Scholarly article on topic 'Fixed point-type results for a class of extended cyclic self-mappings under three general weak contractive conditions of rational type'

Fixed point-type results for a class of extended cyclic self-mappings under three general weak contractive conditions of rational type Academic research paper on "Mathematics"

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Academic research paper on topic "Fixed point-type results for a class of extended cyclic self-mappings under three general weak contractive conditions of rational type"

O Fixed Point Theory and Applications

a SpringerOpen Journal

RESEARCH Open Access

Fixed point-type results for a class of extended cyclic self-mappings under three general weak contractive conditions of rational type

Manuel De la Sen1* and Ravi P Agarwal2

* Correspondence: wepdepam@lg.

instituto de Investigación y Desarrollo de Procesos, Universidad delPais Vasco, Campus of Leioa (Bizkaia) - Aptdo. 644-Bilbao, 48080-Bilbao, Spain Fulllist of author information is available at the end of the article

Abstract

This article discusses three weak ^-contractive conditions of rational type for a class of 2-cyclic self-mappings defined on the union of two non-empty subsets of a metric space to itself. If the space is uniformly convex and the subsets are non-empty, closed, and convex, then the iterates of points obtained through the self-mapping converge to unique best proximity points in each of the subsets.

1. Introduction

A general contractive condition has been proposed in [1,2] for mappings on a partially ordered metric space. Some results about the existence of a fixed point and then its uniqueness under supplementary conditions are proved in those articles. The rational contractive condition proposed in [3] includes as particular cases several of the previously proposed ones [1,4-12], including Banach principle [5] and Kannan fixed point theorems [4,8,9,11]. The rational contractive conditions of [1,2] are applicable only on distinct points of the considered metric spaces. In particular, the fixed point theory for Kannan mappings is extended in [4] by the use of a non-increasing function affecting the contractive condition and the best constant to ensure a fixed point is also obtained. Three fixed point theorems which extended the fixed point theory for Kannan mappings were stated and proved in [11]. More attention has been paid to the investigation of standard contractive and Meir-Keeler-type contractive 2-cyclic self-mappings T-A U B ® A UB defined on subsets A,B £ X and, in general, /»-cyclic self-mappings

T : U(ep Ai ^ Uiep Ai defined on any number of subsets A, c X, i e p :={1,2.....p}

where (X,d) is a metric space (see, for instance [13-22]). More recent investigation about cyclic self-mappings is being devoted to its characterization in partially ordered spaces and also to the formal extension of the contractive condition through the use of more general strictly increasing functions of the distance between adjacent subsets. In particular, the uniqueness of the best proximity points to which all the sequences of iterates of composed self-mappings T2: AUB ® A U B converge is proven in [14] for the extension of the contractive principle for cyclic self-mappings in uniformly convex Banach spaces (then being strictly convex and reflexive [23]) if the subsets A,B c X in the metric space (X,d), or in the Banach space (X,|| ||), where the 2-cyclic self-mappings are defined, are both non-empty, convex and closed. The research in [14] is

Springer

© 2011 De la Sen and Agarwal; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

centred on the case of the cyclic self-mapping being defined on the union of two subsets of the metric space. Those results are extended in [15] for Meir-Keeler cyclic contraction maps and, in general, for the self-mapping T : (Jiep Ai ^ (Jiep Ai be a p (> 2) -cyclic self-mapping being defined on any number of subsets of the metric space with p := {1,2..., p}. Also, the concept of best proximity points of (in general) non-self-mappings S,T:A ® B relating non-empty subsets of metric spaces in the case that such maps do not have common fixed points has recently been investigated in [24,25]. Such an approach is extended in [26] to a mapping structure being referred to as K-cyclic mapping with contractive constant k < 1/2. In [27], the basic properties of cyclic self-mappings under a rational-type of contractive condition weighted by point-to-point-dependent continuous functions are investigated. On the other hand, some extensions of Krasnoselskii-type theorems and general rational contractive conditions to cyclic self-mappings have recently been given in [28,29] while the study of stability through fixed point theory of Caputo linear fractional systems has been provided in [30]. Finally, promising results are being obtained concerning fixed point theory for multivalued maps (see, for instance [31-33]).

This manuscript is devoted to the investigation of several modifications of rational type of the ^-contractive condition of [21,22] for a class of 2-cyclic self-mappings on non-empty convex and closed subsets A,B c X. The contractive modification is of rational type and includes the nondecreasing function associated with the ^-contrac-tions. The existence and uniqueness of two best proximity points, one in each of the subsets A,B c X, of 2-cyclic self-mappings T: Au B ® A U B defined on the union of two non-empty, closed, and convex subsets of a uniformly convex Banach spaces, is proven. The convergence of the sequences of iterates through T: Au B ® A U B to one of such best proximity points is also proven. In the case that A and B intersect, both the best proximity points coincide with the unique fixed point in the intersection of both the sets.

2. Basic properties of some modified constraints of 2-cyclic ^-contractions

Let (X,d) be a metric space and consider two non-empty subsets A and B of X. Let T: AU B ® A U B be a 2-cyclic self-mapping, i.e., T(A) £ B and T(B) £ A. Suppose, in addition, that T: Au B ® A U B is a 2-cyclic modified weak ^-contraction (see [21,22]) for some non-decreasing function ^:R0+ ® R0+ subject to the rational modified ^-contractive constraint:

d (.v, T.v) d (y, Ty) I d (a; T.v) d (y, Ty)

d{TX'T},)^al d(x,y) d(x,y)

Vx, y (=x) e A U B

(d(x y) - <p (d(x, y))) + V(D); (21)

D := dist (A, B) := inf {d(x, y) : x e A, y e B} (2.2)

D < Um sup d (T"+1.v, Tnx) < Um (hnd Ix, Tx) + ——tl^l—^ m)] = ip ID); V.v € AUB (2.3)

n^rn 1 - k )

Note that (2.1) is, in particular, a so-called 2-cyclic ^-contraction if a = 0 and ^>(i) = (1-a)t for some real constant a e [0,1) since ^:R0+ ® R0+ is strictly increasing [1]. We refer to "modified weak ^-contraction" for (2.1) in the particular case a > 0, b > 0, a + b < 1,

and ^:R0+ ® R0+ being non-decreasing as counterpart to the term ^-contraction (or via an abuse of terminology "modified strong ^-contraction") for the case of ^:R0+ ® r0+ in (2.1) being strictly increasing. There are important background results on the properties of weak contractive mappings (see, for instance, [1,2,34] and references therein). The so-called "^-contraction", [1,2], involves the particular contractive condition obtained from (2.1) with a = 0, b = 1, and ^:R0+ ® R0+ being strictly increasing, that is,d (Tx, Ty) < d (x, y) — y (d (x, y)) + y (D); Wx e A U B,

In the following, we refer to 2-cyclic self-maps TA U B ® A UB simply as cyclic self-maps. The following result holds:

Lemma 2.1. Assume that TA U B ® A UB is a modified weak ^-contraction, that is, a cyclic self-map satisfying the contractive condition (2.1) subject to the constraints min (a, b) > 0 and a + b <1 with ^:R0+ ® R0+ being non-decreasing. Then, the following properties hold:

(i) Assume that ^(D) > D

D < d (Tn+1x, Tnx) < kd (Tx,x) + (1 - k) y(D); Wn e N0 := N U {0}, Wx e A U B (2.4) D < lim inf d (Tn+m+1 x, Tn+mx) < lim sup d (Tn+m+1x, Tn+mx) < y(D); Wx e AUB Wm e N0 (2.5)

n—TO n—TO

and limsup d(Tn+m+1 x Tn+mx) < y(D) if d * 0 If ^(D) = D = 0 then

n—TO

3 lim d (Tn+m+1 x, Tn+mx) = 0; Vx e A U B, Vm e N0.

n—>TO v '

(ii) Assume that d(x,Tx) < m(x) for any given xe A U B. Then

, ( n ) km (x) 1 — k ^

d (T"x, x) < --r + (D); Vx e A U B, Wn e N (2.6)

1 - k k

If d(x,Tx) is finite and, in particular, if x and Tx in A U B are finite then the sequences {T"x} nen0 and {Tn+1 x} neno are bounded sequences where T" e A and Tn+1x e B if x e A and n is even, T" e B and T"+1 e B if x e B and n is even.

