The convergence of modified Mann-Ishikawa iterations when applied to an asymptotically pseudocontractive map

Abstract

We prove that under minimal conditions the modified Mann and Ishikawa iterations converge when dealing with an asymptotically pseudocontractive map. We give an affirmative answer to the open question from C.E. Chidume and H. Zegeye, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354–366.

Authors

Stefan M. Soltuz
Tiberiu Popoviciu Institute of Numerical Analysis, Romanian Academy

Keywords

Asymptotically hemicontractive map; Modified Mann; Modified Ishikawa iteration

Paper coordinates

Stefan M. Soltuz, The convergence of modified Mann-Ishikawa iterations when applied to an asymptotically pseudocontractive map, Austral. J. Math Anal. Appl., Volume 4, Issue 2, Article 16, pp. 1-8, 2007.

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The Australian Journal of Mathematical
Analysis and Applications

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1449-5910

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[3] C.E. CHIDUME, Convergence theorems for asymptotically pseudocontractive mappings, Nonlinear Analysis, 49 (2002), 1–11.
[4] C.E. CHIDUME and H. ZEGEYE, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354–366.
[5] D.I. IGBOKWE, Iterative construction of fixed points of asymptotically pseudocontractive maps, Panamer. Math. J., 13 (2003), 83–97.
[6] S. ISHIKAWA, Fixed points by a new iteration method, Proc. Amer. Math. Soc., 44 (1974), 147-150.
[7] T. KATO, Nonlinear semigroup and evolution equations, J. Math. Soc. Japan, 19(1967), 508–520.
[8] W.R. MANN, Mean value in iteration, Proc. Amer. Math. Soc., 4 (1953), 506–510.
[9] M.O. OSILIKE, Iterative approximation of fiexd points of asymptotically demicontractive mappings, Indian J. Pure Appl. Math., 29 (1998), 1291–1300.
[10] M.O. OSILIKE and D.I. IGBBOKWE, Convergence theorems for asymptotically pseudocontractive maps, Bull. Korean Math. Soc., 39 (2002), 389–399.
[11] B.E. RHOADES and ΒΈSTEFAN M. ΒΈSOLTUZ, The equivalence between the convergences of Ishikawa and Mann iterations for asymptotically pseudocontractive map, J. Math. Anal. Appl., 283 (2003), 681–688.
[12] B.K. SHARMA and D.R. SAHU, Existence and approximation results for asymptotically pseudocontractive mappings, Indian J. Pure Appl. Math., 31 (2000), 185–196.
[13] J. SCHU, Iterative construction of fixed points of asymptotically nonexpansive mappings, J. Math. Anal. Appl., 158 (1991), 407–413.
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THE CONVERGENCE OF MODIFIED MANN-ISHIKAWA ITERATIONS WHEN APPLIED TO AN ASYMPTOTICALLY PSEUDOCONTRACTIVE MAP

Departamento de Matematicas, Universidad de Los Andes, Carrera 1, No. 18A-10, Bogota, Colombia, and "T. Popoviciu" Institute of Numerical Analysis, Cluj-Napoca, Romania, smsoltuz@gmail.com
URL:http://www.uniandes.edu.co
Abstract

We prove that under minimal conditions the modified Mann and Ishikawa iterations converge when dealing with an asymptotically pseudocontractive map. We give an affirmative answer to the open question from C.E. Chidume and H. Zegeye, Approximate fixed point sequences and convergence theorems for asymptotically pseudocontractive mappings, J. Math. Anal. Appl., 278 (2003), 354-366.

Received 26 January, 2007; accepted 5 September, 2007; published 30 November, 2007. Communicated by: M. Mariani

1. Introduction

Let XX be an arbitrary real Banach space and J:Xβ†’2Xβˆ—J:X\rightarrow 2^{X^{*}} the normalized duality mapping given by

Jx:={f∈Xβˆ—:⟨x,f⟩=β€–xβ€–β€–fβ€–,β€–fβ€–=β€–xβ€–},βˆ€x∈X.Jx:=\left\{f\in X^{*}:\langle x,f\rangle=\|x\|\|f\|,\|f\|=\|x\|\right\},\forall x\in X. (1.1)

In [13] the following class of maps was introduced:
Definition 1.1. Let XX be a normed space and BB a subset of XX. A map TT is said to be asymptotically pseudocontractive if there exists a sequence {Kn},Kn∈[1,∞),βˆ€nβˆˆβ„•,limnβ†’βˆžKn=\left\{K_{n}\right\},K_{n}\in[1,\infty),\forall n\in\mathbb{N},\lim_{n\rightarrow\infty}K_{n}= 1 , and there exists j(xβˆ’y)∈J(xβˆ’y)j(x-y)\in J(x-y) such that

