# On externally complete subsets and common fixed points in partially ordered sets

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## fixed points in partially ordered sets

1*

### and Mohamed A Khamsi

1,2

* Correspondence: abusbeih@kfupm.edu.sa 1Department of Mathematics & Statistics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Full list of author information is available at the end of the article

Abstract

In this study, we introduce the concept of externally complete ordered sets. We discuss the properties of such sets and characterize them in ordered trees. We also prove some common fixed point results for order preserving mappings. In particular, we introduce for the first time the concept of Banach Operator pairs in partially ordered sets and prove a common fixed point result which generalizes the classical De Marr’s common fixed point theorem.

2000 MSC: primary 06F30; 46B20; 47E10.

Keywords:partially ordered sets, order preserving mappings, order trees, hypercon-vex metric spaces, fixed point

1. Introduction

This article focuses on the externally complete structure, a new concept that was initi-ally introduced in metric spaces as externiniti-ally hyperconvex sets by Aron-szajn and Panitchpakdi in their fundamental article  on hyperconvexity. This idea developed from the original work of Quilliot  who introduced the concept of generalized metric structures to show that metric hyperconvexity is in fact similar to the complete lattice structure for ordered sets. In this fashion, Tarski’s fixed point theorem  becomes Sine and Soardi’s fixed point theorems for hyperconvex metric spaces [4,5]. For more on this, the reader may consult the references [6-8].

We begin by describing the relevant notation and terminology. Let (X,≺) be a par-tially ordered set andM ⊂X a non-empty subset. Recall that an upper (resp. lower) bound forMis an element pÎ Xwithm≺p(resp.p≺m) for eachmÎM; the least-upper (resp. greatest-lower) bound ofMwill be denoted supM (resp. infM). A none-mpty subsetMof a partially ordered setX will be called Dedekind complete if for any nonempty subsetA⊂M, sup A(resp inf A) exists in Mprovided Ais bounded above (resp. bounded below) inX. Recall that M ⊂ X is said to be linearly ordered if for every m1, m2 Î M we havem1 ≺ m2 or m2 ≺m1. A linearly ordered subset of X is

called a chain. For anymÎXdefine

(←,m] ={xX;xm} and [m,→) ={xX;mx}.

Recall that a connected partially ordered setXis called a tree ifXhas a lowest point e, and for everymÎX, the subset [e, m] is well ordered.

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A subset Yof a partially ordered setXis called convex if the segment [x, y] = {zÎX; x ≺z ≺y} ⊂Ywhenever x, y Î Y. A mapT: X ® X is order preserving (also called monotone, isotone, or increasing) ifT(x)≺T(y) wheneverx≺y.

2. Externally complete sets

Inspired by the success of the concept of the externally hyperconvex subsets intro-duced by Aronszajn and Panitchpakdi , we propose a similar concept in partially ordered sets.

Definition 2.1. Let(X,≺)be a partially ordered set. A subset M of X is called exter-nally complete if and only if for any family of points (xa)Г in X such that I()∩I()=∅for anya,bÎΓ,andI(xα)∩M=∅,we have

αI()

M=∅,

Where I(x) = (¬,x]or I(x) = [x,®).

The family of all nonempty externally complete subsets of X will be denoted by EC(X).

Proposition 2.1.Let X be a partially ordered set. Then, any MEC(X)is Dedekind complete and convex.

Proof. Let AMEC(X)be nonempty and bounded above in X. The setU(A) = {b Î X; A⊂(¬,b]} is not empty sinceAis bounded above. It is clear that the families (I (a))aÎA, whereI(a) = [a, ®) and (I(b))bÎU(A), whereI (b) = (¬, b], intersect 2-by-2.

Moreover, we haveI(a)∩M=∅andI(b)∩M=∅, for any (a, b)Î A×U(A). SinceM is inEC(X), we conclude that

J=

a∈AI(a)

bU(A)I(b)

M=∅.