Proof: Take y = Tx so that Ty = T2x. Since ^:R0+ ® R0+ is non-decreasing ^(x) > ^(D) for x > D, one gets for any x e A and any Tx e B or for any x e B and any Tx e A:

(1 — a) d (T2x, Tx) < —ay (d (Tx, T2xj) + 3 [d (x, Tx) — y (d (x, Tx))] + y (D) = 3d (x, Tx) + y (D) — ay (d (Tx, T2xj) — 3y (d (x, Tx)); Wx e A U B (2 7)

/ n ) 1 _ a — 3

d (T x, Tx) < kd (x, Tx) +--<p (D) = kd (x, Tx) + (1 - k) cp (D); Wx e AUB

1 — a

if Tx * x where k :=- < 1, since T:AU B —> A U B is cyclic, d{x,Tx) > D and <j>:

1 — a

R0+ ® R0+ is increasing. Then

d (Tn+1 x, Tnx) < knd (x, Tx) + (1 — kn) y(D); Wx e A U B; Wn e N (2 . 8)

F(D) > D ^ 0 since min (a, b) > 0 and a + b <1 Proceeding recursively from (2.8), one gets for any m e n:

D < d (Tn+1x, Tnx) < knd (Tx, x) + y(D)(1 — k) ^ k^ < kd (Tx, x) + y (D) (1 — k") (2 . 9a)

< kd (Tx, x) + (1 - k)p(D) < kd (Tx, x) + p(D) < d (Tx, x) + p (D); Vx e A U B (2.9b)

(/n+m—1 \ \

kn+md (Tx, x) + p(D)(1 - k)( U)\

\ ™ '' (2.10)

1 - kn+m

< (p (D) (1 — k) lim -— = <p (D); Vx e A U B

n—TO ^ 1 — k J

F(D) > D * 0 and if F(D) = D = 0 then the 3 lim d (Tn+m+1 x, Tn+mx) =0; Vx e A U

n—TO x '

B. Hence, Property (i) follows from (2.9) and (2.10) since 0(D) > D and d(x,Tx) > D; Vx e A U B, since T:AU B ® A U B is a 2-cyclic self-mapping and 0:R0+ ® R0+ is non-decreasing. Now, it follows from triangle inequality for distances and (2.9a) that:

d (Tnx, x) < 1 d (Ti+1x, Tix) < (j^—1 k')d (x, Tx) + p (D) (j^— (1 — kf))

k (1 — kn—1) f^n—1 x , k (1 — kn—1) (1 — k) (1 — (1 — k)n—1) /„UN

£ \_h d(x,Tx) + qnD)(2_,il H-'-'j £ ,, 'dix,Tx) +-5---'-tpiD) (2.11)

k 1 - k

£ --d (x, Tx) + ——ip (D) < cc, Vx t A U B, Vn t N

1 — k k

which leads directly to Property (ii) with {T11x} neN0 {Tn+1 x} neNo sequences for any finite x e A U B. □

Concerning the case that A and B intersect, we have the following existence and uniqueness result of fixed points: Theorem 2.2. If 0(D) = D = 0 (i.e., A°nB° * 0) then 3 lim d (Tn+m+1 x, Tn+mx) = 0

T n—TO x '

kd (x Tx)

and d(T"x,x) < - ; Vx e AuB. Furthermore, if (X,d) is complete and A and B are

1 — k

non-empty closed and convex then there is a unique fixed point z e AnB of T:AuB ® AuB to which all the sequences {Tnx} ne_N„ which are Cauchy sequences, converge; Vx e AUB.

Proof: It follows from Lemma 2.1(i)-(ii) for 0(D) = D = 0 It also follows that

lim d (Tn+m+1x, Tn+mx) = lim (kn) d (Tm+2x, Tm+1 x) = 0; Vx e AUB, Vm e n0 what

n—TO ' n—TO V ' 0

implies n Im d (Tn+m+1 x, Tn+mx) =0 so that {T"x} neN0 is a Cauchy sequence, Vx e AUB, then being bounded and also convergent in AnB as n ® since (X,d) is complete and A and B are non-empty, closed, and convex. Thus, ,}imJTnx =z e A ^B and

z = lim T"+1 x = T ( lim T"+1 x) = Tz, since the iterate composed self-mapping T'AB ®

n—>TO \n—>TO '

AUB, Vn e N0 is continuous for any initial point x e AUB (since it is contractive, then Lipschitz continuous in view of (2.9a) with associate Lipschitz constant 0 < k <1 for 0(D) = D = 0). Thus, z e AnB is a fixed point of T:AUB ® AUB. Its uniqueness is proven by contradiction. Assume that there are two distinct fixed points z and y of T:AUB ® AUB in AnB. Then, one gets from (2.1) that either 0 <d(Tz,Ty) < b(d(z,y)-0(d(z,y))) < Mz.y) <d (z>y) or d(Tz,Ty) = d(z,y) = 0 what contradicts d(z,y) > 0 since z * y. Then, d(Tz,Ty) < b(d (x,y) < j8d(x,y) <d(z>y) what leads to the contradiction lim^ d (Tnz, Tny) = 0 = d(z, y) > 0. Thus, z = y. Hence, the theorem. □ Now, the contractive condition (2.1) is modified as follows:

d (Tx, Ty) < a0

d (x, Tx) d (y, Ty) I d (x, Tx) d (y, Ty)

d x , y d x , y

+P0 (d (x, ^ — <p (d (x, ^)+<p (D) (2.12)

for x) e X, where min (a0, bo) ^ 0, min (a0, bo) > 0, and a0 + bo < 1- Note that in the former contractive condition (2-1), a + b <1- Thus, for any non-negative real constants a < a0 and b < b0, (2-12) can be rewritten as

d (Tx, Ty ) < a

+ (ao - a)

d (x, Tx) d (y, Ty) / d (x, Tx) d (y, Ty)

d (л:, у) ' '' \ d (л:, у)

d (x, Tx) d y, Ty d (x, Tx) d y, Ty

, / (л; у) ' '' \ d (л:, у)

+ e (d(x, y) - v (d(x, y))) + <p(D)

(2. 13)

+ (в0 - в) (d (x, y) - v (d (x, y))) ; Vx, y e A U B.

The following two results extend Lemma 2.1 and Theorem 2.2 by using constants a0 and bo in (2.1) whose sum can equalize unity a0+bo = 1.