⟨Tnxβˆ’Tny,j(xβˆ’y)βŸ©β‰€Knβ€–xβˆ’yβ€–2,βˆ€x,y∈B,βˆ€nβˆˆβ„•.\left\langle T^{n}x-T^{n}y,j(x-y)\right\rangle\leq K_{n}\|x-y\|^{2},\forall x,y\in B,\forall n\in\mathbb{N}. (1.2)

If there exists xβˆ—x^{*} such that Txβˆ—=xβˆ—Tx^{*}=x^{*}, by setting y:=xβˆ—y:=x^{*} in (1.2) we get

⟨Tnxβˆ’xβˆ—,j(xβˆ’xβˆ—)βŸ©β‰€Knβ€–xβˆ’xβˆ—β€–2,βˆ€x,y∈B,βˆ€nβˆˆβ„•;\left\langle T^{n}x-x^{*},j\left(x-x^{*}\right)\right\rangle\leq K_{n}\left\|x-x^{*}\right\|^{2},\forall x,y\in B,\forall n\in\mathbb{N}; (1.3)

such a map is called asymptotically hemicontractive.

The modified Mann iteration, (see [8]), is defined by

un+1=(1βˆ’Ξ±n)un+Ξ±nTnun.u_{n+1}=\left(1-\alpha_{n}\right)u_{n}+\alpha_{n}T^{n}u_{n}. (1.4)

The modified Ishikawa iteration is defined, (see [6]), by

xn+1\displaystyle x_{n+1} =(1βˆ’Ξ±n)xn+Ξ±nTnyn,\displaystyle=\left(1-\alpha_{n}\right)x_{n}+\alpha_{n}T^{n}y_{n}, (1.5)
yn\displaystyle y_{n} =(1βˆ’Ξ²n)xn+Ξ²nTnxn.\displaystyle=\left(1-\beta_{n}\right)x_{n}+\beta_{n}T^{n}x_{n}.

The sequences {Ξ±n}βŠ‚(0,1),{Ξ²n}βŠ‚[0,1)\left\{\alpha_{n}\right\}\subset(0,1),\left\{\beta_{n}\right\}\subset[0,1) satisfy

limnβ†’βˆžΞ±n=limnβ†’βˆžΞ²n=0,βˆ‘n=1∞αn=+∞.\lim_{n\rightarrow\infty}\alpha_{n}=\lim_{n\rightarrow\infty}\beta_{n}=0,\sum_{n=1}^{\infty}\alpha_{n}=+\infty. (1.6)

We shall give here the most general result concerning the convergence of Mann and Ishikawa iterations dealing with a uniformly Lipschitzian and asymptotically pseudocontractive map. Our result generalizes the main results from [3], [4], [10] and [12]. We also give an affirmative answer to the open question from [4] if the Mann or Ishikawa iteration converges when applied to an asymptotically pseudocontractive (respectively an asymptotically hemicontractive map), in more general spaces than Hilbert spaces.

2. Preliminaries

We recall the following auxiliary results.
Lemma 2.1. [7] Let XX be a Banach space and x,y∈Xx,y\in X. Then

β€–x‖≀‖x+ryβ€–\|x\|\leq\|x+ry\| (2.1)

for all r>0r>0 if and only if there exists j(x)∈J(x)j(x)\in J(x) such that ⟨y,j(x)⟩β‰₯0\langle y,j(x)\rangle\geq 0.

Lemma 2.2 [11] Let BB be a nonempty subset of a Banach space XX and let T:B→BT:B\rightarrow B be a map. Then the following conditions are equivalent:
(i) TT is an asymptotically pseudocontractive map,
(ii) for kn∈[1,∞),βˆ€nβˆˆβ„•k_{n}\in[1,\infty),\forall n\in\mathbb{N}, we have

β€–xβˆ’y‖≀‖xβˆ’y+r[(knIβˆ’Tn)xβˆ’(knIβˆ’Tn)y]β€–,βˆ€x,y∈B,βˆ€r>0.\|x-y\|\leq\left\|x-y+r\left[\left(k_{n}I-T^{n}\right)x-\left(k_{n}I-T^{n}\right)y\right]\right\|,\forall x,y\in B,\forall r>0. (2.2)

Definition 2.1. Let XX be a normed space and BB a subset of XX, then the map T:Bβ†’BT:B\rightarrow B is a uniformly Lipschitzian map if for some Lβ‰₯1L\geq 1, we have β€–Tnxβˆ’Tny‖≀Lβ€–xβˆ’yβ€–,βˆ€x,y∈B,βˆ€nβˆˆβ„•\left\|T^{n}x-T^{n}y\right\|\leq L\|x-y\|,\forall x,y\in B,\forall n\in\mathbb{N}.