Let mÎJ. Then, for anyaÎA, we havea≺m. So,mis an upper bound ofA. Letb be any upper bound ofA, thenb ÎU(A). Hence,m≺bwhich forcesmto be the least upper bound of A, i.e.m= supA. Similarly, one can prove that infAalso exists and belongs toM providedAis bounded below inX. Next, we prove thatMis convex. Let x, y ÎM. Obviously, if xand yare not comparable, then[x,y] =∅and we have nothing to prove. So, assume x≺y. LetaÎ [x, y]. Obviously, we have (¬,a]⋂ [a, ®) = {a}. And, since(←,a]∩M=∅and[a,→)∩M=∅, then

(←,a]∩[a,→)∩M={a} ∩M=∅.

This obviously implies that aÎM, i.e. [x, y]⊂M, which completes the proof of our proposition.

Example 2.1. LetN= {0, 1,...}.we consider the order 0≺2≺4≺··· and0≺1≺3≺ ···,and no even number (different from 0) is comparable to any odd number. Then, (N,

≺)is a tree. The set M= {0, 1, 2}is inEC(N).Note that M is convex and is not linearly ordered.

In the next result, we characterize the externally complete subsets of trees.

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Proof. Let MEC(X). Then,Mis convex and Dedekind complete. LetCbe a none-mpty chain ofM. Letc1,c2ÎC, then we havec1≺c2 orc2≺c1. Hence,

[c1,→)∩[c2,→)∩M=∅.

Since MEC(X)

J=

cC[c,→)

M=∅.

Obviously, any cÎ Jis an upper bound ofC. Since Mis Dedekind complete, sup C exists in M. Assume conversely thatMis a convex, and Dedekind complete subset of X such that any chain in M has an upper bound inM. Letx, y Î X such that there exist m1,m2 ÎM withx≺m1 andm2 ≺y. DefineP(x) = inf{mÎM;x≺m}, andP(y)

= sup{mÎM;m≺y}. BothP(x) andP(y) exist and belong toM sinceMis Dedekind complete. Let (xi)iÎIinXbe such that for any iÎIthere existsmiÎ Msuch that xi≺

mi. Also, we have[xi,→)∩[xj,→)=∅, for anyi, j ÎI. This condition forces the set {xi;iÎ I} to be linearly ordered sinceXis a tree. Consider the subsetMI= {P(xi);iÎ

I} ofM. It is easy to check thatMIis linearly ordered. Since any linearly ordered subset

ofM is bounded above, there existsmÎM such thatP(xi)≺mfor anyiÎI. Sincexi ≺P(xi) then

m

iI[xi,→)

M=∅.

Next, let (yj)jÎJ inX such that for anyjÎ J there existsmj Î Msuch that mj≺yj.

Consider the subsetMJ = {P(yj);jÎ J} of M. SinceXis a tree, the set MJ is bounded

below, som0 = infMJexists inM. It is obvious thatm0≺P(yj)≺yjfor anyjÎJ. This

implies

m0∈

j∈J(←,yj]

M=∅.

Finally, assume that we have (xi)iÎIand (yj)jÎJ inX such that the subsets ([xi,®))iÎI,

and ((¬, yj])jÎJ intersect 2-by-2 and[xi,→)∩M=∅and(←,yj]∩M=∅for any (i, j) Î I×J. As before, set

mI= sup{P(xi);iI} and mJ= inf{P(yj);jJ}.

For any (i, j)ÎI×J,we haveP(xi)≺P(yj), which impliesmI≺mJ. Obviously we have

[mI,mJ]⊂

i∈I[xi,→)

i∈I(←,yj]

M=∅.

Hence, Mis inEC(X).

The above proof suggests that externally complete subsets are proximinal. In fact in , the authors introduced externally hyperconvex subsets as an example of proximinal sets other than the admissible subsets, i.e. intersection of balls. Before we state a simi-lar result, we need the following definitions.

Definition 2.2.Let X be a partially ordered set. Let M be a nonempty subset of X. Define the lower and upper cones by

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and

Cu(M) ={xX;there exists mM such that mx}.

