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Mathematical Theory and Modeling                                                                               www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012




           Common fixed point for two weakly compatible pairs
      of self-maps through property E. A. under an implicit relation
                                                          T. Phaneendra
                                                    Applied Analysis Division
                                                   School of Advanced Sciences
                                    VIT-University, Vellore - 632 014, Tamil Nadu, INDIA
                                                  E–mail: drtp.indra@gmail.com


                                                           Swatmaram
                                                    Department of Mathematics
                                             Chatanya Bharathi Institute of Technology
                                               Hyderabad – 500 075 (A. P.), INDIA
                                                E–mail: ramuswatma@yahoo.com
Abstract
We prove some generalizations of the results of Renu Chug-Sanjay Kumar, the author with necessary corrections, and
of Rhoades et al. for weakly compatible self-maps satisfying the property E. A. under an implicit relation.
Key Words: Compatible and Weakly Compatible Self-maps, Property E. A., Contractive Mmodulus, Common Fixed
Point


1      INTRODUCTION


Throughout this paper, ( X , d ) denotes a metric space. If x ∈ X and A is a self-map on X, we write Ax for the A-image
of x, A(X) for the range of A and AS for the composition of self-maps A and S. Also IR + will denote the set of all non
negative real numbers. A point p of the space X is a fixed point of a self-map A if and only if Ap = p. A self-map A on
X with the choice d ( Ax, Ay) ≤ a d ( x, y) for all x, y ∈ X is known as a contraction, provided 0 ≤ a < 1 . According to the
celebrated Banach contraction principle (BCP), every contraction on a complete metric space has a unique fixed
point. It is easy to see that a discontinuous self-map cannot be a contraction and hence contraction principle cannot
ensure a fixed point for it even if X is complete. However, various generalizations of BCP have been established by
weakening the contraction condition, relaxing the completeness of the underlying space and/or extending to two or
more self-maps under additional assumptions. To mention a few are the works of Fisher ([2]-[3]), Fisher and Khan
[4], Chang [5], Ciric [6], Das and Naik [7], Jungck [8], Pant [11] and Singh and Singh [18].


Self-maps S and A on X are said to be weakly commuting [17] if d ( ASx, SAx) ≤ a d (Sx, Ax) for all x, y ∈ X . As a
further generalization for commuting maps, Gerald Jungck [9] proposed the compatibility as in the following lines:
Definition 1.1 Self-maps S and A on X are compatible if
                  lim d (SAxn , ASxn ) = 0                                                                               (1)
                  n∞ ∞
                    →
whenever     xn    n =1
                        ⊂X such that
           lim Axn = lim Sxn = t for some t ∈ X.                                                                         (2)
           n→∞           n→∞
                     ∞
If there is no    xn n = 1 in X   with the choice (2), S and T are called vacuously compatible.
                                                                                                               ∞
Remark 1.1         Self-maps S and A on X are not (non-vacuously) compatible if there is a sequence       xn   n =1
                                                                                                                    in

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Mathematical Theory and Modeling                                                                                           www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


X with choice (2) but lim d (SAxn , ASxn ) ≠ 0 or + ∞.
                             n →∞
Obviously every commuting pair of self-maps is a weakly commuting one, and a weakly commuting pair is
necessarily compatible. But neither reverse implication is true [9].


The following notion is due to Aamri and Moutawakil [1]:

                                                                                               ∞
Definition 1.2 Self-maps S and T satisfy the property E. A. if there is an                xn   n =1
                                                                                                    ⊂   X with the choice (2).


In this paper CX denotes the class of all non-compatible pairs of self-maps on X, while C* , the class of all pairs of
                                                                                         X
self-maps on X which satisfy the property E. A.

                                                                                                        ∞
In view of Remark 1.1, non-compatibility implies the existence of the sequence                     xn   n =1
                                                                                                               with the choice (2).
Hence the class       C*
                       X     is potentially wider than CX. We also note that vacuously compatible self-maps do not satisfy
the property E. A.


We call a point x ∈ X , a coincidence point of self-maps S and A on X if Tx = Sx, while y ∈ X is a point of their
coincidence w. r. t. x if Tx = Sx = y. Taking xn = x for all n, from (1) and (2), it follows that STx = TSx
whenever x ∈ X is such that Tx = Sx. Hence we have
Definition 1.3 Self-maps which commute at their coincidence points are weakly compatible maps [10] which are
also called coincidentally commuting or partially commuting [16].
Being weakly compatible and possessing property E. A. are independent conditions of each other (Pathak                                et al.
[12])


In this paper φ : IR5 → IR+ is an upper semi-continuous (written shortly as usc) non decreasing in each coordinate
                    +
variable, and for ξ > 0 satisfy the conditions:
   (i)       max{φ(ξ, ξ,0,2ξ,0), φ(ξ, ξ,0, ,0,2ξ)} ≤ ξ
   (ii)      max{φ(ξ, ξ,0, αξ,0), φ(ξ, ξ,0, ,0, αξ)} < ξ when α < 2
  (iii)      γ(t ) = φ(ξ, ξ, α1ξ, α 2ξ, α3ξ) < ξ if α1 + α 2 + α3 = 4 .


Remark 1.2           ξ ≤ φ(ξ, ξ, ξ, ξ, ξ) implies that ξ = 0 .



With this notion Renu Chug and Sanjay Kumar [14] proved the following result for two weakly compatible pairs of
self-maps:
Theorem 1.1 Let A, B, S and T be self-maps on X satisfying the inclusions
                           A( X ) ⊂ T ( X ) ,                                                                               (3-a)
                           B( X ) ⊂ S ( X )                                                                                 (3-b)
and the inequality
         d ( Ax, By) ≤ φ(d (Sx,Ty), d ( Ax, Sx), d (By,Ty), d ( Ax,Ty), d ( By, Sx)) for all x, y ∈ X .                        (4)
Suppose that:
    (a)        X is complete

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Mathematical Theory and Modeling                                                                                                      www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


      (b)     The pairs ( A, S ) and (B,T ) are weakly compatible.
Then all the four maps A, B, S and T have a unique common fixed point.


         The authors of Theorem 1.1 asserted that there is a point u ∈ X such that z = Su under the inclusion
(3-b) (Line 4 from the bottom, page 244). We point out that their assertion is not true and that the inclusion (3-b)
plays no role to obtain the point u. However it will be true if we assume that the map B (and hence S) is onto.
            In view of this suggestion we restate Theorem 1.1 as follows:
Theorem 1.2 Let A, B, S and T be self-maps on X satisfying the inclusions (3-a), (3-b), the inequality (4), (a) and (b)
of Theorem 1.1. If
      (c)    either A or B is onto,
then all the four maps A, B, S and T have a unique common fixed point.


     In this paper we obtain the conclusion of Theorem 1.1, by relaxing the completeness of the space X and using
the property E. A. (see below). Our result will be a generalization of those some of the authors.