Lemma 2.3. [14] Let {Ξ¨n}\left\{\Psi_{n}\right\} be a nonnegative sequence satisfying

Ξ¨n+1≀(1βˆ’Ξ»n)Ξ¨n+Οƒn\Psi_{n+1}\leq\left(1-\lambda_{n}\right)\Psi_{n}+\sigma_{n} (2.3)

where Ξ»n∈(0,1),βˆ‘n=1∞λn=+∞\lambda_{n}\in(0,1),\sum_{n=1}^{\infty}\lambda_{n}=+\infty and Οƒn=o(Ξ»n)\sigma_{n}=o\left(\lambda_{n}\right). Then limnβ†’βˆžΞ¨n=0\lim_{n\rightarrow\infty}\Psi_{n}=0.

3. Main Result

Theorem 3.1. Let BB be a closed convex subset of an arbitrary Banach space XX and (un)n\left(u_{n}\right)_{n} defined by (1.4) with (Ξ±n)n\left(\alpha_{n}\right)_{n} and (Ξ²n)n\left(\beta_{n}\right)_{n} satisfying (1.6). Let TT be an asymptotically pseudocontractive (or asymptotically hemicontractive) and uniformly Lipschitzian map with Lβ‰₯1L\geq 1 self-map of BB. If u0∈Bu_{0}\in B, then the modified Mann iteration (1.4) strongly converges to the nearest xβˆ—x^{*} fixed point of TT.

Proof. From (1.4) we obtain

un\displaystyle u_{n} =un+1+Ξ±nunβˆ’Ξ±nTnun\displaystyle=u_{n+1}+\alpha_{n}u_{n}-\alpha_{n}T^{n}u_{n} (3.1)
=(1+Ξ±n2)un+1+Ξ±n(Ξ±nknIβˆ’Tn)un+1+\displaystyle=\left(1+\alpha_{n}^{2}\right)u_{n+1}+\alpha_{n}\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}+
βˆ’(1+kn)Ξ±n2un+1+Ξ±nun+Ξ±n(Tnun+1βˆ’Tnun)\displaystyle-\left(1+k_{n}\right)\alpha_{n}^{2}u_{n+1}+\alpha_{n}u_{n}+\alpha_{n}\left(T^{n}u_{n+1}-T^{n}u_{n}\right)
=(1+Ξ±n2)un+1+Ξ±n(Ξ±nknIβˆ’Tn)un+1+\displaystyle=\left(1+\alpha_{n}^{2}\right)u_{n+1}+\alpha_{n}\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}+
βˆ’(1+kn)Ξ±n2[un+Ξ±n(Tnunβˆ’un)]+Ξ±nun+Ξ±n(Tnun+1βˆ’Tnun)\displaystyle-\left(1+k_{n}\right)\alpha_{n}^{2}\left[u_{n}+\alpha_{n}\left(T^{n}u_{n}-u_{n}\right)\right]+\alpha_{n}u_{n}+\alpha_{n}\left(T^{n}u_{n+1}-T^{n}u_{n}\right)
=(1+Ξ±n2)un+1+Ξ±n(Ξ±nknIβˆ’Tn)un+1+(1+kn)Ξ±n3(unβˆ’Tnun)+\displaystyle=\left(1+\alpha_{n}^{2}\right)u_{n+1}+\alpha_{n}\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}+\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
+[1βˆ’(1+kn)Ξ±n]Ξ±nun+Ξ±n(Tnun+1βˆ’Tnun)\displaystyle+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}u_{n}+\alpha_{n}\left(T^{n}u_{n+1}-T^{n}u_{n}\right)

By using Tnxβˆ—=xβˆ—T^{n}x^{*}=x^{*} we observe that

xβˆ—=(1+Ξ±n2)xβˆ—+Ξ±n(Ξ±nknIβˆ’Tn)xβˆ—+[1βˆ’(1+kn)Ξ±n]Ξ±nxβˆ—x^{*}=\left(1+\alpha_{n}^{2}\right)x^{*}+\alpha_{n}\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}x^{*} (3.2)

From (3.1) and (3.2) we get

xβˆ—βˆ’un\displaystyle x^{*}-u_{n} (3.3)
=(1+Ξ±n2)(xβˆ—βˆ’un+1)+\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)+\displaystyle+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)+
+[1βˆ’(1+kn)Ξ±n]Ξ±n(xβˆ—βˆ’un)+(1+kn)Ξ±n3(Tnunβˆ’un)+\displaystyle+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left(x^{*}-u_{n}\right)+\left(1+k_{n}\right)\alpha_{n}^{3}\left(T^{n}u_{n}-u_{n}\right)+
+Ξ±n(Tnunβˆ’Tnun+1)\displaystyle+\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)