Lemma 2.3. Assume that T:AuB ® AuB is a cyclic self-map satisfying the contractive condition (2.13) with min(a0, b0) > 0, a0 + b0 < 1, and ^:R0+ ® R0+ is non-decreasing. Assume also that

<p (d(Tx,x))>d(Tx,x)--M0; Ух e AU В (2.14)

1 - a - в

For some non-negative real constants M0 < -—-——D, a < a0 and ¡3 < bo with a

+ b <1. Then, the following properties hold:

(i) D < lim sup d (Tn+m+1 x, Tn+mx) < v(D) + (a0 + в0 - a - в) D; Vx e A U B, Vm e N0 (2 . 15)

for any arbitrarily small e e R+.

(ii) If ф(р) = (1+a+b-a0-b0)D then 3 lim^ d(Tn+m+1x, Tn+mx) = D; Vx e AuB, Vm e N0.

(iii) If d(x,Tx) is finite and, in particular, if x and Tx are finite then the sequence {Tnx} neNo and {Tn+1 x} neNo are bounded sequences, where T"x e A and T"+1x e B if x e A and n is even and T"x e B and T"+1x e B if x e A and n is even.

Proof: Since ф^0+ ® R0+ is non-decreasing then ф^) > ф(Д) for x(e R0+) > D. Note

also that M0 < -—-——D implies the necessary condition ф{с1{Тх,х)) > 0 and (2.14) 1-a

implies that 0 < ф(О) < D. Note also for y = Tx and Ty = Tx2 and (2.14), since ф^) >ф (D) for x >D, that for x e AuB, one gets from (2.14):

1 — a

<p (d (T2x, Tx)) > d (T2x, Tx) - ---Mo) Ухе AUB (2.16)

leading from (2.14) to

(a0 - a) [d (T2x, Tx) - ф (d (T2x, T^] + (fi0 - в) (d (Tx, x) - ф (d (Tx, x)))

1 - a (2 . 17)

< M: = (a0 + fto - a - P) ---Mo

1 - a - в

and M < (a0 + p0-a-P)D since M0 < 1 ~ g ~ One gets from (2.13) and (2.17)

the following modifications of (2.9) and (2.10) by taking y = Tx, Ty = T2x, and successive iterates by composition of the self-mapping TA u B ® A uB:

n+m-1 \ \

(2.20)

D < d (Tn+1x, Tnx) < knd (Tx, x) + {<p{D) + M) (1 - k)( J^k) < knd (Tx, x) + (l - kn) {<p{D) + M)

\i=o ' (218)

< kd (Tx, x) + <(D) + M; Vx s A U B, Vn s No : = N U {0}

D < lim sup d (Tn+1x, Tnx) < < (D)+M < < (D)+(ao + $0 - a - $) D; Vx s AUB, Vm s N0 (2.19)

D < lim supd (Tn+m+1x, Tn+mx)

< lim | kn+md (Tx, x) + (< (D) + (ao + $o - a - $) D)(1 - k) ( V ki

n^w \ \ ^-' , ,

\ \ i=0 / /

< <(D) + (ao + $ o - a - $) D; Vx s A U B, Vm s No

and Property (i) has been proven. Property (ii) follows from (2.20) directly by replacing 0(D) = (1+a+b-a0-b0)D in (2.15). To prove Property (iii), note from (2.18) that

d (Tnx, x) <J2n-1 d (Ti+1x, Tix) n-1 k') d (x, Tx) + (< (D) + M) {j^(1 - k'fj

k (l - kn-1) /„n-1 \ .

< V i_k >d (x, Tx) + (tp (D) + M) (1 "

k (l - kn-1) (1 - k) (l - (1 - k)n-1)

< —-:—J-d (x, Tx) +-^---'- (cp (D)+M)

k , 1 - k

<--d(x, Tx) + —— (<p (D) +M) < oo; Vx eAUB, Vn e N.

1 -k k

Hence, {Tnx} nsNo and {Tn+1 x} nsNo are bounded for any finite x e AuB. Property (iii) has been proven. Hence, the lemma. □ Theorem 2.4. If 0(D) = D = 0 then 3 lim d (Tn+m+1 x, Tn+mx) = o; Vx e AuB.

Furthermore, if (X,d) is complete and both A and B are non-empty, closed, and convex then there is a unique fixed point z e AnB of T:A u B ® A uB to which all the sequences {T"x} nsNo, which are Cauchy sequences, converge; VxeAuB.

Proof gwz'de/z'ne: It is identical to that of Theorem 2.2 by using 0(D) = D = M0 M = 0 and the fact that from (2.17) a0 = a and b0 = b with 0 < a + b < 1 if there is a pair (x,Tx) e A x BuB x A such that d(Tx,x) = 0(d(Tx,x)); d(T2x,Tx) = 0(d(T2x,Tx)); Vx e AuB. Hence, the theorem. □

Remark 2.5. Note that Lemma 2.2 (ii) for 0(D) < D (0(D) <D if a + b < a0 + b0 < 1) leads to an identical result as Lemma 2.1 (i) for 0(D) = D and a + b <1 consisting in proving that 3 Imm d{Tn+m+1 x, Tn+mx) = D. This result is similar to a parallel obtained for standard 2-cyclic contractions [2,5,8]. □

Remark 2.6. Note from (2.7) that Lemma 2.1 is subject to the necessary condition D < 0(D) since d(T2x,Tx) > D and; Vx e AuB. On the other hand, note from Lemma 2.2,

Equation (2.14) that <p (D) > D--M0, and one also gets from (2.18) for n = 1

1 - a - $

the dominant lower-bound <p (D) > D — M > D--Mo (»o + Po — <* — P), that

1 - a - $

is, D < tp (D) +--Mo («o + fio ~ a — fi) which coincides with the parallel con-

1 - a - $

straint obtained from Lemma 2.1 if a0 + b0 = a + b. □

Remark 2.7. Note that Lemmas 2-2 and 2-3 apply for non-decreasing functions ® R0+- The case of ^:R0+ ® R0+ being monotone increasing, then unbounded, is also included as it is the case of ^:R0+ ® R0+ being bounded non-decreasing- □ Now, modify the modified cyclic ^-contractive constraint (2-1) as follows:

d (Tx, Ty) < a ' ^—— + p (d (x, y) — tp (d (x, y))) d(x, y)

/ ( )\ (2 • 21) / d (x, Tx) d (y, Ty) \

+ (l-a)<p[-?—^ + (p (D); Vx e A U B

d x, y

Thus, the following parallel result to Lemmas 2-1 and 2-2 result holds under a more restrictive modified weak ^-contraction Assume that T:A U B ® A UB is modified weak ^-contraction subject to ^:R0+ ® R0+ subject to the constraint

limsup (x — (p (x)) > ———-— and having a finite limit: x—+to 1 - a - p

Lemma 2.8- Assume that TA U B ® A UB is a cyclic self-map satisfying the contractive condition (2-21) with min (a,b) ^ 0, a + b <1, and ^:R0+ ® R0+ is non-decreasing having a finite limit Ji^ V (x) = V and subject to ^(0) = 0 Assume also that ^:R0+ ®

R0+ satisfies hmsup (x — (p (x)) > --. Then, the following properties hold:

x—+to 1 - a — p

(i) The following relations are fulfilled:

1 if (D) < D < d (T"*\x, Tnx) < y(D) < < co; Vn € N, Vx € AUB (2.22)