2     MAIN RESULT
First we prove
Theorem 2.1 Let A, B, S and T be self-maps on X satisfying (3-a), (3-b), and the inequality (4).
Suppose that
      (d)     either (S , A) ∈ C* or (B,T ) ∈ C* ,
                                X              X
      (e)     one of S ( X ), A( X ), T ( X ) and B(X ) is a complete subspace of X.
If the condition (b) of Theorem 1.2 holds good, then A, B, S and T will have a unique common fixed point.

                                                                             ∞
Proof. First suppose that (S , A) ∈ C* . Then there is a xn
                                     X                                       n =1
                                                                                  ⊂   X such that lim Axn = lim Sxn = p for
                                                                                                        n→∞               n→∞
                                                                                                          ∞
some p ∈ X. By virtue of the inclusion (3-a), we can find another sequence yn                             n =1
                                                                                                               of     points in X such that
Axn = Tyn for all n so that
               lim Axn = lim Sxn = lim Tyn = p.                                                                                               (5)
              n→∞                n→∞               n→∞


Observe that q = lim Byn also equals p. In fact, writing x = xn and y = yn in the inequality (4), we see that
                      n→∞
       d ( Axn , Byn ) ≤ φ ( d (Sxn ,Tyn ), d ( Axn , Sxn ), d (Byn , Tyn ), d ( Axn ,Tyn ), d (Byn , Sxn ))
Applying the limit as n → ∞ in this, and using (5) and the usc of φ,
              d ( p, q) ≤ φ (0,0, d (q, p),0, d (q, p)) ≤ φ (d ( p, q), d ( p, q), d ( p, q), d ( p, q), d ( p, q))
so that or d ( p, q) = 0 or p = q , in view of Remark 1.2. Thus
        lim Axn = lim Sxn = lim Byn = lim Tyn = p.                                                                                            (6)
       n→∞               n→∞                n→∞                n→∞
We first show that p is a common coincidence point for (S , A) and (B,T ) . That is
       Ap = Sp = Bp = Tp .
(7)


Case (a): Suppose that T (X ) is complete.
Then p ∈ T (X ) so that Tu = p for some u ∈ X. But then from (4), we see that

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Mathematical Theory and Modeling                                                                                                     www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


                             d ( Axn , Bu) ≤ φ (d (Sxn ,Tu ), d ( Axn , Sxn ), d (Bu,Tu ), d ( Axn ,Tu ), d (Bu, Sxn ))
Applying the limit as n → ∞ in this, and using Tu = p and upper semi-continuity of φ followed by its non
decreasing nature in each coordinate variable, we get
       d ( p, Bu) ≤ φ (0,0, d (Bu, p),0, d (Bu, p)) ≤ φ (d ( Bu, p), d ( Bu, p), d (Bu, p), d (Bu, p), d (Bu, p))
or d ( p, Bu) = 0 Bu = p . Thus u is a coincidence point, and hence p is a point of coincidence for B and T . That is
        Bu = Tu = p.                                                                              (8)
Again from the inclusion (3-b), we see that p = Bu = Sr for some r ∈ X .
Since φ is non decreasing, from (4) and (8) we get
       d ( Ar, Sr) = d ( Ar, Bu) ≤ φ (d (Sr,Tu), d ( Ar, Sr), d (Bu,Tu), d ( Ar,Tu), d (Bu, Sr))
                                       = φ (0, d ( Ar , p), 0, d ( Ar , p), 0) ≤ φ (d ( Ar , p), d ( Ar, p), d ( Ar, p), d ( Ar, p), d ( Ar, p))
so that Ar = Sr = p , in view of Remark 1.2. That is p is a point of coincidence for A and S w. r. t. r. Thus
             Ar = Sr = Bu = Tu = p.                                                                                                     (9)
Since (A, S) and (B, T) are weakly compatible pairs, (7) follows from (9).


Case (b): Suppose that A(X ) is complete.
Then p ∈ A( X ) ⊂ T ( X ) , in view of (3-a) and hence (7) follows from Case (a).
Case (c): Suppose that S (X ) is complete.
Then p∈ S ( X ) so that Sw = p for some w ∈ X. But then from (4), we see that
                             d ( Aw, Byn ) ≤ φ (d (Sw,Tyn ), d ( Aw, Sw), d (Byn ,Tyn ), d ( Aw,Tyn ), d (Byn , Sw))
Applying the limit as n → ∞ , and using (6), Sw = p and upper semi-continuity of φ, followed by its non decreasing
nature in each coordinate variable, this gives
       d ( Aw, p) ≤ φ (0, d ( Aw, p), 0, d ( Aw, p), 0) ≤ φ (d ( Aw, p), d ( Aw, p), d ( Aw, p), d ( Aw, p), d ( Aw, p))
or d ( p, Aw) = 0 or Aw = p . Thus w is a coincidence point and p is a point of coincidence for A and S
w. r. t. w. That is
             Aw = Sw = p .                                                                                                            (10)
Again from the inclusion (3-a), we see that p = Aw = Tq for some q ∈ X .
Since φ is non decreasing, from (4) and (10) we get
d ( p, Bq) = d ( Aw, Bq) ≤ φ (d (Sw, Tq), d ( Aw, Sw), d ( Bq, Tq), d ( Aw, Tq), d (Bq, Sw))
                              = φ (0,0, d ( Bq, p), 0, d ( Bq, p)) ≤ φ (d (Bq, p), d ( Bq, p), d (Bq, p), d (Bq, p), d ( Bq, p))
so that p = Bq in view of Remark 1.2.
Thus         Aw = Sw = Bq = Tq = p.                                                                                                   (11)
Now (7) will follow from (b) and (11).
Case (d): Suppose that B( X ) is complete.
Then p ∈ B( X ) ⊂ S ( X ) , in view of (3-b) and hence (7) follows from Case (c).


To establish that p is a common fixed point for all the four maps, we again use (4) with x = r and y = p so that
       d ( Ar, Bp) ≤ φ (d (Sr,Tp), d ( Ar, Sr ), d (Bp,Tp), d ( Ar ,Tp), d (Bp, Sr )) .
Again since φ is non decreasing, this together with (7) and (8) gives
       d ( p, Bp) ≤ φ (d ( p, Bp),0, 0, d ( p, Bp), d (Bp, p)) ≤ φ (d ( p, Bp), d ( p, Bp), d ( p, Bp), d ( p, Bp), d ( p, Bp))


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Mathematical Theory and Modeling                                                                                                  www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


so that from Remark 1.2, it follows that Bp = p and hence p is a common fixed point for A, B, S and T, which in
fact is a point of their common coincidence.
Now suppose that (B,T ) ∈ C* . Then exchanging the roles of (S , A) and (B,T ) ; of (3-a) and (3-b); and of
                              X
T (X ) and S (X ) in the above proof, we can similarly obtain the conclusion.
Uniqueness of the common fixed point follows easily from the choice of φ and (4).                                                         