The norm of the sum of the first two terms on the right-hand side of 3.3) is equal to

(1+Ξ±n2)β€–(xβˆ—βˆ’un+1)+Ξ±n1+Ξ±n2((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)β€–.\left(1+\alpha_{n}^{2}\right)\left\|\left(x^{*}-u_{n+1}\right)+\frac{\alpha_{n}}{1+\alpha_{n}^{2}}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)\right\|. (3.4)

Using (2.1) with

x\displaystyle x :=xβˆ—βˆ’un+1,\displaystyle:=x^{*}-u_{n+1}, (3.5)
y\displaystyle y :=(Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1,\displaystyle:=\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1},
r\displaystyle r :=Ξ±n1+Ξ±n2,\displaystyle:=\frac{\alpha_{n}}{1+\alpha_{n}^{2}},

we obtain

β€–(1+Ξ±n2)(xβˆ—βˆ’un+1)+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)β€–\displaystyle\left\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)\right\| (3.6)
β‰₯(1+Ξ±n2)β€–xβˆ—βˆ’un+1β€–.\displaystyle\geq\left(1+\alpha_{n}^{2}\right)\left\|x^{*}-u_{n+1}\right\|.

From (3.3) and (3.6) it follows that

β€–xβˆ—βˆ’unβ€–\displaystyle\left\|x^{*}-u_{n}\right\| (3.7)
β‰₯?β€–(1+Ξ±n2)(xβˆ—βˆ’un+1)+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)β€–+\displaystyle\stackrel{{\scriptstyle?}}\left\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)\right\|+
+[1βˆ’(1+kn)Ξ±n]Ξ±nβ€–xβˆ—βˆ’unβ€–βˆ’(1+kn)Ξ±n3β€–Tnunβˆ’unβ€–+\displaystyle+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left\|x^{*}-u_{n}\right\|-\left(1+k_{n}\right)\alpha_{n}^{3}\left\|T^{n}u_{n}-u_{n}\right\|+
βˆ’Ξ±nβ€–Tnunβˆ’Tnun+1β€–\displaystyle-\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|
β‰₯(1+Ξ±n2)β€–xβˆ—βˆ’un+1β€–+[1βˆ’(1+kn)Ξ±n]Ξ±nβ€–xβˆ—βˆ’unβ€–+\displaystyle\geq\left(1+\alpha_{n}^{2}\right)\left\|x^{*}-u_{n+1}\right\|+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left\|x^{*}-u_{n}\right\|+
βˆ’(1+kn)Ξ±n3β€–Tnunβˆ’unβ€–βˆ’Ξ±nβ€–Tnunβˆ’Tnun+1β€–\displaystyle-\left(1+k_{n}\right)\alpha_{n}^{3}\left\|T^{n}u_{n}-u_{n}\right\|-\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|

We shall prove later the first inequality from 3.7). Supposing that 3.7) holds, we obtain

(1+Ξ±n2)β€–xβˆ—βˆ’un+1β€–\displaystyle\left(1+\alpha_{n}^{2}\right)\left\|x^{*}-u_{n+1}\right\| (3.8)
≀{1βˆ’[1βˆ’(1+kn)Ξ±n]Ξ±n}β€–xβˆ—βˆ’unβ€–+(1+kn)Ξ±n3β€–Tnunβˆ’unβ€–+\displaystyle\leq\left\{1-\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\right\}\left\|x^{*}-u_{n}\right\|+\left(1+k_{n}\right)\alpha_{n}^{3}\left\|T^{n}u_{n}-u_{n}\right\|+
+Ξ±nβ€–Tnunβˆ’Tnun+1β€–\displaystyle+\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|

Also, we know that

β€–unβˆ’Tnunβ€–\displaystyle\left\|u_{n}-T^{n}u_{n}\right\| ≀‖Tnunβˆ’Tnxβˆ—β€–+β€–xβˆ—βˆ’unβ€–\displaystyle\leq\left\|T^{n}u_{n}-T^{n}x^{*}\right\|+\left\|x^{*}-u_{n}\right\| (3.9)
≀Lβ€–xβˆ—βˆ’unβ€–+β€–xβˆ—βˆ’unβ€–\displaystyle\leq L\left\|x^{*}-u_{n}\right\|+\left\|x^{*}-u_{n}\right\|
=(L+1)β€–xβˆ—βˆ’unβ€–\displaystyle=(L+1)\left\|x^{*}-u_{n}\right\|