2 — a — p 1 — a — p 1 — a — p

1 ~ 01 ~ ^ v (D) <D < lim supd (T"+1 x, T"x) < V (D) < 2 ~ " ~ < oo; V.v s AUB (2.23)

2 — a — p n—TO 1 — a — p 1 — a — p

(ii) If, furthermore, ^:R0+ ® R0+ is, in addition, sub-additive and d(x,Tx) is finite (in particular, if x and Tx are finite) then the sequences {Tnx} neN0 and {Tn+1 x} neNo are both bounded, where T"x e A and T"+1x e B if x e A and n is even and T"x e B and Tn+1x e A if x e B and n is even- If ^:R0+ ® R0+ is identically zero then 3 lim d (Tn+m+1 x, Tn+mx) = 0; Vx e AUB-

n—>TO f

Proof: One gets directly from (2-21):

(1 — a) (d (T2x, Tx) — v (d (T2x, Tx))) < pd (T2x, Tx)~p (d (T2x, Tx))+v (D); Vx e AUB (2 • 24)

or, equivalently, one gets for k =- < 1 that

1 — a

d (T2x, Tx) - <p (d (T2x, Tx)) < kd (T2x, Tx) - <p (d (T2x, Tx)) + p^) V.teAUB (2.25) leading to

0 < D — V(D) < lim inf (d (Tn+1x, Tnx) — v (d (Tn+1x, Tnx)))

n—TO

< Urn sup (d (Tn+1x, Tnx) -q>(d (Tn+1x, Tnx))) < -^^-= V{D) ; Vx e A U B

(2 ^6)

what implies the necessary condition <p (D) > -^-D leading to

2 — a — p

D 2 — a — p

>1 if D * 0 and then

p(D) 1 — a — p

lim inf (d (Tn+1 x, Vx) — p (d (Tn+1 x, Tnx))) > D — p (D) > 0. Vx e AuB. Also, since

Iimsup (x — <p (x)) > —^^—. \/x e by construction, then d(T"+1x,T"x) is

x—1 — a — p

bounded; Vn e N since, otherwise, a contradiction to (2.24) holds. Since 0:Ro+ ® R0+ is non-decreasing and has a finite limit p > p (x) > 0; Vx Ro+ (p = 0 if and only if 0:Ro+ ® Ro+ is identically zero), thus p > p (D) > 0. Then, (2.22)-(2.23) hold and Property (i) has been proven. On the other hand, one gets from (2.25), since 0:Ro+ ® r0+ is sub-additive and nondecreasing and has a finite limit, that:

d (Tnx, x) — p (d (Tnx, x)) < n—1 d (Ti+1 x, Tix) — p (d (Ti+1 x, Tix))^j

- (JLm k'){d{x'Tx) {x'Tx))) + (JLM -fe<))

k (1 — kn—1) , p(D) n—1 , \ i

< -(d (x, Tx) -P(d (x, Tx))) + f- V (1 - k) V (2.27)

1 — k 1 — a i=i /

„ Ml-fe"-1)^ (1 — fe) (l — (1 — fe) p (D)

< —--—-d(x,Tx) +-^---1-

1 — k k 1 — a

k , 1 — k p (D)

<-- (d(x,Tx) -p(d(x,Tx))) + <co; VieAUB, VneN

1 — k k 1 — a

lim supd (T"x, x) <-- (d(x,Tx) — ip (d(x,Tx))) + —-——-+w < oo; Vx e AUB (2.28)

n—^ 1 — k k 1 — a

Then the sequences {Tnx} neN0 and {Tn+1 x} neN0 are both bounded for any x e AuB. Hence, the first part of Property (ii). If 0:Ro+ ® R0+ is identically zero then p> = p (x, Tx) = 0; Vx e AuB so that 3 lim^ d(Tn+m+1 x, Tn+mx) = 0 from (2.23). Hence, the lemma. □

The existence and uniqueness of a fixed point in A n B if A and B are non-empty, closed, and convex and (X,d) is complete follows in the subsequent result as its counterpart in Theorem 2.2 modified cyclic 0-contractive constraint (2.21):

Theorem 2.9. if (X,d) is complete and A and B intersect and are non-empty, closed, and convex then there is a unique fixed point z e AnB of T:A u B ® A uB to which all the sequences {Tnx} neN0, which are Cauchy sequences, converge; Vx e AuB. □

Remark 2.7. Note that the nondecreasing function 0:Ro+ ® Ro+ of the contractive condition (2.21) is not monotone increasing under Lemma 2.5 since it possesses a finite limit and it is then bounded. □

Remark 2.8. The case of TA u B ® A uB being a ^-contraction, namely, d(Tx,Ty) < d(x,y)-0(d(x,y)) + 0(D) with strictly increasing 0:Ro+ ® Ro+; Vx e AuB, [1,2] implies, since 0(x) = 0 if and only if x = 0, implies the relation

d (Tx, Ty) < p 1d (x, y) + p(D) < d (x, y) + p(D); Vx, y (= x) e A U B (2.29)

for some real constant 0 < bi = bi(x>y) < 1; Vx,y(^x) e AuB so that proceeding recursively:

d (Tn+1x, Tnx) < ]"[. i [fii]d (Tx, x)+y ■ j FL ■ i [P^) < d (Tx' x)+hv D ' Vx e AUB (2 ■ 30)

D < lim sup d (Tn+1x, T'\x) < Y<P (P); Vx e A U B (2.31)

n^i 1 — ft

where p ■= lim m■ ^ ^ \ < 1 and 3 lim^T^1 x, Tnx) = 0; Vx e AuB if <(D) =

D = 0, and one gets from Lemma 2.1(iii) that {Tnx} neN0 and {Tn+1 x} neNo are Cauchy sequences which converge to a unique fixed point in AnB if A and B are non-empty, closed, and convex and (X,d) is complete [1]. □

Remark 2.9. Note that the constraint (2.1) implies in Lemma 2.1 and Theorem 2.2 that (1-a-b) <(D) < (1-a-b)D what implies <(D) < D if max (a,b) > 0 since 0 < a + b <1. However, such a constraint in Lemma 2.3 and Theorem 3.4 implies that (1-a0-b0) <(D) < (1-ao-bo)D. □

3. Properties for the case that A and B do not intersect

This section considers the contractive conditions (2.1) and (2.21) for the case AnB * 0 For such a case, Lemmas 2.1, 2.3, and 2.8 still hold. However, Theorems 2.2, 2.4, and 2.9 do not further hold since fixed points in AnB cannot exist. Thus, the investigation is centred in the existence of best proximity points. It has been proven in [1] that if T: A u B ® A uB is a cyclic ^-contraction with A and B being weakly closed subsets of a reflexive Banach space (X,|| ||) then, 3(x,y) e A x B such that D = d(x,y) = | |x-y|| where d:R0+ ® R0+ is a norm-induced metric, i.e., x and y are best proximity points. Also, if T:A u B ® A uB is a cyclic contraction 3 (x,y) e A x B such that D = d(x,y) if A is compact and B is approximatively compact with respect to A with both A and B

being subsets of a metric space (X,d) (i.e., if r}imHT2nx, ^ = d(B,^ := infd{z,y) for

some y e A and x e B then the sequence {Tnx} neN0 has a convergent subsequence [14]). Theorem 2.2 extends via Lemma 2.1 as follows for the case when A and B do not intersect, in general:

