $x,y\in X$. Then T and S have a unique common fixed point.
for all $x,y\in X$. Then T has a unique fixed point.
3.2 Cyclic contraction on uniform space
- (i)
$T({A}_{1})\subseteq {A}_{2}$ and $T({A}_{2})\subseteq {A}_{1}$;
- (ii)there exists a function $\psi \in \mathrm{\Psi}$ such that$p(Tx,Ty)\le \psi (p(x,y)),\phantom{\rule{1em}{0ex}}\mathit{\text{for all}}(x,y)\in {A}_{1}\times {A}_{2}.$
Then T has a unique fixed point that belongs to ${A}_{1}\cap {A}_{2}$.
for all $x,y\in Y$. Thus T is an α-ψ-contractive mapping.
Let $(x,y)\in Y\times Y$ such that $\alpha (x,y)\ge 1$. If $(x,y)\in {A}_{1}\times {A}_{2}$, from (i), $(Tx,Ty)\in {A}_{2}\times {A}_{1}$, which implies that $\alpha (Tx,Ty)\ge 1$. If $(x,y)\in {A}_{2}\times {A}_{1}$, from (i), $(Tx,Ty)\in {A}_{1}\times {A}_{2}$, which implies that $\alpha (Tx,Ty)\ge 1$. Thus in all cases, we have $\alpha (Tx,Ty)\ge 1$. This implies that T is α-admissible.
Also, from (i), for any $a\in {A}_{1}$, we have $(a,Ta)\in {A}_{1}\times {A}_{2}$, which implies that $\alpha (a,Ta)\ge 1$.
which implies that $x\in {A}_{1}\cap {A}_{2}$. Thus we can easily get from the definition of α that $\alpha ({x}_{n},x)\ge 1$ for all n.
Finally, let $x,y\in Fix(T)$. From (i), this implies that $x,y\in {A}_{1}\cap {A}_{2}$. So, for any $z\in Y$, we have $\alpha (z,x)\ge 1$ and $\alpha (z,y)\ge 1$. Thus condition (H) is satisfied.
Now, all the hypotheses of Theorem 2.6 are satisfied, and we deduce that T has a unique fixed point that belongs to ${A}_{1}\cap {A}_{2}$ (from (i)). □
Declarations
Acknowledgements
The authors thank the anonymous referees for their remarkable comments, suggestions, and ideas that helped to improve this paper.
Authors’ Affiliations
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