Using (1.4), 3.9 and the fact that TT is a uniformly Lipschitzian map, we obtain

β€–Tnun+1βˆ’Tnunβ€–\displaystyle\left\|T^{n}u_{n+1}-T^{n}u_{n}\right\| ≀Lβ€–un+1βˆ’unβ€–\displaystyle\leq L\left\|u_{n+1}-u_{n}\right\| (3.10)
=Ξ±nLβ€–unβˆ’Tnunβ€–\displaystyle=\alpha_{n}L\left\|u_{n}-T^{n}u_{n}\right\|
≀αnL(L+1)β€–xβˆ—βˆ’unβ€–\displaystyle\leq\alpha_{n}L(L+1)\left\|x^{*}-u_{n}\right\|

From (3.8), (3.9) and (3.10), by using (1+Ξ±n2)βˆ’1≀1,βˆ€nβˆˆβ„•\left(1+\alpha_{n}^{2}\right)^{-1}\leq 1,\forall n\in\mathbb{N}, we get

β€–xβˆ—βˆ’un+1β€–\displaystyle\left\|x^{*}-u_{n+1}\right\| (3.11)
≀{1βˆ’[1βˆ’(1+kn)Ξ±n]Ξ±n}β€–xβˆ—βˆ’unβ€–+\displaystyle\leq\left\{1-\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\right\}\left\|x^{*}-u_{n}\right\|+
+(1+kn)Ξ±n3(L+1)β€–xβˆ—βˆ’unβ€–+Ξ±n2L(L+1)β€–xβˆ—βˆ’unβ€–\displaystyle+\left(1+k_{n}\right)\alpha_{n}^{3}(L+1)\left\|x^{*}-u_{n}\right\|+\alpha_{n}^{2}L(L+1)\left\|x^{*}-u_{n}\right\|

The condition limnβ†’βˆžΞ±n=0\lim_{n\rightarrow\infty}\alpha_{n}=0 implies the existence of n0βˆˆβ„•n_{0}\in\mathbb{N}, such that

Ξ±n2≀118(1+L),βˆ€nβ‰₯n0\alpha_{n}^{2}\leq\frac{1}{18(1+L)},\forall n\geq n_{0} (3.12)

Condition (3.12) assures the following inequalities, βˆ€nβ‰₯n0\forall n\geq n_{0},

Ξ±n\displaystyle\alpha_{n} ≀13((1+kn)+L(1+L))\displaystyle\leq\frac{1}{3\left(\left(1+k_{n}\right)+L(1+L)\right)} (3.13)
Ξ±n2\displaystyle\alpha_{n}^{2} ≀13(1+L)(1+kn)\displaystyle\leq\frac{1}{3(1+L)\left(1+k_{n}\right)}
Ξ±n\displaystyle\alpha_{n} ≀13\displaystyle\leq\frac{1}{3}

Using (3.11) and (3.13) we observe that

{1βˆ’[1βˆ’(1+kn)Ξ±n]Ξ±n}+(1+kn)Ξ±n3(L+1)+Ξ±n2L(L+1)\displaystyle\left\{1-\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\right\}+\left(1+k_{n}\right)\alpha_{n}^{3}(L+1)+\alpha_{n}^{2}L(L+1) (3.14)
=1βˆ’Ξ±n+Ξ±n[((1+kn)+L(1+L))Ξ±n+(1+kn)(L+1)Ξ±n2]\displaystyle=1-\alpha_{n}+\alpha_{n}\left[\left(\left(1+k_{n}\right)+L(1+L)\right)\alpha_{n}+\left(1+k_{n}\right)(L+1)\alpha_{n}^{2}\right]
≀1βˆ’Ξ±n+Ξ±n(13+13)=1βˆ’13Ξ±n\displaystyle\leq 1-\alpha_{n}+\alpha_{n}\left(\frac{1}{3}+\frac{1}{3}\right)=1-\frac{1}{3}\alpha_{n}

Relations (3.11), (3.14), lead us to

β€–xβˆ—βˆ’un+1‖≀(1βˆ’13Ξ±n)β€–xβˆ—βˆ’unβ€–,βˆ€nβ‰₯n0\left\|x^{*}-u_{n+1}\right\|\leq\left(1-\frac{1}{3}\alpha_{n}\right)\left\|x^{*}-u_{n}\right\|,\forall n\geq n_{0} (3.15)

Setting in (2.3) from lemma 2.3

Ξ¨n\displaystyle\Psi_{n} :=β€–xβˆ—βˆ’unβ€–,βˆ€nβ‰₯n0\displaystyle:=\left\|x^{*}-u_{n}\right\|,\forall n\geq n_{0} (3.16)
Ξ»n\displaystyle\lambda_{n} :=13Ξ±n,βˆ€nβ‰₯n0\displaystyle:=\frac{1}{3}\alpha_{n},\forall n\geq n_{0}
Οƒn\displaystyle\sigma_{n} :=0,βˆ€nβˆˆβ„•\displaystyle:=0,\forall n\in\mathbb{N}

we get

limnβ†’βˆžΞ¨n=limnβ†’βˆžβ€–xβˆ—βˆ’unβ€–=0\lim_{n\rightarrow\infty}\Psi_{n}=\lim_{n\rightarrow\infty}\left\|x^{*}-u_{n}\right\|=0 (3.17)