Theorem 3.1. Assume that TA u B ® A uB is a modified weak ^-contraction, that is, a cyclic self-map satisfying the contractive condition (2.1) subject to the constraints min (a,b) - 0 and a +b <1 with <:R0+ ® R0+ being nondecreasing with <(D) = D. Assume also that A and B are non-empty closed and convex subsets of a uniformly convex Banach space (X,|| ||). Then, there exist two unique best proximity points z e A, y e B of T:A u B ® A uB such that Tz = y, Ty = z to which all the sequences generated by iterations of T:A u B ® A uB converge for any x e AuB as follows. The sequences {T2nx}neNg and {T2n+1 x}neN converge to z and y for all x e A, respectively, to y and z for all x e B. If An B * 0 then z = y e AnB is the unique fixed point of T: A u B ® A uB

Proof: If D = 0, i.e., A and B intersect then this result reduces to Theorem 2.2 with the best proximity points being coincident and equal to the unique fixed point. Consider the case that A and B do not intersect, that is, D > 0 and take x e AuB. Assume with no loss in generality that x e A. It follows, since A and B are non-empty and closed, A is convex and Lemma 3.1 (i) that:

[d (T2n+1x, T2nx) ^ D ; d (T2n+1x, T2n+2x) ^ D] ^ d (r2(n+p)x, T2nx) ^ 0 as n ^ œ (3 . 1)

(proven in Lemma 3.8 [14]). The same conclusion arises if x e B since B is convex. Thus, {T2nx}neNg is bounded [Lemma 2.1 (ii)] and converges to some point z = z(x), being potentially dependently on the initial point x, which is in A if x e A, since A is closed, and in B if x e B since B is closed. Take with no loss in generality the norm-induced metric and consider the associate metric space (X,d) which can be identified with (X, || ||) in this context. It is now proven by contradiction that for every e e R+, there exists n0 e N0 such that d(T2mx,T2n+ x) < D + e for all m >n > n0. Assume the contrary, that is, given some e e R+, there exists n0 e N0 such that d (T2mkx, T2nk+1 x) > D + s for all mk >nk > n0 Vk e N0. Then, by using the triangle inequality for distances:

D + s < d (T2mkx, T2nk+1x) < d (T2mkx, T2mk+2x) + d (T2mk+2x, T2nk+1x) as n ^œ (3 . 2)

One gets from (3.1) and (3.2) that

lim inf (d (T2mkx, T2mk+2x) + d (T2mk+2x, T2nk= liminf d (T2mk+2x, T2nk+1x) > D+s (3 . 3)

k^œ k^œ

Now, one gets from (3.1), (3.3), ^(D) > D, and Lemma 2.1 (i) the following contradiction:

D + s < limsup d(T2mk+2x, T2nkt1 x) < limsupd (T2"k+2x, T2nkt1x) + limsup d (T2mk+2x, T2nk+2x)

k^œ ni^œ k^œ

= limsup d (T2nk+2x, T2nk+1x) = D

As a result, d(T2mx,T2n+1x) < D + s for every given s e R+ and all m >n > n0 for some existing n0 e N0. This leads by a choice of arbitrarily small s to

D < lim sup d (T2mx, T2n+1 x) < D ^ 3lim d (T2mx, T2n+1 x) = D

(3 .5)

But {T2n

"MneNo is a Cauchy sequence with a limit z = T2z in A (respectively, with a limit y = T2y in B) if x e A (respectively, if x e B) such that D = ||Tz-z|| = d(z,Tz) (Proposition 3.2 [14]). Assume on the contrary that x e A and {T2nx} neN0 ^ z = T2z as n ® ^ so that T2z-Tz = z-Tz * z-y so that since A is convex and (X,|| ||) is uniformly convex Banach space, then strictly convex, one has

/T2z + z \ D = d(z,Tz) = d I—---Tzj =

T2z — Tz z — Tz -+-

T2z — Tz

z — Tz

D D < - + - = D 2 2

(3 .6)

which is a contradiction so that z = T2z is a best approximation point in A of TA U B ® A UB. In the same way, {T2nx}neNg is a Cauchy sequence with a limit T2y = y e B which is a best approximation point in B of TA U B ® A UB if x e B since B is convex and (X,|| ||) is strictly convex. We prove now that y = Tz. Assume, on the contrary that y * Tz with y = T2y,Tz = T3z e B, z = T2z e A, d(z,y) >D, d(Tz,Ty) > D, d(Tz,z) = d(Ty,y) = D, and ^(D) = D. One gets from (2.1) since R0+ ® R0+ is non-decreasing the following contradiction:

D < d (z, y) = d (T2z, T2y) < ,

d (T2z, Tz) d (T2y, Ty) ( d (T2z, Tz) d (T2y, Ty) ■ — ip

d Tz, Ty

d Tz, Ty

(3 .7)

(d (Tz, Ty) — pp (d (Tz, Ty))) + D (a + ft) D + (1 — a — fi) D = D

Thus, z = Ty = T2z = Xy and y = Tz = X^y = Xz are the best proximity points of XX: A U B ® A UB in A and B. Finally, we prove that the best proximity points z e A and y e B are unique. Assume that zi(* z2) e A are two distinct best proximity points of TA U B ® A UB in A. Thus, Tz1(^ Xz2) e B are two distinct best proximity points in B. Otherwise, Tz1 = Xz2 ^ T2z1 = T2z2 ^ z1 = z2, since z1 and z1 are best proximity points, contradicts z1 * z2. One gets from Lemma 2.1(i) and d(Tz1,T2z2) = d(Tz2,T2z1) = d(z1,Tz2) = d(z2,Tz1) = D. Through a similar argument to that concluding with (3.6) with the convexity of A and the strict convexity of (X,|| ||), guaranteed by its uniform convexity, one gets the contradiction:

T2zi - Tzi

Z2 - TZ2

< - + - = D (3.8)

...... 2 2

since Xaz1-Tz1 ^ Tz1-z1. Thus, z1 is the unique best proximity point in A while Tz1 is the unique best proximity point in B. □

In a similar way, Theorem 2.4 extends via Lemma 2.3 as follows from the modification (2.12) of the contractive condition (2.1): Theorem 3.2. Assume the following hypotheses:

(1) TA U B ® A UB is a modified weak ^-contraction, that is, a cyclic self-map satisfying the contractive condition (2.12) subject to the constraints mi« (ao,bo) ^ 0, mi« (ao,bo) > 0, and ao + bo ^ 1.

(2) d>: R0+ —> R0+ is non-decreasing subject to (p (d(Tx,x)) > d(Tx,x)---

1 — a — p

Vx e AuB and 0(D) = (1 + a+b-a0-b0)D for some non-negative real constants

M0 < -—-——D, 0 < a < a0 and 0 < ¡3 < ¡30 with a + ¡3 < 1. 1—a

(3) A and B are non-empty closed and convex subsets of a uniformly convex Banach space (X,|| ||).

Then, there exist two unique best proximity points z e A, y e B of T:A u B ® A uB such that Tz = y, Ty = z to which all the sequences generated by iterations of T:A u B ® A uB converge for any x e AuB as follows. The sequences {T2nx}neNo and {T2n+1 x}neN converge to z and y for all x e A, respectively, to y and z for all x e B If A nB * 0 then z = y e AnB is the unique fixed point of T:A u B —> A uB.