Remark 2.1 If A is onto then A( X ) = X . Therefore the completeness of X implies the completeness of A(X ) .
Similarly the completeness of X implies the completeness of B(X ) whenever B is onto.
Remark 2.2 We can prove that if self-maps A, B, S and T on X satisfy the inclusions (3-a), (3-b) and the inequality
(4), and X is complete, then both ( A, S ) ∈ C* and (B,T ) ∈ C* .
                                              X               X
                                                                                                          ∞
In fact for any x0 ∈ X , the inclusions (3 a-b) generate a sequence of points xn                          n =1
                                                                                                                 in X with the choice
           Ax2n−2 = Tx2n −1 , Bx2n −1 = Sx2n for all n ≥ 1.                                                                             (12)
                                                                     ∞                      ∞
From the proof of Theorem 1.2, it follows that                  Ax2n n = 1    and      Ax2n n = 1      are Cauchy sequences.
If X is complete, these will converge to some z ∈ X. That is
      lim Ax2n−2 = lim Tx2n−1 = lim Bx2n−1 = lim Sx2n = p.                                                                               (13)
      n→∞                   n→∞                   n→∞                   n→∞
With x2n = xn and x2n−1 = yn from (13), we see that
            *              *

                  *         *
            lim Axn = lim Sxn = p                            *         *
                                                   and lim Byn = lim Tyn = p,
           n→∞               n→∞                          n→∞                n→∞
proving that the pairs (S , A) and (B,T ) satisfy the property E. A.
Remark 2.3 In view of Remarks 2.1 and 2.2, we see that Theorem 1.2 follows as a particular case of our result
(Theorem 2.1).


It is possible to relax the condition (b), and drop the inclusions in Theorem 2.1 for three self-maps
A, B and T.
     In deed we prove the following
Theorem 2.2 Let A, B and T be self-maps on X satisfying the inequality
                                   d ( Ax, By) ≤ φ(d (Tx, Ty), d ( Ax, Tx), d ( By,Ty), d ( Ax,Ty), d (By, Tx))
                                 for all x, y ∈ X .                                                                                     (14)
Suppose that T ( X ) is complete subspace of X and
          (f)     either of the pairs ( A,T ) and (B,T ) is both (weakly compatible and belongs to the class C* ).
                                                                                                              X
Then A, B and T will have a unique common fixed point.

                                                                                                       ∞
Proof. Suppose that the pair ( A,T ) ∈ C* . Then there exists a sequence
                                        X                                                         xn   n =1
                                                                                                            ⊂   X such that
            lim Axn = lim Txn = p for some p ∈ X.                                                                                       (15)
           n→∞               n→∞
Let q = lim Bxn . Then q = p. In fact, writing x = y = xn in the inequality (14),
          n→∞
      d ( Axn , Bxn ) ≤ φ ( d (Txn ,Txn ), d ( Axn ,Txn ), d (Bxn ,Txn ), d ( Axn, Txn ), d (Bxn, Txn ))
Applying the limit as n → ∞ in this and using (15) and upper semi-continuity of φ,
      d ( p, q) ≤ φ (0,0, d (q, p),0, d (q, p)) ≤ φ (d ( p, q), d ( p, q), d ( p, q), d ( p, q), d ( p, q))
so that d ( p, q) = 0 or p = q , in view of Remark 1.2. Thus
                lim Axn = lim Txn = lim Bxn = p.                                                                                         (16)
             n→∞                n→∞               n→∞




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Mathematical Theory and Modeling                                                                                                    www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


Suppose that T ( X ) is complete. Then p ∈ T ( X ) so that Tu = p for some u ∈ X. But then from (14),
                              d ( Axn , Bu) ≤ φ (d (Txn ,Tu), d ( Axn , Txn ), d (Bu, Tu), d ( Axn ,Tu), d ( Bu,Txn )) .
Applying the limit as n → ∞ in this, and using Tu = p and upper semi-continuity of φ followed by its non decreasing
nature in each coordinate variable, we get
d ( p, Bu) ≤ φ (0,0, d ( Bu, p),0, d ( Bu, p)) ≤ φ (d (Bu, p), d (Bu, p), d (Bu, p), d (Bu, p), d (Bu, p)) or d ( p, Bu) = 0
or Bu = p . Thus u is a coincidence point of B and T. That is Bu = Tu = p .
Since φ is non decreasing, from (14), we get
       d ( Au, Bu ) ≤ φ (d (Tu,Tu ), d ( Au, Su ), d ( Bu,Tu), d ( Au,Tu ), d ( Bu,Tu))
                        = φ (0, d ( Au, p), 0), d ( Au, p), 0) ≤ φ (d ( Au, p), d ( Au, p), d ( Au, p), d ( Au, p), d ( Au, p))
so that Au = Tu in view of Remark 1.2.


Thus          Au = Bu = Tu = p .                                                                                                            (17)
Since (A, T) is weakly compatible pair, from (17) we get that Ap = Tp .
Again since φ is non decreasing, from (14) and using Ap = Tp , we get
       d ( Ap, Bp) ≤ φ (d (Tp, Tp), d ( Ap,Tp), d (Bp, Tp), d ( Ap,Tp), d (Bp, Tp))
                        = φ (0,0, d ( Ap, Bp), 0, d ( Ap, Bp)) ≤ φ (d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp))
so that Ap = Bp in view of Remark 1.2.
Thus Ap = Bp = Tp .                                                                                                                         (18)


Finally p will be a common fixed point for the three maps. For, writing x = u and y = p in (14) and then using
(17) and (18) we get
       d ( p, Tp) = d ( Au, Bp) ≤ φ (d (Tu, Tp), d ( Au, Tp), d (Bp, Tp), d ( Au, Tp), d ( Bp, Tu))
                  = φ (d ( p, Tp), d ( p, Tp), 0, d ( p, Tp), d (Tp, p)) ≤ φ (d ( p, Tp), d ( p, Tp), d ( p, Tp), d (Tp, p), d ( p, Tp))
Again since φ is non decreasing, this with Remark 1.2, implies that Bp = p and hence p is a common fixed point
for A, B, and T.
Similarly if ( A,T ) is a weakly compatible pair satisfying the property E.A., it follows that A, B, and T have a
common fixed point. Uniqueness of the common fixed point follows easily from the choice of φ and (14).
                                                                                            



Taking A = B in Theorem 2.2 we have
Corollary 2.1 Let A and T be self-maps on X satisfying the inequality
       d ( Ax, Ay) ≤ φ(d (Tx, Ty), d ( Ax,Tx), d ( Ay,Ty ), d ( Ax, Ay), d ( Ay,Tx)) for all x, y ∈ X .                                    (19)
If T ( X ) is complete subspace of X, ( A,T ) is weakly compatible and ( A,T ) ∈ C* , then A and T will have a unique
                                                                                  X
common fixed point.


Given x0 ∈ X and self-maps A and T on X, if there exist points x1, x2, x3,... in X with Axn −1 = Txn for n ≥ 1 , the
               ∞
sequence Axn n =1 defines a ( A,T ) -orbit or simply an orbit at x0. The space X is ( A,T ) -orbitally complete [13]
at x0 ∈ X if every Cauchy sequence in some orbit at x0 converges in X .
Note that every complete metric space is orbitally complete at each of its points. However there are incomplete
metric spaces which are orbitally complete at some point of it [13].