We prove now the first inequality from 3.7 . Set in 3.7

a\displaystyle a =(1+Ξ±n2)(xβˆ—βˆ’un+1)\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right) (3.18)
aβ€²\displaystyle a^{\prime} =(1+Ξ±n2)(xβˆ—βˆ’un+1)+\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)\displaystyle+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)
b\displaystyle b =[1βˆ’(1+kn)Ξ±n]Ξ±n(xβˆ—βˆ’un)\displaystyle=\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left(x^{*}-u_{n}\right)
c\displaystyle c =(1+kn)Ξ±n3(Tnunβˆ’un)\displaystyle=\left(1+k_{n}\right)\alpha_{n}^{3}\left(T^{n}u_{n}-u_{n}\right)
d\displaystyle d =Ξ±n(Tnun+1βˆ’Tnun)\displaystyle=\alpha_{n}\left(T^{n}u_{n+1}-T^{n}u_{n}\right)

to obtain

β€–aβ€²+b+c+dβ€–\displaystyle\left\|a^{\prime}+b+c+d\right\| (3.19)
=βˆ₯xβˆ—βˆ’unβˆ₯=βˆ₯(1+Ξ±n2)(xβˆ—βˆ’un+1)+\displaystyle=\left\|x^{*}-u_{n}\right\|=\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)+\displaystyle+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)+
+[1βˆ’(1+kn)Ξ±n]Ξ±n(xβˆ—βˆ’un)+(1+kn)Ξ±n3(Tnunβˆ’un)+\displaystyle+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left(x^{*}-u_{n}\right)+\left(1+k_{n}\right)\alpha_{n}^{3}\left(T^{n}u_{n}-u_{n}\right)+
+Ξ±n(Tnunβˆ’Tnun+1)βˆ₯\displaystyle+\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)\|
β‰₯?β€–(1+Ξ±n2)(xβˆ—βˆ’un+1)+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)β€–+\displaystyle\stackrel{{\scriptstyle?}}\left\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)\right\|+
+[1βˆ’(1+kn)Ξ±n]Ξ±nβ€–xβˆ—βˆ’unβ€–βˆ’(1+kn)Ξ±n3β€–Tnunβˆ’unβ€–+\displaystyle+\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left\|x^{*}-u_{n}\right\|-\left(1+k_{n}\right)\alpha_{n}^{3}\left\|T^{n}u_{n}-u_{n}\right\|+
βˆ’Ξ±nβ€–Tnunβˆ’Tnun+1β€–\displaystyle-\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|
=β€–aβ€²β€–+β€–bβ€–βˆ’β€–cβ€–βˆ’β€–dβ€–\displaystyle=\left\|a^{\prime}\right\|+\|b\|-\|c\|-\|d\|

We shall now prove (3.19) using the following relations

β€–aβ€²+b+c+dβ€–+β€–cβ€–+β€–dβ€–\displaystyle\left\|a^{\prime}+b+c+d\right\|+\|c\|+\|d\| (3.20)
β‰₯β€–aβ€²+b+c+dβ€–+β€–c+dβ€–\displaystyle\geq\left\|a^{\prime}+b+c+d\right\|+\|c+d\|
β‰₯?β€–aβ€²β€–+β€–bβ€–\displaystyle\stackrel{{\scriptstyle?}}\left\|a^{\prime}\right\|+\|b\|
β‰₯β€–aβ€–+β€–bβ€–.\displaystyle\geq\|a\|+\|b\|.

The last inequality from (3.20) is given by (2.1), that is β€–aβ€²β€–β‰₯β€–aβ€–\left\|a^{\prime}\right\|\geq\|a\|. We further prove that

β€–aβ€²+b+c+dβ€–+β€–c+dβ€–β‰₯?β€–aβ€²β€–+β€–bβ€–.\left\|a^{\prime}+b+c+d\right\|+\|c+d\|\stackrel{{\scriptstyle?}}\left\|a^{\prime}\right\|+\|b\|. (3.21)