Owi/ine of />roof It is similar to that of Theorem 3.1 since (3.1) to (3.3) still hold, (3.4) and (3.5) still hold as well from Lemma 2.3(ii) as well as the results from the contradictions (3.6)-(3.8). □

The following result may be proven using identical arguments to those used in the proof of Theorem 3.1 by using Lemma 2.8 starting with its proven convergence property (2.23) for distances:

Theorem 3.3. Assume that T:A u B — A uB is a cyclic self-map satisfying the contractive condition (2.21) with min (a,b) ^ 0, a + b <1, and 0: Ro+ — Ro+ is non-decreasing having a finite limit 0 (x) = 0 and subject to 0(0) = 0 Assume also that

0: R0+ —> R0+ satisfies limsup (x — <p (x)) > -. Finally, assume that A and B are

x—+TO 1 — a — p

non-empty closed and convex subsets of a uniformly convex Banach space (X,|| ||). Then, there exist two unique best proximity points z e A, y e B of T:A u B — A uB

such that Tz = y, Ty = z to which all the sequences generated by iterations of TA U B ® A UB converge for any x e AuB as follows. The sequences {T2nx}neNg and {T2n+1 x}neN converge to z and y for all x e A, respectively, to y and z for all x e B. If AnB * 0 then z = y e AnB is the unique fixed point of TA U B ® A UB. □

Example 3.4. The first contractive condition (2.1) is equivalent to

d (Tx, T2x) < {Pd Tx) + <P(D)~ a<p (d (Tx, T2x)) - p<p (d (x, Tx))). (3.9)

To fix ideas, we first consider the trivial particular case ^(x) ^ 0 (^ ^(D) = 0); Vx e R0 +. This figures out that TA U B ® A UB is a strict contraction if AnB is non-empty and closed, min (a,b) > 0, and a + b <1. Then, it is known from the contraction principle that there is a unique fixed point in AnB. Note that in this case R0+ ® 0. If a + b =1 then TA U B ® A UB is non-expansive fulfilling d(Tp+1x,Tpx) = d(x,Tx); Vx e AUB, Vp e z0+. The convergence to fixed points cannot be proven. It is of interest to see if TA U B ® A UB being a weak contraction with R0+ ® R0+ being non-decreasing guarantees the convergence to a fixed point if a + b = 1 and ^(0) = D = 0 according to the modified contractive condition (2.12). In this case, if ^(x) >0; Vx e R+ then convergence to a fixed point is still potentially achievable since

d (Tx, T2x) < d (x, Tx)--(aip (d (Tx, T2x)) + fiip (d (x, Tx))) < d (x, Tx) if x = Tx.

Now, consider the discrete scalar dynamic difference equation of respective state and control real sequences {xk\kez0+ and {uk\kez0+ and dynamics and control parametrical real sequences {ak}kez0+ and {bk = 0}keZ0+, respectively:

xk+i = akxk + bkUk + nk', Vk e Zo+, xo e R (3 . 11)

where {xk}keZo+, of general term defined by xck := (x0, x1r ... ,xk), is a sequence of real kth tuples built with state values up till the kth sampled value such that the real sequence {nk}keZo+ with nk = nk 0 is related to non-perfectly modeled effects which can include, for instance, contributions of unmodeled dynamics (if the real order of the difference equation is larger than one), parametrical errors (for instance, the sequences of parameters are not exactly known), and external disturbances. It is assumed that upper- and lower-bounding real sequences {nk}keZo+ and {n0}keZo are known which satisfy nk = nk 0 > nk > n0 = n0 (xk); Vk e z0+. Define a 2-cyclic self-mapping T:A U B ® A UB with T(A)£ B and T(B)£ A for some sets A c R0+ := {z e R:z > 0} and B c R0- := {z e R:z < 0} being non-empty bounded connected sets containing {0}, so that D = 0, such that Txk = xk+i; Vk e z0+ for the control sequence {uk}keZo+ lying in some appropriate class to be specified later on. Note from (3.11) that

xk+2 = akxk+i + bk+i Uk+i + nk+i (3 12)

= ak+iakxk + ak+ibkUk + bk+iUk+i + ak+ink + nk+i, Vk e Zo+, xo e A U B

An equivalent expression to (3.9) if ^(D) = D = 0 is by using the Euclidean distance:

ap (\xu+1\ + \xk+2\)+p<p(\xk\ + \xu+1\) < p (|xfe| + |xfe+11)—(1 — a) (\xk+1\ + \xk+2\); Vk e Z0+ (3.13)

Consider different cases as follows by assuming with no loss in generality that the parametrical sequences {<%}kez0+ and {bk}kez0+ are positive: (a) D = 0 Then

xk+2 = dk+1dkxk + dk+1 bkUk + bk+1Uk+1 + dk+1Vk + nk+1; Vk e Z0+, x0 e A U B 0.M)

Note that if xk > 0 then xk+i < 0 and xk+2 < 0 if

akxk + m ak+1 (rjk — akxk — bkUk) + rjk+1 . ,

Uk <----< 0; uh+1 >---; Vfe e Z0+ (3.15)

bk bk+1

If xk < 0 then xk+1 > 0 and xk+2< 0 if

rk — akxk ak+1 (m + akxk + bkUk) + rjk+1 w, ,„ ,,,

Uk >-:-; Uh+1 <--:-; Vfe e Z0+ (3.16)

Thus, if x0 > 0 then the control law is

a2kx2k + nj2k a2k+1 (nj2k — a2kx2k — b2ku2k) + nj2k+1 ,,, ,0 „x

i'2k <--:-< 0; u2k+l > -:-; vfe e Z0+ (3.17)

b2k b2k+1

and if x0 < 0 then

nj2k — a2kx2k a2k+1 (nj2k + a2kx2k + b2ku2k) + Tj2k+1 w, ™ /0 1 ox

uik >-:-; "2fe+i <--:-; vfe e z0+ (3.18)

b2k b2k+1

The stabilization and convergence of the state sequence to zero is achieved by using a control sequence that makes compatible (3.16) and (3.17) with (3.13). First, assume x0 < 0 and rewrite the controls (3.17) in equivalent equality form as:

a2kx2k + V2k + B2k a2k+1 (V2k — a2kx2k — b2ku2k) + V2k+1+ B2k+1 „ /oTCW

u2i; =----; I/21-+1 =--,-; vfe e Z0+ (3.19)

b2k b2k+1

for any non-negative real sequence {fik}keZ0+ to be defined so that (3.13) holds. Then (3.11) and (3.14) lead to:

—£2k = — j2k — S2k < x2k+1 = a2kx2k + b2ku2k + V2k = V2k — V2k — e2k < S2k < 0

(3.20)

^ e2_k > \x2k+1\ > e2k; Vk e Z0+

e<0k+1 = 2 (aik+1 nj2k + j2k+1) + e2k+1

> x2k+2 = a2k+1a2kx2k + a2k+1b2ku2k + b2k+1 u2k+1 + a2k+1n2k + n2k+1

(3:21)