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Mathematical Theory and Modeling                                                                                   www.iiste.org
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Vol.2, No.6, 2012



We give the following generalization of Corollary 2.1, the proof of which can be obtained on similar lines of that of
Theorem 2.2.


Theorem 2.3 Let A and T be self-maps on X satisfying the inequality (14) and ( A,T ) ∈ C* . Suppose that any one of
                                                                                        X
the following conditions holds good:
         (g)     the subspace A( X ) is orbitally complete at x0 ∈ X
         (h)      T ( X ) is orbitally complete at x0 ∈ X .
Then there is a coincidence point z for A and T. Further if A and T are weakly compatible, then the point of
coincidence of A and T w. r. t. z will be a unique common fixed point for them.


In the remainder of the paper ψ : IR+ → IR+ represents a contractive modulus, due to Solomon Leader [19] with the
choice ψ(t ) < t for t > 0.



Corollary 2.2 (Theorem 2, [13])           Let A and T be self-maps on X satisfying the inclusion
                     A( X ) ⊂ T ( X )                                                                              (20)
and the inequality
     d ( Ax, Ay) ≤ ψ(max d (Tx, Ty), d ( Ax, Tx), d ( Ay, Ty) 1 [ d ( Ax, Ty) + d ( Ay, Tx)]) for all x, y ∈ X ,
                                                              2
                                                                                                                   (21)
where ψ is non decreasing and upper semi continuous contractive modulus.
Suppose that one of the conditions (g), (h) and (i) holds good, where
         (i)   the space X is orbitally complete at some x0 ∈ X and T is onto
Then there is a coincidence point z for A and T. Further if A and T are weakly compatible, then the point of
coincidence of A and T w. r. t. z will be a unique common fixed point for them.


Proof. We note that the condition (i) implies (g). We set
         φ(ξ1, ξ2, ξ3, ξ4, ξ5) = ψ(max{ξ1, ξ2, ξ3, 1 [ξ4 + ξ5]}) for all ξi , i = 1,2, ..., 5 .
                                                   2
Then (14) reduces to (21).


Let x0 ∈ X be arbitrary. By virtue of the inclusion (20), we can construct an ( A,T ) - orbit x0 with choice
                                                           ∞
Axn −1 = Txn for n ≥ 1 . Then the sequence           Axn   n =1
                                                                  is a Cauchy sequence in this orbit at X.


In view of Remark 2.2, A and T satisfy the property E. A. Hence the conclusion follows from Theorem 2.3.


Now write B = A in Theorem 2.1, we get
Theorem 2.4 Let A, S and T be self-maps on X satisfying the inclusion
                     A( X ) ⊂ [S ( X ) I T ( X )]                                                                  (22)
and the inequality
           d ( Ax, Ay) ≤ φ(d (Sx,Ty), d ( Ax, Sx), d ( Ay, Ty), d ( Ax,Ty), d ( Ay, Sx)) for all x, y ∈ X .        (23)
Suppose that

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Mathematical Theory and Modeling                                                                                              www.iiste.org
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Vol.2, No.6, 2012


           (j)    either of the pairs ( A, S ) and ( A,T ) satisfies the property E. A.
           (k)    one of A( X ), B( X ) and T ( X ) is a compete subspace of X.
If ( A, S ) and ( A,T ) are weakly compatible, then A, B, S and T will have a unique common fixed point.


Since every compatible pair is weakly compatible, we have the following sufficiency part of Main theorem of [15].


Corollary 2.3 Let A, S and T be self-maps on X satisfying (20) and the inequality
        d ( Ax, Ay) ≤ max{d (Sx,Ty), d ( Ax, Sx), d ( Ay,Ty), 1 [d ( Ax,Ty) + d ( Ay, Sx)]}
                                                              2
                            − ω(max{d (Sx,Ty), d ( Ax, Sx), d ( Ay,Ty), 1 [d ( Ax,Ty) + d ( Ay, Sx)]}) for all x, y ∈ X ,
                                                                        2
                                                                                                                                  (24)
where ω : IR+ → IR+ is continuous and 0 < ω(t ) < t for t > 0 . Suppose that X is complete, A is continuous and
( A, S ) and ( A,T ) are compatible. Then A, S and T will have a unique common fixed point.
Note that (23) reduces to (24) with
                                                                                              
     φ(ξ1, ξ2 , ξ3, ξ4 , ξ5 ) = maxξ1, ξ2 , ξ3, [ξ4 + ξ5 ] − ω maxξ1, ξ2 , ξ3, [ξ4 + ξ5 ]  for all ξi , i = 1,2, ..., 5 .
                                                1                                 1
                                                                                          
                                               2                              2          
                                                                                                 ∞
Given x0 ∈ X , due to the inclusion (22), we can construct the sequence                    Axn   n =1
                                                                                                        with choice
                    Ax2n − 2 = Tx2n−1, Ax2n −1 = Tx2n for n ≥ 1 ,
which is a Cauchy sequence in the complete space X and hence converges in it.
In view of Remark 2.2, A and T satisfy the property E. A.


References


[1]     Aamri M. A. and Moutawakil D. El. (2002). Some new common fixed point theorems under strict contractive
        conditions, J. Math. Anal. Appl., 270: 181-188
[2]     Brian Fisher (1979). Mappings with a common fixed point, Math. Sem. Notes, 7, 81 – 83
[3]     Brian Fisher (1983). Common fixed points of four mappings, Bull. Inst. Math. Acad. Sinica, 11,
        103 – 113
[4]     Brian Fisher and Khan M. S. (1982). Some fixed point theorems for commuting mappings, Math. Nachr., 108,
        323 – 326
[5]     Chang S. S. (1981). A common fixed point theorem for commuting maps, Proc. Amer. Math. Soc. 83, 645 –
        652
[6]     Ciric Lj. B. (1974). A Generalization of Banach Contraction Principle, Proc. Amer. Math. Soc. 45 (2), 267-27
[7]     Das K. M. and Naik V. (1981). Common fixed point theorems for commuting maps on metric space, Proc.
        Amer. Math. Soc. 83, 645 – 652
[8]     Gerald Jungck (1976). Commuting mappings and fixed points, Amer. Math. Monthly, 83, 261 – 263
[9]     Gerald Jungck (1986). Compatible maps and common fixed points, Int. J. Math. & Math. Sci.,
        9, 771-779
[10]    Gerald Jungck (1996) Common fixed points for non continuous and non self-mappings on a metric space, Far
        East J. Math. Sci. 4 (2), 199-212
[11]    Pant R. P. (1986). Common fixed points of two pairs of commuting mappings, Indian J. Pure Appl. Math. Sci.
        17 (1986), 187 – 192


                                                                       8
Mathematical Theory and Modeling                                                                     www.iiste.org
ISSN 2224-5804 (Paper)    ISSN 2225-0522 (Online)
Vol.2, No.6, 2012