By using

xβˆ—βˆ’un+1=xβˆ—βˆ’un+Ξ±n(unβˆ’Tnun),x^{*}-u_{n+1}=x^{*}-u_{n}+\alpha_{n}\left(u_{n}-T^{n}u_{n}\right), (3.22)

we obtain

β€–aβ€²β€–\displaystyle\left\|a^{\prime}\right\| =βˆ₯(1+Ξ±n2)(xβˆ—βˆ’un+1)+\displaystyle=\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+ (3.23)
+Ξ±n((Ξ±nknIβˆ’Tn)xβˆ—βˆ’(Ξ±nknIβˆ’Tn)un+1)βˆ₯\displaystyle+\alpha_{n}\left(\left(\alpha_{n}k_{n}I-T^{n}\right)x^{*}-\left(\alpha_{n}k_{n}I-T^{n}\right)u_{n+1}\right)\|
=βˆ₯(1+Ξ±n2)(xβˆ—βˆ’un+1)+knΞ±n2(xβˆ—βˆ’un+1)\displaystyle=\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+k_{n}\alpha_{n}^{2}\left(x^{*}-u_{n+1}\right)
βˆ’Ξ±n(Tnxβˆ—βˆ’Tnun+1)βˆ₯\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}\right)\|
=βˆ₯(1+kn)Ξ±n2(Ξ±nunβˆ’Ξ±nTnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{2}\left(\alpha_{n}u_{n}-\alpha_{n}T^{n}u_{n}\right)+
+(1+kn)Ξ±n2(xβˆ—βˆ’un)+(xβˆ—βˆ’un+1)+\displaystyle+\left(1+k_{n}\right)\alpha_{n}^{2}\left(x^{*}-u_{n}\right)+\left(x^{*}-u_{n+1}\right)+
βˆ’Ξ±n(Tnxβˆ—βˆ’Tnun+1)βˆ₯\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}\right)\|
=βˆ₯(1+kn)Ξ±n3(unβˆ’Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
βˆ’Ξ±n(Tnxβˆ—βˆ’Tnun+1)+(1+kn)Ξ±n2(xβˆ—βˆ’un)+\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}\right)+\left(1+k_{n}\right)\alpha_{n}^{2}\left(x^{*}-u_{n}\right)+
+(xβˆ—βˆ’un)+Ξ±n(unβˆ’Tnun)βˆ₯\displaystyle+\left(x^{*}-u_{n}\right)+\alpha_{n}\left(u_{n}-T^{n}u_{n}\right)\|
=βˆ₯(1+kn)Ξ±n3(unβˆ’Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
βˆ’Ξ±n(Tnxβˆ—βˆ’Tnun+1βˆ’Tnxβˆ—+Tnun)+(1+kn)Ξ±n2(xβˆ—βˆ’un)+\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}-T^{n}x^{*}+T^{n}u_{n}\right)+\left(1+k_{n}\right)\alpha_{n}^{2}\left(x^{*}-u_{n}\right)+
+(xβˆ—βˆ’un)+Ξ±n(unβˆ’Tnun)+Ξ±n(βˆ’Tnxβˆ—+Tnun)βˆ₯\displaystyle+\left(x^{*}-u_{n}\right)+\alpha_{n}\left(u_{n}-T^{n}u_{n}\right)+\alpha_{n}\left(-T^{n}x^{*}+T^{n}u_{n}\right)\|
=βˆ₯(1+kn)Ξ±n3(unβˆ’Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
βˆ’Ξ±n(βˆ’Tnun+1+Tnun)+(1+kn)Ξ±n2(xβˆ—βˆ’un)+\displaystyle-\alpha_{n}\left(-T^{n}u_{n+1}+T^{n}u_{n}\right)+\left(1+k_{n}\right)\alpha_{n}^{2}\left(x^{*}-u_{n}\right)+
+(xβˆ—βˆ’un)+Ξ±n(unβˆ’xβˆ—)βˆ₯.\displaystyle+\left(x^{*}-u_{n}\right)+\alpha_{n}\left(u_{n}-x^{*}\right)\|.

The last equality is true because Tnxβˆ—=xβˆ—T^{n}x^{*}=x^{*}. Finally, we have

β€–aβ€²β€–\displaystyle\left\|a^{\prime}\right\| =βˆ₯(1+kn)Ξ±n3(unβˆ’Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+ (3.24)
βˆ’Ξ±n(Tnunβˆ’Tnun+1)+\displaystyle-\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)+
+(1βˆ’Ξ±n+(1+kn)Ξ±n2)(xβˆ—βˆ’un)βˆ₯\displaystyle+\left(1-\alpha_{n}+\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left(x^{*}-u_{n}\right)\|
β‰€β€–βˆ’((1+kn)Ξ±n3(unβˆ’Tnun)+Ξ±n(Tnunβˆ’Tnun+1))β€–+\displaystyle\leq\left\|-\left(\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)\right)\right\|+
+(1βˆ’Ξ±n+(1+kn)Ξ±n2)β€–xβˆ—βˆ’unβ€–\displaystyle+\left(1-\alpha_{n}+\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left\|x^{*}-u_{n}\right\|
=β€–xβˆ—βˆ’unβ€–+\displaystyle=\left\|x^{*}-u_{n}\right\|+
+β€–βˆ’((1+kn)Ξ±n3(unβˆ’Tnun)+Ξ±n(Tnunβˆ’Tnun+1))β€–+\displaystyle+\left\|-\left(\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)\right)\right\|+
βˆ’(1βˆ’(1+kn)Ξ±n2)β€–xβˆ—βˆ’unβ€–\displaystyle-\left(1-\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left\|x^{*}-u_{n}\right\|
=β€–aβ€²+b+c+dβ€–+β€–c+dβ€–βˆ’β€–bβ€–.\displaystyle=\left\|a^{\prime}+b+c+d\right\|+\|c+d\|-\|b\|.