The cone generated by M will be defined byC(M) =Cl(M)∪Cu(M).

Theorem 2.2. Let X be a partially ordered set and M a nonempty externally com-plete subset of X. Then, there exists an order preserving retractP:C(M)→Msuch that

(1)for anyxCl(M)we have x≺P(x),and (2)for anyxCu(M)we have P(x)≺x.

Proof. First setP(m) =mfor any mÎM. Next, letxCl(M). We have

x∈[x,→)∩

x≺m(←,m]

,

wheremÎM. Using the external completeness ofM, we get

M∩[x,→)∩

xm(←,m]

=∅.

It is easy to check that this intersection is reduced to one point. Set

M∩[x,→)

x≺m(←,m]

={P(x)}.

Similarly, letxCu(M). We have

x∈(←,x]∩

mx[m,→)

,

wheremÎM. Using the external completeness ofM, we get

M∩(←,x]∩

m≺x[m,→)

=∅.

It is easy to check that this intersection is reduced to one point. Set

M∩(←,x]∩

mx[m,→)

={P(x)}.

In particular, this prove (1) and (2). In order to finish the proof of the theorem, let us show that P is order preserving. Indeed, let x,yC(M)with x ≺ y. If yCl(M), then xCl(M). Since y ≺P(y) thenx ≺P(y) which impliesP(x) ≺P(y). Similarly, if xCu(M), thenyCu(M)and again it is easy to showP(x)≺P(y). Assume xCl(M)

and yCu(M). Since

x∈[x,→)∩

x≺m(←,m]

∩(←,y]

and Mis externally complete, we have

M∩[x,→)∩

xm(←,m]

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wheremÎM. But[x,→)∩

xm(←,m]

={P(x)}, this forcesP(x)Î(¬, y]. By

defi-nition of P(y), we getP(x) ≺P(y). In fact, we proved that x≺ P(x)≺P(y)≺y. This completes the proof of the theorem.

We have the following result.

Theorem 2.3. Let X be a partially ordered set and M a nonempty externally com-plete subset of X. Assume that X has a supremum or an infimum, then there exists an order preserving retract P:X ®M which extends the retract ofC(M)into M.

Proof. LetP:C(M)→Mbe the retract defined in Theorem 2.2. Assume first thatX has a supremum e. Then, X=Cl(C(M)). Indeed, for any x Î X, we have x ≺e and eCu(M). Because x∈ ∩x≺z(←,z], wherezCu(M), and M is externally complete, we

get M

xz(←,z]

=∅, where zCu(M). Hence, there existsm ÎM such that m≺

z, for anyzCu(M)such thatx ≺z. Using the properties ofP, we getm ≺P(z), for any zCu(M)such thatx≺z. SinceM is Dedekind complete, inf{P(z); zCu(M)such thatx≺z} exists. Set

˜

P(x) = inf{P(z);zCu(M) such thatxz}.

First note that if xC(M), then for anyzCu(M)such thatx≺zwe haveP(x)≺P (z). This will imply P(x)≺ ˜P(x). If xCu(M), then by definition of P˜, we have

˜

P(x)≺P(x). Hence, P(x)≺ ˜P(x). If xCl(M), then by definition of P˜, we have ˜

P(x)≺P(P(x))sincex≺P(x). Hence, P˜(x)≺P(x)which implies again P(x)≺ ˜P(x). So, ˜

P extendsP. Let us show that P˜ is order preserving. Indeed, let x, yÎ Xsuch that x≺ y. Since

{P(z);zCu(M) such thatyz} ⊂ {P(z);zCu(M) such thatxz}

we have

inf{P(z);zCu(M) such thatxz} ≺inf{P(z);zCu(M) such thatyz},

or P˜(x)≺ ˜P(y). In order to finish the proof of Theorem 2.3, consider the case when X has an infimum, saye. Then, X=Cl(C(M)). As for the previous case, define

˜

P(x) = sup{P(z);zCl(M) such thatzx}.