[12]   Pathak H. K., Lopez R. R. and Verma R. K. (2007). A common fixed point theorem using implicit relation and
       Property E. A. in Metric Spaces, Filomat 21 (2), 211-234
[13]   Phaneendra T. (2004). Coincidence points of two weakly compatible self-maps and common fixed point
       theorem through orbits, Indian J. Math. 46 (2-3), 173-180
[14]   Renu Chugh and Sanjay Kumar(2001). Common fixed points for weakly compatible maps, Proc. Indian Acad.
       Sci. (Math. Sci.) 111(2), 241-247.
[15]   Rhoades B. E., Kalishankar Tiwary and Singh G. N. (1995). A common fixed point theorem for compatible
       mappings, Indian J. pure appl. Math. 26 (5), 403-409
[16]   Sastry K. P. R., Ismail S. and Murthy I. S. R. K. (2004). Common fixed points for two
       self-maps under strongly partially commuting condition, Bull. Cal. Math. Soc. 96 (1), 1-10
[17]   Sessa S. (1982). On a weak commutativity condition of mappings in fixed point considerations, Publ. Inst.
       Math. (Beograd), 32 (46), 149-153
[18]   Singh S. L. and Singh S. P. (1980). A fixed point theorem, Indian J. Pure Appl. Math. 11(12), 1584-1586
[19]   Solomon Leader (1977). Fixed Points Operators on Metric Spaces with Conditional Uniform Equivalence of
       Orbits, J. Math. Anal. Appl., 61, 466-474




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Common fixed point for two weakly compatible pairs of self maps through property e. a. under an implicit relation