= a2k+1 (nj2k + n2k) + nj2k+1 + n2k+1 + e2k+1 > e2k+1 > 0 ^ e<0k+1 > \x2k+2 \ > e2k+1; Vk e Z0+

for the given controls (3.19). Then, (3.13) becomes for x0 e A:

ap\\X2k+l \ + \x2k+2\)+pp (\x2k\ + \x2i+1\) + (1 — a) (\x2k+1\ + \x2k+2 \) < p (\x2i\ + \x2k+\\) ; Vk e Z0+ (3.22)

which is guaranteed from (3.20) and (3.21), without a need for directly testing the solution of the difference equation, if the sequence {uk}keZ0+ can be chosen to have zero limit while satisfying:

av {s0k + e0k+i) + Pv is2k-i + 82k) + (i - a) (82k + s2k+i) < P (s2k-i + £2k); Vk e Zo+ (3 . 23)

for some upper-bounding sequence {®0}keZg satisfying (3.20) and (3.21) and some given non-decreasing function ^:R0+ ® R0+. This implies that xk ® 0 as k ® which is the unique fixed point of T:A U B ® A UB, by using the proposed control law (3.19). Note the following:

(1) Even, although {£k}keZo+ converges to zero, it is not required for the contribution of the non-perfectly modeled part of the model to converge to zero. It can suffice, for instance, n2k ^ ij2k; n2k+i ^ - (ij2k+i + 2a2k+i ij2k) as k ® ^ It is not necessary that {nk}keZo+ be convergent fulfilling \ijk\ ^ \nk\ ^ ij < ^ as k ® ^ for some non-negative real ij = ij 0. However, there are particular cases in this framework as, for instance, \jk\ ^ \nk\ ^ 0 as k ® ^ or nk ^ ij > 0; a2k+1 ® 1 as k ®

(2) The constraints (3.23) imply ^(x) = 0 for x e [0,x0] and some x0 e R0+ but not that ^:R0+ ® R0+ is strictly increasing or that ^(x) = 0 if and only if x = 0.

If Xq e B, then x^ < 0, take uo > -:-leading to X\ > 0 The above stabiliza-

tion/convergence condition (3.23) still holds with the replacement k ® k-1 for any k e z+.

(b) Now, assume that D >0, A:={z e R+ :z > D/2}, and B:={z e R+ :z < -D/2} are bounded subsets of R and reconsider the above Case b modified so that T:AUB ® AUB the sequence {£k}keZo+ is subject to s2k > D/2, s2k ® D/2 as k and ^(D) = D = dist (A,B) Also, the stabilization constraints (3.22) and (3.23) become modified as follows:

av (\x2k+i \ + \x2k+2\) + PV (\x2k\ + \x2k+i\) + (i - a) (\x2k+i\ + \x2k+2\)

(3 24)

< D + P (\x2k\ + \x2k+i \), Vk e Zo+

ap (s0k + s0k+i)+pp Sk—1 + s0k)+(1 — a) (s0k + sOk+i) < D+P Sk—i + S2k), Vk e Zo+ (3 . 25)

the second one being a sufficient condition for the first one to hold. Note that x2k and x2k+i both converge to best proximity points as k ® ^ If x0 > D/2 then x2k ® D/2 and x2k+1 ® D/2 as k ® ^ and if x0 < -D/2 then x2k ® -D/2 and x2k+1 ® D/2. Note that Case a is a particular version of Case b for D = 0.

(c) The conditions (3.23) and (3.25) can be generalized to the nonlinear potentially non-perfectly modeled difference equation:

xk+i = akg (xu) + bkuk + nk; Vk e Zo+, xo e Rn (3 . 26)

for some function g : r ® r leading to the nonlinear real sequence {gk = g (xk)}keZo. Proceed by replacing the controls (3.19) by their counterparts obtained correspondingly with right-hand side replacements xk ®gk = g(xk) by choosing the sequence \-Sk}keZo+ with ® [D,^) satisfying ^(x) = D for x e [D,D + x0] and some x0 e

R0+ so that (3.25) holds.

(d) Consider the nth-order nonlinear dynamic system:

xk+i = auxu + Bu, vk e Zo+, xo e Rn (3 . 27)

for some matrix function sequences sampling point-wise defined by Ak = Ak(xk) and Bk = Bk(xk) of images in Rnxn and Rnxm, respectively; Vk e Zo+. Proceeding recursively with (3.27) over n consecutive samples, one gets

x(k+1)n = pkxkn + rkUkn; Vk e Z0+, x0 e Rn (3.28)

with Fk = Fk(xkn) and rk = ^t(xkn) as:

n( k+1 \u— 1

[Ai]; rk :=

B(k+1)n-l'- A(k+i)n-lB(k+i)n-2'- ••• ^(j=kkkn'+1 1 [Aj] Bk

(3.29)

with the extended nm control real vector sequence over n consecutive samples being defined by um = ûk7AXk) := (u{k+i)n-i, "(fe+im-2, ■■■, Ukn) T- Consider solutions of (3.28) lying alternately in a non-empty closed bounded connected subset A of the first closed orthant of Rnand in B = -A for each couple of subsequent samples for some extended control sequence {uklkeZo+ in Rm, for some integer 1 < m < n, assumed to exist. A unique such a control sequence exist, if for instance, the controllability condition rank rk = n; Vk e z0+ holds for each matrix sequence Ak = Ak(xk) by achieving:

X(k+i)n = VkXkn + rkûkn = - AkXkn; vfc e Zo+, xo e A (3.30)

with Ak = Ak(xkn); Vk e z0+ defining some prefixed positive real matrix sequence taking values in Rnxnwith at least a non-zero entry per row. The closed-loop control objective (3.30) is achievable by the feedback control sequence:

Ukn = -rkT (rkrT) (Vk + Ak) Xkn, vk e Zo+; xo e A (3.31)

Thus, a modified constraint of the type (3.22), or (3.23), ensures that the solution of (3.28), subject to the extended control (3.31), lies alternately in A and B for each two consecutive samples for x0 e A and converges to zero, while a modification of (3.24), or (3.25), ensures that the solution lies alternately in B and A and converges to zero, provided that Akx0 e Au(-A); Vx0 e Au(-A), Vk e z0+, i.e., Au(-A) is Ak-invariant, Vk e z0+. Furthermore, A and B are both A2k-invariant. Such a modifications are got directly by replacing x(-) ® A(-)x(-), e(-) ® A(-)e(-) Note that the constraints (3.22), (3.23), (3.24), and (3.25) now become n-vector constraints. The Euclidean distances are now replaced by any Minkowski distance of order p (p-norm-induced distance for some real p > 1) in rn as for instance, 1-norm-induced distance d\ (x, y) = ^n=l \xi — yi, 2-

¡2 \!/2

norm-induced (i.e., Euclidean) distance d2 (x, y) = rn=1 \x — , p-norm-induced

distance dp (x, y) = ^J] 'n=1\xi — yif^1P, or infinity-norm-induced distance

(x, y)='n=1 \x—yi\p) 11 = max (\xi—yi\).

Acknowledgements

The authors are gratefulto the Spanish Ministry of Education for its partialsupport of this study through Grant DPI2009-07197. They are also gratefulto the Basque Government for its support through Grants IT378-10 and SAIOTEK S-PE08UN15 and 09UN12. Finally, they thank the Associate Editor and the reviewers for their usefulcomments to improve the first version of the manuscript.

Author details

instituto de Investigación y Desarrollo de Procesos, Universidad delPais Vasco, Campus of Leioa (Bizkaia) - Aptdo. 644-Bilbao, 48080-Bilbao, Spain 2Department of Mathematics, Texas A&M University - Kingsville, 700 University Blvd., Kingsville, TX 78363-8202, USA

Authors' contributions

Both the authors contributed equally and significantly in writing this paper. Allauthors read and approved the final

manuscript.