It is easy to show that P˜(x)exists. In a similar proof, one can show that P˜ extendsP and is order preserving.

Since a tree has an infimum, we get the following result.

Corollary 2.1.Let X be a tree and M a nonempty externally complete subset of X. Then, there exists an order preserving retract P:X ®M.

A similar result for externally hyperconvex subsets of metric trees maybe found in .

3. Common fixed point

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question are not the conclusions which maybe known but the proofs as developed in the metric setting. Maybe one of the most beautiful results known in the hyperconvex metric spaces is the intersection property discovered by Baillon . The boundedness assumption in Baillon’s result is equivalent to the complete lattice structure in our set-ting. Indeed, any nonempty subset of a complete lattice has an infimum and a supre-mum. We have the following result in partially ordered sets.

Theorem 3.1.Let X be a partially ordered set. Let(Xb)Γbe a decreasing family of nonempty complete lattice subsets of X, where Γis a directed index set. Then,⋂bÎΓ Xb is not empty and is a complete lattice.

Proof. Consider the family

F= ⎧ ⎨ ⎩

β

Aβ;Aβ is a nonempty interval inXβ and (Aβ) is decreasing ⎫ ⎬ ⎭.

F is not empty sinceβXβF. In a complete lattice, any decreasing family of nonempty intervals has a nonempty intersection and it is an interval. Therefore, F satisfies the assumptions of Zorn’s lemma. Hence, for everyDF, there exists a mini-mal element AF such that A⊂ D. We claim that if ∏bÎΓAbis minimal, then each

Abis a singleton. Indeed, let us fix b0 ÎΓ. We know that Ab0 = [mb0,Mb0]. Consider

the new family

Bβ

Aβ

{xXβ;mβ0 ≺x0}

ifβ0≺βorβnot comparable toβ0, ifββ0

Our assumptions on (Xb) and (Ab) imply that(Bβ)∈F. Moreover, we haveBb⊂Ab for any bÎΓ. Since∏bÎΓAbis minimal, we getBb=Abfor anybÎ Γ. In particular,

we have

={x;0 ≺x0}

for b≺b0. IfAb= [mb,Mb], then we must havemb=mb0andMb=Mb0. Therefore,

we proved the existence ofm, MÎXsuch thatAb= {xÎ Xb;m≺x≺M}, for anybÎ Γ. It is easy from here to show that in fact we havem=M by the minimality of∏Γ Ab, which proves our claim. Clearly, we havemÎAbfor anybÎ Γwhich impliesK=

⋂bÎΓ Xb is not empty. Next, we will prove thatKis a complete lattice. LetA⊂Kbe

nonempty. We will only prove that sup Aexists in K. The proof for the existence of the infimum follows identically. For anybÎΓ, we haveA⊂Xb. SinceXbis a complete lattice, then mb = sup A exists in Xb. The interval [mb, ®) is a complete lattice. Clearly, the family ([mb, ®))Γis decreasing. From the above result, we know that

⋂bÎΓ[mb,®) is not empty. Therefore, there existsmÎ Ksuch that a≺mfor anyaÎ

A. SetB= {mÎK; a≺mfor any aÎ A}. For anybÎΓ, defineMb= infBinXb. Set

Xβ=

aA,bB

[a,b]Xβ = [mβ,Mβ]∩Xβ.

Then, Xβis a nonempty complete sublattice ofXb. It is easy to see that the family

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β

Xβ∗ ={supA}

in⋂bÎΓXb. The proof of Theorem 3.1 is therefore complete.

As a consequence of this theorem, we obtain the following common fixed point result.

Theorem 3.2.Let X be a complete lattice. Then, any commuting family of order pre-serving mappings (Ti)iÎI,Ti:X® X, has a common fixed point. Moreover, if we denote

by Fix((Ti)) the set of the common fixed points, then Fix((Ti)) is a complete sublattice of

X.