  • 1. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 Common fixed point for two weakly compatible pairs of self-maps through property E. A. under an implicit relation T. Phaneendra Applied Analysis Division School of Advanced Sciences VIT-University, Vellore - 632 014, Tamil Nadu, INDIA E–mail: drtp.indra@gmail.com Swatmaram Department of Mathematics Chatanya Bharathi Institute of Technology Hyderabad – 500 075 (A. P.), INDIA E–mail: ramuswatma@yahoo.com Abstract We prove some generalizations of the results of Renu Chug-Sanjay Kumar, the author with necessary corrections, and of Rhoades et al. for weakly compatible self-maps satisfying the property E. A. under an implicit relation. Key Words: Compatible and Weakly Compatible Self-maps, Property E. A., Contractive Mmodulus, Common Fixed Point 1 INTRODUCTION Throughout this paper, ( X , d ) denotes a metric space. If x ∈ X and A is a self-map on X, we write Ax for the A-image of x, A(X) for the range of A and AS for the composition of self-maps A and S. Also IR + will denote the set of all non negative real numbers. A point p of the space X is a fixed point of a self-map A if and only if Ap = p. A self-map A on X with the choice d ( Ax, Ay) ≤ a d ( x, y) for all x, y ∈ X is known as a contraction, provided 0 ≤ a < 1 . According to the celebrated Banach contraction principle (BCP), every contraction on a complete metric space has a unique fixed point. It is easy to see that a discontinuous self-map cannot be a contraction and hence contraction principle cannot ensure a fixed point for it even if X is complete. However, various generalizations of BCP have been established by weakening the contraction condition, relaxing the completeness of the underlying space and/or extending to two or more self-maps under additional assumptions. To mention a few are the works of Fisher ([2]-[3]), Fisher and Khan [4], Chang [5], Ciric [6], Das and Naik [7], Jungck [8], Pant [11] and Singh and Singh [18]. Self-maps S and A on X are said to be weakly commuting [17] if d ( ASx, SAx) ≤ a d (Sx, Ax) for all x, y ∈ X . As a further generalization for commuting maps, Gerald Jungck [9] proposed the compatibility as in the following lines: Definition 1.1 Self-maps S and A on X are compatible if lim d (SAxn , ASxn ) = 0 (1) n∞ ∞ → whenever xn n =1 ⊂X such that lim Axn = lim Sxn = t for some t ∈ X. (2) n→∞ n→∞ ∞ If there is no xn n = 1 in X with the choice (2), S and T are called vacuously compatible. ∞ Remark 1.1 Self-maps S and A on X are not (non-vacuously) compatible if there is a sequence xn n =1 in 1
  • 2. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 X with choice (2) but lim d (SAxn , ASxn ) ≠ 0 or + ∞. n →∞ Obviously every commuting pair of self-maps is a weakly commuting one, and a weakly commuting pair is necessarily compatible. But neither reverse implication is true [9]. The following notion is due to Aamri and Moutawakil [1]: ∞ Definition 1.2 Self-maps S and T satisfy the property E. A. if there is an xn n =1 ⊂ X with the choice (2). In this paper CX denotes the class of all non-compatible pairs of self-maps on X, while C* , the class of all pairs of X self-maps on X which satisfy the property E. A. ∞ In view of Remark 1.1, non-compatibility implies the existence of the sequence xn n =1 with the choice (2). Hence the class C* X is potentially wider than CX. We also note that vacuously compatible self-maps do not satisfy the property E. A. We call a point x ∈ X , a coincidence point of self-maps S and A on X if Tx = Sx, while y ∈ X is a point of their coincidence w. r. t. x if Tx = Sx = y. Taking xn = x for all n, from (1) and (2), it follows that STx = TSx whenever x ∈ X is such that Tx = Sx. Hence we have Definition 1.3 Self-maps which commute at their coincidence points are weakly compatible maps [10] which are also called coincidentally commuting or partially commuting [16]. Being weakly compatible and possessing property E. A. are independent conditions of each other (Pathak et al. [12]) In this paper φ : IR5 → IR+ is an upper semi-continuous (written shortly as usc) non decreasing in each coordinate + variable, and for ξ > 0 satisfy the conditions: (i) max{φ(ξ, ξ,0,2ξ,0), φ(ξ, ξ,0, ,0,2ξ)} ≤ ξ (ii) max{φ(ξ, ξ,0, αξ,0), φ(ξ, ξ,0, ,0, αξ)} < ξ when α < 2 (iii) γ(t ) = φ(ξ, ξ, α1ξ, α 2ξ, α3ξ) < ξ if α1 + α 2 + α3 = 4 . Remark 1.2 ξ ≤ φ(ξ, ξ, ξ, ξ, ξ) implies that ξ = 0 . With this notion Renu Chug and Sanjay Kumar [14] proved the following result for two weakly compatible pairs of self-maps: Theorem 1.1 Let A, B, S and T be self-maps on X satisfying the inclusions A( X ) ⊂ T ( X ) , (3-a) B( X ) ⊂ S ( X ) (3-b) and the inequality d ( Ax, By) ≤ φ(d (Sx,Ty), d ( Ax, Sx), d (By,Ty), d ( Ax,Ty), d ( By, Sx)) for all x, y ∈ X . (4) Suppose that: (a) X is complete 2
  • 3. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 (b) The pairs ( A, S ) and (B,T ) are weakly compatible. Then all the four maps A, B, S and T have a unique common fixed point. The authors of Theorem 1.1 asserted that there is a point u ∈ X such that z = Su under the inclusion (3-b) (Line 4 from the bottom, page 244). We point out that their assertion is not true and that the inclusion (3-b) plays no role to obtain the point u. However it will be true if we assume that the map B (and hence S) is onto. In view of this suggestion we restate Theorem 1.1 as follows: Theorem 1.2 Let A, B, S and T be self-maps on X satisfying the inclusions (3-a), (3-b), the inequality (4), (a) and (b) of Theorem 1.1. If (c) either A or B is onto, then all the four maps A, B, S and T have a unique common fixed point. In this paper we obtain the conclusion of Theorem 1.1, by relaxing the completeness of the space X and using the property E. A. (see below). Our result will be a generalization of those some of the authors. 2 MAIN RESULT First we prove Theorem 2.1 Let A, B, S and T be self-maps on X satisfying (3-a), (3-b), and the inequality (4). Suppose that (d) either (S , A) ∈ C* or (B,T ) ∈ C* , X X (e) one of S ( X ), A( X ), T ( X ) and B(X ) is a complete subspace of X. If the condition (b) of Theorem 1.2 holds good, then A, B, S and T will have a unique common fixed point. ∞ Proof. First suppose that (S , A) ∈ C* . Then there is a xn X n =1 ⊂ X such that lim Axn = lim Sxn = p for n→∞ n→∞ ∞ some p ∈ X. By virtue of the inclusion (3-a), we can find another sequence yn n =1 of points in X such that Axn = Tyn for all n so that lim Axn = lim Sxn = lim Tyn = p. (5) n→∞ n→∞ n→∞ Observe that q = lim Byn also equals p. In fact, writing x = xn and y = yn in the inequality (4), we see that n→∞ d ( Axn , Byn ) ≤ φ ( d (Sxn ,Tyn ), d ( Axn , Sxn ), d (Byn , Tyn ), d ( Axn ,Tyn ), d (Byn , Sxn )) Applying the limit as n → ∞ in this, and using (5) and the usc of φ, d ( p, q) ≤ φ (0,0, d (q, p),0, d (q, p)) ≤ φ (d ( p, q), d ( p, q), d ( p, q), d ( p, q), d ( p, q)) so that or d ( p, q) = 0 or p = q , in view of Remark 1.2. Thus lim Axn = lim Sxn = lim Byn = lim Tyn = p. (6) n→∞ n→∞ n→∞ n→∞ We first show that p is a common coincidence point for (S , A) and (B,T ) . That is Ap = Sp = Bp = Tp . (7) Case (a): Suppose that T (X ) is complete. Then p ∈ T (X ) so that Tu = p for some u ∈ X. But then from (4), we see that 3