Competing interests

The authors declare that they have no competing interests.

Received: 12 September 2011 Accepted: 21 December 2011 Published: 21 December 2011

References

1. Harjani, J, Lopez, B, Sadarangani, B: A fixed point theorem for mappings satisfying a contractive condition of rational type of partially ordered metric space. Abstr Appl Anal 2010 (2010). Article ID 190701, 8. doi:10.1155/2010/19070i

2. Harjani, J, Sadarangani, K: Fixed point theorems for weakly contractive mappings in partially ordered sets. Nonlinear Anal Theory Methods Appl. 71(7-8), 3403-3410 (2009). doi:10.1016/j.na.2009.01.240

3. Bhardwaj, R, Rajput, SS, Yadava, RN: Application of fixed point theory in metric spaces. Thai J Math. 5(2), 253-259 (2007)

4. Enjouji, Y, Nakanishi, Y, Suzuki, T: A generalization of Kannan's fixed point theorem. Fixed Point Theory Appl 2009 (2009). Article ID 192872, 10. doi:10.1155/2009/192872

5. Banach, S: Sur les operations dans les ensembles abstracts et leur application aux équations intégrales. Fundam. Mathematicae. 3, 133-181 (1922)

6. Chatterjee, SK: Fixed point theorems. Comptes Ren Acad Bulgaria Sci. 25, 727-730 (1972)

7. Fisher, B: A fixed point theorem for compact metric spaces. Publ Math Debrecen. 25, 193-194 (1978)

8. Kannan, R: Some results on fixed points. Bull Calcutta Math Soc. 60, 71-76 (1968)

9. Kannan, R: Some results on fixed points-II. Am. Math. Monthly. 76, 405-408 (1969). doi:10.2307/2316437

10. Reich, S: Some remarks concerning contraction mappings. Can Math Bull. 14, 121-124 (1971). doi:10.4153/CMB-1971-024-9

11. Kikkawa, M, Suzuki, T: Some similarity between contractions and Kannan mappings. Fixed Point Theory Appl 2008 (2008). Article ID 649749, 8. doi:10.1155/2009/649749

12. Subrahmanyam, PV: Completeness and fixed points. Monatshefte fur Mathematik. 80(4), 325-330 (1975). doi:10.1007/ BF01472580

13. Kirk, WA, Srinivasan, PS, Veeramani, P: Fixed points for mappings satisfying cyclical contractive conditions. Fixed Point Theory. 4(1), 79-89 (2003)

14. Eldred, AA, Veeramani, P: Existence and convergence of best proximity points. J Math Anal Appl. 323, 1001-1006 (2006). doi:10.1016/j.jmaa.2005.10.081

15. Karpagam, S, Agrawal, S: Best proximity point theorems for p-cyclic Meir-Keeler contractions. Fixed Point Theory Appl 2009 (2009). Article ID 197308, 9. doi:10.1155/2009/197308

16. Di Bari, C, Suzuki, T, Vetro, C: Best proximity points for cyclic Meir-Keeler contractions. Nonlinear Anal Theory Methods Appl. 69(11), 3790-3794 (2008). doi:10.1016/j.na.2007.10.014

17. De la Sen, M: Linking contractive self-mappings and cyclic Meir-Keeler contractions with Kannan self-mappings. Fixed Point Theory Appl 2010 (2010). Article ID 572057, 23. doi:10.1155/2010/572057

18. De la Sen, M: Some combined relations between contractive mappings, Kannan mappings, reasonable expansive mappings and T-stability. Fixed Point Theory Appl 2009 (2009). Article ID 815637, 25. doi:10.1155/2009/815637

19. Suzuki, T: Some notes on Meir-Keeler contractions and L-functions. Bull. Kyushu Inst Tech. 53, 12-13 (2006)

20. Derafshpour, M, Rezapour, S, Shahzad, N: On the existence of best proximity points of cyclic contractions. Adv Dyn Syst Appl. 6(1), 33-40 (2011)

21. Rezapour, Sh, Derafshpour, M, Shahzad, N: Best proximity points of cyclic ^-contractions on reflexive Banach spaces. Fixed Point Theory and Appl 2010 (2010). Article ID 946178, 7. doi:10.1155/2010/046178

22. Al-Thagafi, MA, Shahzad, N: Convergence and existence results for best proximity points. Nonlinear Anal: Theory, Methods Appl. 70(10), 3665-3671 (2009). doi:10.1016/j.na.2008.07.022

23. Qin, X, Kang, SM, Agarwal, RP: On the convergence of an implicit iterative process for generalized asymptotically quasi non-expansive mappings. Fixed Point Theory Appl 2010 (2010). Article ID 714860, 19. doi:10.1155/2010/7145860

24. Sadiq Basha, S, Shahzad, N, Jeyaraj, R: Common best proximity points: global optimization of multi-objective functions. App Math Lett. 24(6), 883-886 (2011). doi:10.1016/j.aml.2010.12.043

25. Shahzad, N, Sadiq Basha, S, Jeyaraj, R: Common best proximity points: global optimal solutions. J Optim Theory Appl. 148(1), 69-78 (2011). doi:10.1007/s10957-010-9745-7

26. Sadiq Basha, S, Shahzad, N, Jeyaraj, R: Optimal approximate solutions of fixed point equations. Abstr Appl Anal (2011). Article ID 174560, 9 (2011)

27. De la Sen, M, d Agarwal, RP: Some fixed point-type results for a class of extended cyclic self-mappings with a more general contractive condition. Fixed Point Theory Appl. 59 (2011)

28. De la Sen, M: On the extensions of Krasnoselskii-type theorems to p-cyclic self-mappings in Banach spaces. Discrete Dyn Nat Soc 2011 (2011). Article ID 568072, 19. doi:10.1155/2011/568072

29. De la Sen, M: On a general contractive condition for cyclic self-mappings. J Appl Math 2011 (2011). Article ID 542941, 17. doi:10.1155/2011/542941

30. De la Sen, M: About robust stability of Caputo linear fractional dynamic systems with time delays through fixed point theory. Fixed Point Theory Appl 2011 (2011). Article ID 867932, 19. doi:10.1155/2011/867932

31. Dhompongsa, S, Yingtaweesittikul, H: Fixed points for multivalued mappings and the metric completeness. Fixed Point Theory Appl 2009 (2011). Article ID 972395, 15. doi:10.1155/2009/972395

32. Doric, D, Lazovic, R: Some Suzuki-type fixed point theorems for generalized multivalued mappings and applications. Fixed Point Theory Appl 2011, 40 (2011). doi:10.1186/1687-1812-2011-40. doi:10.1186/1687-1812-2011-40

33. Kikkawa, M, Suzuki, T: Three fixed point theorems for generalized contractions with constants in complete metric spaces. Nonlinear Anal: Theory, Methods Appl. 69(9), 2942-2949 (2008). doi:10.1016/j.na.2007.08.064

34. Nashine, HK, Altun, I: Fixed point theorems for generalized weakly contractive condition in ordered metric spaces. Fixed Point Theory Appl 2011 (2011). Article ID 132367, 20. doi:10.1155/2011/132367

doi:10.1186/1687-1812-2011-102

Cite this article as: De la Sen and Agarwal: Fixed point-type results for a class of extended cyclic self-mappings under three general weak contractive conditions of rational type. Fixed Point Theory and Applications 2011 2011:102.

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