Proof. First note that Tarski fixed point theorem  implies that any finite commut-ing family of order preservcommut-ing mappcommut-ings T1,T2, ..,Tn,Ti:X ®X, has a common fixed

point. Moreover, if we denote by Fix((Ti)) the set of the common fixed points, i.e.Fix

((Ti)) = {xÎ M;Ti(x) =x i= 1,..,n}, is a complete sublattice ofX. LetΓ = {b;b is a

finite nonempty subset ofI}. Clearly,Γ is downward directed (where the order onΓ is the set inclusion). For any b ÎΓ, the set Fbof common fixed point set of the

map-pings Ti,i Îb, is a nonempty complete sublattice of X. Clearly, the family (Fb)bÎΓ is

decreasing. Theorem 3.1 implies that ⋂bÎΓFbis nonempty and is a complete sublattice

ofX. The proof of Theorem 3.2 is therefore complete.

The commutativity assumption maybe relaxed using a new concept discovered in  (see also [12-15]. Of course, this new concept was initially defined in the metric setting, therefore we need first to extend it to the case of partially ordered sets.

Definition 3.1. Let X be a partially ordered set. The ordered pair(S, T)of two self-maps of the set X is called a Banach operator pair, if the set Fix(T) is S-invariant, namely S(Fix(T)) ⊆Fix(T).

We have the following result whose proof is easy.

Theorem 3.3.Let X be a complete lattice. Let T:X ®X be an order preserving map-ping. Let S: X®X be an order preserving mapping such that(S, T)is a Banach opera-tor pair. Then, Fix(S, T) =Fix(T)⋂Fix(S)is a nonempty complete lattice.

In order to extend this conclusion to a family of mappings, we will need the follow-ing definition.

Definition 3.2.Let T and S be two self-maps of a partially ordered set X. The pair (S, T)is called symmetric Banach operator pair if both (S, T) and(T, S)are Banach operator pairs, i.e., T(Fix(S))⊆Fix(S)and S(Fix(T))⊆Fix(T).

We have the following result which can be seen as an analogue to De Marr’s result  without compactness assumption of the domain.

Theorem 3.4. Let X be a partially ordered set. LetTbe a family of order preserving mappings defined on X. Assume any two mappings from Tform a symmetric Banach operator pair. Then, the family Thas a common fixed point provided one map from Thas a fixed point set which is a complete lattice. Moreover, the common fixed point setFix(T)is a complete lattice.

Proof. LetT0∈T be the map for which Fix(T0) =X0is a nonempty complete lattice.

Since any two mappings fromT form a symmetric Banach operator pair, then for any TT, we haveT(X0)⊂X0. SinceX0 is a complete lattice,T has a fixed point inX0.

The fixed point set of T inX0 isFix(T)⋂ Fix(T0) and is a complete sublattice ofX0.

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complete lattice, thenS has a fixed point in Fix(T)⋂Fix(T0). The fixed point set ofS

inFix(T)⋂Fix(T0) isFix(S)⋂Fix(T)⋂Fix(T0) which is a complete sublattice ofX0. By

induction, one will prove that any finite subfamily T1, ..., Tn ofT has a nonempty

common fixed point set Fix(T1)⋂···⋂Fix(Tn)⋂ X0 which is a complete sublattice of

X0. Theorem 3.1 will then imply that

TT Fix(T)∩X0is not empty and is a complete

lattice. Since

Fix(T)∩X0=Fix(T),

we conclude that Fix(T)is a nonempty complete lattice.

Acknowledgements

The authors were grateful to King Fahd University of Petroleum & Minerals for supporting research project IN 101008.

Author details

1

Department of Mathematics & Statistics, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia2Department of Mathematical Sciences, The University of Texas at El Paso, El Paso, TX 79968, USA

Authors’contributions

All authors participated in the design of this work and performed equally. All authors read and approved the final manuscript.

Competing interests

The authors declare that they have no competing interests.

Received: 21 August 2011 Accepted: 6 December 2011 Published: 6 December 2011

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doi:10.1186/1687-1812-2011-97

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