  • 4. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 d ( Axn , Bu) ≤ φ (d (Sxn ,Tu ), d ( Axn , Sxn ), d (Bu,Tu ), d ( Axn ,Tu ), d (Bu, Sxn )) Applying the limit as n → ∞ in this, and using Tu = p and upper semi-continuity of φ followed by its non decreasing nature in each coordinate variable, we get d ( p, Bu) ≤ φ (0,0, d (Bu, p),0, d (Bu, p)) ≤ φ (d ( Bu, p), d ( Bu, p), d (Bu, p), d (Bu, p), d (Bu, p)) or d ( p, Bu) = 0 Bu = p . Thus u is a coincidence point, and hence p is a point of coincidence for B and T . That is Bu = Tu = p. (8) Again from the inclusion (3-b), we see that p = Bu = Sr for some r ∈ X . Since φ is non decreasing, from (4) and (8) we get d ( Ar, Sr) = d ( Ar, Bu) ≤ φ (d (Sr,Tu), d ( Ar, Sr), d (Bu,Tu), d ( Ar,Tu), d (Bu, Sr)) = φ (0, d ( Ar , p), 0, d ( Ar , p), 0) ≤ φ (d ( Ar , p), d ( Ar, p), d ( Ar, p), d ( Ar, p), d ( Ar, p)) so that Ar = Sr = p , in view of Remark 1.2. That is p is a point of coincidence for A and S w. r. t. r. Thus Ar = Sr = Bu = Tu = p. (9) Since (A, S) and (B, T) are weakly compatible pairs, (7) follows from (9). Case (b): Suppose that A(X ) is complete. Then p ∈ A( X ) ⊂ T ( X ) , in view of (3-a) and hence (7) follows from Case (a). Case (c): Suppose that S (X ) is complete. Then p∈ S ( X ) so that Sw = p for some w ∈ X. But then from (4), we see that d ( Aw, Byn ) ≤ φ (d (Sw,Tyn ), d ( Aw, Sw), d (Byn ,Tyn ), d ( Aw,Tyn ), d (Byn , Sw)) Applying the limit as n → ∞ , and using (6), Sw = p and upper semi-continuity of φ, followed by its non decreasing nature in each coordinate variable, this gives d ( Aw, p) ≤ φ (0, d ( Aw, p), 0, d ( Aw, p), 0) ≤ φ (d ( Aw, p), d ( Aw, p), d ( Aw, p), d ( Aw, p), d ( Aw, p)) or d ( p, Aw) = 0 or Aw = p . Thus w is a coincidence point and p is a point of coincidence for A and S w. r. t. w. That is Aw = Sw = p . (10) Again from the inclusion (3-a), we see that p = Aw = Tq for some q ∈ X . Since φ is non decreasing, from (4) and (10) we get d ( p, Bq) = d ( Aw, Bq) ≤ φ (d (Sw, Tq), d ( Aw, Sw), d ( Bq, Tq), d ( Aw, Tq), d (Bq, Sw)) = φ (0,0, d ( Bq, p), 0, d ( Bq, p)) ≤ φ (d (Bq, p), d ( Bq, p), d (Bq, p), d (Bq, p), d ( Bq, p)) so that p = Bq in view of Remark 1.2. Thus Aw = Sw = Bq = Tq = p. (11) Now (7) will follow from (b) and (11). Case (d): Suppose that B( X ) is complete. Then p ∈ B( X ) ⊂ S ( X ) , in view of (3-b) and hence (7) follows from Case (c). To establish that p is a common fixed point for all the four maps, we again use (4) with x = r and y = p so that d ( Ar, Bp) ≤ φ (d (Sr,Tp), d ( Ar, Sr ), d (Bp,Tp), d ( Ar ,Tp), d (Bp, Sr )) . Again since φ is non decreasing, this together with (7) and (8) gives d ( p, Bp) ≤ φ (d ( p, Bp),0, 0, d ( p, Bp), d (Bp, p)) ≤ φ (d ( p, Bp), d ( p, Bp), d ( p, Bp), d ( p, Bp), d ( p, Bp)) 4
  • 5. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 so that from Remark 1.2, it follows that Bp = p and hence p is a common fixed point for A, B, S and T, which in fact is a point of their common coincidence. Now suppose that (B,T ) ∈ C* . Then exchanging the roles of (S , A) and (B,T ) ; of (3-a) and (3-b); and of X T (X ) and S (X ) in the above proof, we can similarly obtain the conclusion. Uniqueness of the common fixed point follows easily from the choice of φ and (4).  Remark 2.1 If A is onto then A( X ) = X . Therefore the completeness of X implies the completeness of A(X ) . Similarly the completeness of X implies the completeness of B(X ) whenever B is onto. Remark 2.2 We can prove that if self-maps A, B, S and T on X satisfy the inclusions (3-a), (3-b) and the inequality (4), and X is complete, then both ( A, S ) ∈ C* and (B,T ) ∈ C* . X X ∞ In fact for any x0 ∈ X , the inclusions (3 a-b) generate a sequence of points xn n =1 in X with the choice Ax2n−2 = Tx2n −1 , Bx2n −1 = Sx2n for all n ≥ 1. (12) ∞ ∞ From the proof of Theorem 1.2, it follows that Ax2n n = 1 and Ax2n n = 1 are Cauchy sequences. If X is complete, these will converge to some z ∈ X. That is lim Ax2n−2 = lim Tx2n−1 = lim Bx2n−1 = lim Sx2n = p. (13) n→∞ n→∞ n→∞ n→∞ With x2n = xn and x2n−1 = yn from (13), we see that * * * * lim Axn = lim Sxn = p * * and lim Byn = lim Tyn = p, n→∞ n→∞ n→∞ n→∞ proving that the pairs (S , A) and (B,T ) satisfy the property E. A. Remark 2.3 In view of Remarks 2.1 and 2.2, we see that Theorem 1.2 follows as a particular case of our result (Theorem 2.1). It is possible to relax the condition (b), and drop the inclusions in Theorem 2.1 for three self-maps A, B and T. In deed we prove the following Theorem 2.2 Let A, B and T be self-maps on X satisfying the inequality d ( Ax, By) ≤ φ(d (Tx, Ty), d ( Ax, Tx), d ( By,Ty), d ( Ax,Ty), d (By, Tx)) for all x, y ∈ X . (14) Suppose that T ( X ) is complete subspace of X and (f) either of the pairs ( A,T ) and (B,T ) is both (weakly compatible and belongs to the class C* ). X Then A, B and T will have a unique common fixed point. ∞ Proof. Suppose that the pair ( A,T ) ∈ C* . Then there exists a sequence X xn n =1 ⊂ X such that lim Axn = lim Txn = p for some p ∈ X. (15) n→∞ n→∞ Let q = lim Bxn . Then q = p. In fact, writing x = y = xn in the inequality (14), n→∞ d ( Axn , Bxn ) ≤ φ ( d (Txn ,Txn ), d ( Axn ,Txn ), d (Bxn ,Txn ), d ( Axn, Txn ), d (Bxn, Txn )) Applying the limit as n → ∞ in this and using (15) and upper semi-continuity of φ, d ( p, q) ≤ φ (0,0, d (q, p),0, d (q, p)) ≤ φ (d ( p, q), d ( p, q), d ( p, q), d ( p, q), d ( p, q)) so that d ( p, q) = 0 or p = q , in view of Remark 1.2. Thus lim Axn = lim Txn = lim Bxn = p. (16) n→∞ n→∞ n→∞ 5
  • 6. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 Suppose that T ( X ) is complete. Then p ∈ T ( X ) so that Tu = p for some u ∈ X. But then from (14), d ( Axn , Bu) ≤ φ (d (Txn ,Tu), d ( Axn , Txn ), d (Bu, Tu), d ( Axn ,Tu), d ( Bu,Txn )) . Applying the limit as n → ∞ in this, and using Tu = p and upper semi-continuity of φ followed by its non decreasing nature in each coordinate variable, we get d ( p, Bu) ≤ φ (0,0, d ( Bu, p),0, d ( Bu, p)) ≤ φ (d (Bu, p), d (Bu, p), d (Bu, p), d (Bu, p), d (Bu, p)) or d ( p, Bu) = 0 or Bu = p . Thus u is a coincidence point of B and T. That is Bu = Tu = p . Since φ is non decreasing, from (14), we get d ( Au, Bu ) ≤ φ (d (Tu,Tu ), d ( Au, Su ), d ( Bu,Tu), d ( Au,Tu ), d ( Bu,Tu)) = φ (0, d ( Au, p), 0), d ( Au, p), 0) ≤ φ (d ( Au, p), d ( Au, p), d ( Au, p), d ( Au, p), d ( Au, p)) so that Au = Tu in view of Remark 1.2. Thus Au = Bu = Tu = p . (17) Since (A, T) is weakly compatible pair, from (17) we get that Ap = Tp . Again since φ is non decreasing, from (14) and using Ap = Tp , we get d ( Ap, Bp) ≤ φ (d (Tp, Tp), d ( Ap,Tp), d (Bp, Tp), d ( Ap,Tp), d (Bp, Tp)) = φ (0,0, d ( Ap, Bp), 0, d ( Ap, Bp)) ≤ φ (d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp), d ( Ap, Bp)) so that Ap = Bp in view of Remark 1.2. Thus Ap = Bp = Tp . (18) Finally p will be a common fixed point for the three maps. For, writing x = u and y = p in (14) and then using (17) and (18) we get d ( p, Tp) = d ( Au, Bp) ≤ φ (d (Tu, Tp), d ( Au, Tp), d (Bp, Tp), d ( Au, Tp), d ( Bp, Tu)) = φ (d ( p, Tp), d ( p, Tp), 0, d ( p, Tp), d (Tp, p)) ≤ φ (d ( p, Tp), d ( p, Tp), d ( p, Tp), d (Tp, p), d ( p, Tp)) Again since φ is non decreasing, this with Remark 1.2, implies that Bp = p and hence p is a common fixed point for A, B, and T. Similarly if ( A,T ) is a weakly compatible pair satisfying the property E.A., it follows that A, B, and T have a common fixed point. Uniqueness of the common fixed point follows easily from the choice of φ and (14).  Taking A = B in Theorem 2.2 we have Corollary 2.1 Let A and T be self-maps on X satisfying the inequality d ( Ax, Ay) ≤ φ(d (Tx, Ty), d ( Ax,Tx), d ( Ay,Ty ), d ( Ax, Ay), d ( Ay,Tx)) for all x, y ∈ X . (19) If T ( X ) is complete subspace of X, ( A,T ) is weakly compatible and ( A,T ) ∈ C* , then A and T will have a unique X common fixed point. Given x0 ∈ X and self-maps A and T on X, if there exist points x1, x2, x3,... in X with Axn −1 = Txn for n ≥ 1 , the ∞ sequence Axn n =1 defines a ( A,T ) -orbit or simply an orbit at x0. The space X is ( A,T ) -orbitally complete [13] at x0 ∈ X if every Cauchy sequence in some orbit at x0 converges in X . Note that every complete metric space is orbitally complete at each of its points. However there are incomplete metric spaces which are orbitally complete at some point of it [13]. 6