The last equality is true because we already know that

β€–xβˆ—βˆ’unβ€–=β€–aβ€²+b+c+dβ€–.\left\|x^{*}-u_{n}\right\|=\left\|a^{\prime}+b+c+d\right\|. (3.25)

Remark 3.1. If limnβ†’βˆžΞ±nβ‰ 0\lim_{n\rightarrow\infty}\alpha_{n}\neq 0, then our Theorem 3.1 holds supposing condition (3.12) is satisfied.

The modified Ishikawa iteration also converges, being equivalent to the modified Mann iteration.

Theorem 3.2. [11] Let BB be a closed convex subset of an arbitrary Banach space X,(xn)nX,\left(x_{n}\right)_{n} and (un)n\left(u_{n}\right)_{n} defined by (1.5) and (1.4) with (Ξ±n)n\left(\alpha_{n}\right)_{n} and (Ξ²n)n\left(\beta_{n}\right)_{n} satisfying (1.6). Let TT be an asymptotically pseudocontractive and uniformly Lipschitzian with Lβ‰₯1L\geq 1 self-map of BB. Let xβˆ—x^{*} be a fixed point of TT. If u0=x0∈Bu_{0}=x_{0}\in B, then the following two assertions are equivalent:
(i) the modified Mann iteration (1.4) strongly converges to xβˆ—x^{*},
(ii) the modified Ishikawa iteration (1.5) strongly converges to xβˆ—x^{*}.

Remark 3.2. Each fixed point has its own basin of attraction. The map TT has no unique fixed point. The starting point is crucial for the convergence of Mann or Ishikawa iteration. For example, take T=IT=I, the identity map on BB, with kn=1,βˆ€nβˆˆβ„•k_{n}=1,\forall n\in\mathbb{N}. Each point of BB becomes a fixed point and the starting point is directly a fixed point.

Theorem 3.1 generalizes the Theorem from [3] because in [3] the set BB is bounded, the space XX is uniformly convex and (Ξ±n)n\left(\alpha_{n}\right)_{n} and (Ξ²n)n\left(\beta_{n}\right)_{n} satisfy some additional conditions. We also generalize Theorem 1 from [12], because the space is smooth and the following conditions are required: βˆ‘(knβˆ’1)<+∞,βˆ‘Ξ±n2<+∞\sum\left(k_{n}-1\right)<+\infty,\sum\alpha_{n}^{2}<+\infty and βˆ‘Ξ²n<+∞\sum\beta_{n}<+\infty. Our Theorem 3.1 generalizes the main results from [2] and [10] because the map TT satisfies the following restrictive condition:

⟨Tnxn+1βˆ’xβˆ—,j(xn+1βˆ’xβˆ—)βŸ©β‰€knβ€–xn+1βˆ’xβˆ—β€–2βˆ’Ο•(β€–xn+1βˆ’xβˆ—β€–),\left\langle T^{n}x_{n+1}-x^{*},j\left(x_{n+1}-x^{*}\right)\right\rangle\leq k_{n}\left\|x_{n+1}-x^{*}\right\|^{2}-\phi\left(\left\|x_{n+1}-x^{*}\right\|\right), (3.26)

where (xn)n\left(x_{n}\right)_{n} is the modified Mann (respectively modified Ishikawa) iterations, xβˆ—x^{*} is a fixed point and Ο•:[0,∞)β†’[0,∞)\phi:[0,\infty)\rightarrow[0,\infty) is a strictly increasing function with Ο•(0)=0\phi(0)=0.

In [1] and [5] the convergence of (1.4) and (1.5) is shown, dealing with an asymptotically pseudocontractive map without being uniformly Lipschitzian. However, in [1] and [5] the assumptions are more restrictive than those from our Theorem 3.1; the Banach space is uniformly smooth, the set BB is bounded, respectively T(B)T(B) is bounded and the map TT satisfies condition (3.26).

References

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2007

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