  • 7. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 We give the following generalization of Corollary 2.1, the proof of which can be obtained on similar lines of that of Theorem 2.2. Theorem 2.3 Let A and T be self-maps on X satisfying the inequality (14) and ( A,T ) ∈ C* . Suppose that any one of X the following conditions holds good: (g) the subspace A( X ) is orbitally complete at x0 ∈ X (h) T ( X ) is orbitally complete at x0 ∈ X . Then there is a coincidence point z for A and T. Further if A and T are weakly compatible, then the point of coincidence of A and T w. r. t. z will be a unique common fixed point for them. In the remainder of the paper ψ : IR+ → IR+ represents a contractive modulus, due to Solomon Leader [19] with the choice ψ(t ) < t for t > 0. Corollary 2.2 (Theorem 2, [13]) Let A and T be self-maps on X satisfying the inclusion A( X ) ⊂ T ( X ) (20) and the inequality d ( Ax, Ay) ≤ ψ(max d (Tx, Ty), d ( Ax, Tx), d ( Ay, Ty) 1 [ d ( Ax, Ty) + d ( Ay, Tx)]) for all x, y ∈ X , 2 (21) where ψ is non decreasing and upper semi continuous contractive modulus. Suppose that one of the conditions (g), (h) and (i) holds good, where (i) the space X is orbitally complete at some x0 ∈ X and T is onto Then there is a coincidence point z for A and T. Further if A and T are weakly compatible, then the point of coincidence of A and T w. r. t. z will be a unique common fixed point for them. Proof. We note that the condition (i) implies (g). We set φ(ξ1, ξ2, ξ3, ξ4, ξ5) = ψ(max{ξ1, ξ2, ξ3, 1 [ξ4 + ξ5]}) for all ξi , i = 1,2, ..., 5 . 2 Then (14) reduces to (21). Let x0 ∈ X be arbitrary. By virtue of the inclusion (20), we can construct an ( A,T ) - orbit x0 with choice ∞ Axn −1 = Txn for n ≥ 1 . Then the sequence Axn n =1 is a Cauchy sequence in this orbit at X. In view of Remark 2.2, A and T satisfy the property E. A. Hence the conclusion follows from Theorem 2.3. Now write B = A in Theorem 2.1, we get Theorem 2.4 Let A, S and T be self-maps on X satisfying the inclusion A( X ) ⊂ [S ( X ) I T ( X )] (22) and the inequality d ( Ax, Ay) ≤ φ(d (Sx,Ty), d ( Ax, Sx), d ( Ay, Ty), d ( Ax,Ty), d ( Ay, Sx)) for all x, y ∈ X . (23) Suppose that 7
  • 8. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 (j) either of the pairs ( A, S ) and ( A,T ) satisfies the property E. A. (k) one of A( X ), B( X ) and T ( X ) is a compete subspace of X. If ( A, S ) and ( A,T ) are weakly compatible, then A, B, S and T will have a unique common fixed point. Since every compatible pair is weakly compatible, we have the following sufficiency part of Main theorem of [15]. Corollary 2.3 Let A, S and T be self-maps on X satisfying (20) and the inequality d ( Ax, Ay) ≤ max{d (Sx,Ty), d ( Ax, Sx), d ( Ay,Ty), 1 [d ( Ax,Ty) + d ( Ay, Sx)]} 2 − ω(max{d (Sx,Ty), d ( Ax, Sx), d ( Ay,Ty), 1 [d ( Ax,Ty) + d ( Ay, Sx)]}) for all x, y ∈ X , 2 (24) where ω : IR+ → IR+ is continuous and 0 < ω(t ) < t for t > 0 . Suppose that X is complete, A is continuous and ( A, S ) and ( A,T ) are compatible. Then A, S and T will have a unique common fixed point. Note that (23) reduces to (24) with   φ(ξ1, ξ2 , ξ3, ξ4 , ξ5 ) = maxξ1, ξ2 , ξ3, [ξ4 + ξ5 ] − ω maxξ1, ξ2 , ξ3, [ξ4 + ξ5 ]  for all ξi , i = 1,2, ..., 5 . 1 1      2    2  ∞ Given x0 ∈ X , due to the inclusion (22), we can construct the sequence Axn n =1 with choice Ax2n − 2 = Tx2n−1, Ax2n −1 = Tx2n for n ≥ 1 , which is a Cauchy sequence in the complete space X and hence converges in it. In view of Remark 2.2, A and T satisfy the property E. A. References [1] Aamri M. A. and Moutawakil D. El. (2002). Some new common fixed point theorems under strict contractive conditions, J. Math. Anal. Appl., 270: 181-188 [2] Brian Fisher (1979). Mappings with a common fixed point, Math. Sem. Notes, 7, 81 – 83 [3] Brian Fisher (1983). Common fixed points of four mappings, Bull. Inst. Math. Acad. Sinica, 11, 103 – 113 [4] Brian Fisher and Khan M. S. (1982). Some fixed point theorems for commuting mappings, Math. Nachr., 108, 323 – 326 [5] Chang S. S. (1981). A common fixed point theorem for commuting maps, Proc. Amer. Math. Soc. 83, 645 – 652 [6] Ciric Lj. B. (1974). A Generalization of Banach Contraction Principle, Proc. Amer. Math. Soc. 45 (2), 267-27 [7] Das K. M. and Naik V. (1981). Common fixed point theorems for commuting maps on metric space, Proc. Amer. Math. Soc. 83, 645 – 652 [8] Gerald Jungck (1976). Commuting mappings and fixed points, Amer. Math. Monthly, 83, 261 – 263 [9] Gerald Jungck (1986). Compatible maps and common fixed points, Int. J. Math. & Math. Sci., 9, 771-779 [10] Gerald Jungck (1996) Common fixed points for non continuous and non self-mappings on a metric space, Far East J. Math. Sci. 4 (2), 199-212 [11] Pant R. P. (1986). Common fixed points of two pairs of commuting mappings, Indian J. Pure Appl. Math. Sci. 17 (1986), 187 – 192 8
  • 9. Mathematical Theory and Modeling www.iiste.org ISSN 2224-5804 (Paper) ISSN 2225-0522 (Online) Vol.2, No.6, 2012 [12] Pathak H. K., Lopez R. R. and Verma R. K. (2007). A common fixed point theorem using implicit relation and Property E. A. in Metric Spaces, Filomat 21 (2), 211-234 [13] Phaneendra T. (2004). Coincidence points of two weakly compatible self-maps and common fixed point theorem through orbits, Indian J. Math. 46 (2-3), 173-180 [14] Renu Chugh and Sanjay Kumar(2001). Common fixed points for weakly compatible maps, Proc. Indian Acad. Sci. (Math. Sci.) 111(2), 241-247. [15] Rhoades B. E., Kalishankar Tiwary and Singh G. N. (1995). A common fixed point theorem for compatible mappings, Indian J. pure appl. Math. 26 (5), 403-409 [16] Sastry K. P. R., Ismail S. and Murthy I. S. R. K. (2004). Common fixed points for two self-maps under strongly partially commuting condition, Bull. Cal. Math. Soc. 96 (1), 1-10 [17] Sessa S. (1982). On a weak commutativity condition of mappings in fixed point considerations, Publ. Inst. Math. (Beograd), 32 (46), 149-153 [18] Singh S. L. and Singh S. P. (1980). A fixed point theorem, Indian J. Pure Appl. Math. 11(12), 1584-1586 [19] Solomon Leader (1977). Fixed Points Operators on Metric Spaces with Conditional Uniform Equivalence of Orbits, J. Math. Anal. Appl., 61, 466-474 9
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