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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[2] S.S. CHANG, J.Y. PARK and Y.J. CHO, Iterative approximations of fixed points for asymptotically nonexpansive mappings in Banach spaces, Bull. Korean Math. Soc., 37 (2000), 109–119.
[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 ¸STEFAN M. ¸SOLTUZ, 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:X→2X∗J:X\rightarrow 2^{X^{*}} the normalized duality mapping given by

J​x:={f∈X∗:⟨x,f⟩=‖x‖​‖f‖,‖f‖=‖x‖},∀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,∞),∀n∈ℕ,limn→∞Kn=\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​(x−y)∈J​(x−y)j(x-y)\in J(x-y) such that

⟨Tn​x−Tn​y,j​(x−y)⟩≤Kn​‖x−y‖2,∀x,y∈B,∀n∈ℕ.\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 x∗x^{*} such that T​x∗=x∗Tx^{*}=x^{*}, by setting y:=x∗y:=x^{*} in (1.2) we get

⟨Tn​x−x∗,j​(x−x∗)⟩≤Kn​‖x−x∗‖2,∀x,y∈B,∀n∈ℕ;\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=(1−αn)​un+αn​Tn​un.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} =(1−αn)​xn+αn​Tn​yn,\displaystyle=\left(1-\alpha_{n}\right)x_{n}+\alpha_{n}T^{n}y_{n}, (1.5)
yn\displaystyle y_{n} =(1−βn)​xn+βn​Tn​xn.\displaystyle=\left(1-\beta_{n}\right)x_{n}+\beta_{n}T^{n}x_{n}.

The sequences {αn}⊂(0,1),{βn}⊂[0,1)\left\{\alpha_{n}\right\}\subset(0,1),\left\{\beta_{n}\right\}\subset[0,1) satisfy

limn→∞αn=limn→∞βn=0,∑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

‖x‖≤‖x+r​y‖\|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)⟩≥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,∞),∀n∈ℕk_{n}\in[1,\infty),\forall n\in\mathbb{N}, we have

‖x−y‖≤‖x−y+r​[(kn​I−Tn)​x−(kn​I−Tn)​y]‖,∀x,y∈B,∀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:B→BT:B\rightarrow B is a uniformly Lipschitzian map if for some L≥1L\geq 1, we have ‖Tn​x−Tn​y‖≤L​‖x−y‖,∀x,y∈B,∀n∈ℕ\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 {Ψn}\left\{\Psi_{n}\right\} be a nonnegative sequence satisfying

Ψn+1≤(1−λn)​Ψn+σn\Psi_{n+1}\leq\left(1-\lambda_{n}\right)\Psi_{n}+\sigma_{n} (2.3)

where λn∈(0,1),∑n=1∞λn=+∞\lambda_{n}\in(0,1),\sum_{n=1}^{\infty}\lambda_{n}=+\infty and σn=o​(λn)\sigma_{n}=o\left(\lambda_{n}\right). Then limn→∞Ψn=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 (αn)n\left(\alpha_{n}\right)_{n} and (βn)n\left(\beta_{n}\right)_{n} satisfying (1.6). Let TT be an asymptotically pseudocontractive (or asymptotically hemicontractive) and uniformly Lipschitzian map with L≥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 x∗x^{*} fixed point of TT.

Proof. From (1.4) we obtain

un\displaystyle u_{n} =un+1+αn​un−αn​Tn​un\displaystyle=u_{n+1}+\alpha_{n}u_{n}-\alpha_{n}T^{n}u_{n} (3.1)
=(1+αn2)​un+1+αn​(αn​kn​I−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}+
−(1+kn)​αn2​un+1+αn​un+αn​(Tn​un+1−Tn​un)\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+αn2)​un+1+αn​(αn​kn​I−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}+
−(1+kn)​αn2​[un+αn​(Tn​un−un)]+αn​un+αn​(Tn​un+1−Tn​un)\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+αn2)​un+1+αn​(αn​kn​I−Tn)​un+1+(1+kn)​αn3​(un−Tn​un)+\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)+
+[1−(1+kn)​αn]​αn​un+αn​(Tn​un+1−Tn​un)\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 Tn​x∗=x∗T^{n}x^{*}=x^{*} we observe that

x∗=(1+αn2)​x∗+αn​(αn​kn​I−Tn)​x∗+[1−(1+kn)​αn]​αn​x∗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

x∗−un\displaystyle x^{*}-u_{n} (3.3)
=(1+αn2)​(x∗−un+1)+\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−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)+
+[1−(1+kn)​αn]​αn​(x∗−un)+(1+kn)​αn3​(Tn​un−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)+
+αn​(Tn​un−Tn​un+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+αn2)​‖(x∗−un+1)+αn1+αn2​((αn​kn​I−Tn)​x∗−(αn​kn​I−Tn)​un+1)‖.\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 :=x∗−un+1,\displaystyle:=x^{*}-u_{n+1}, (3.5)
y\displaystyle y :=(αn​kn​I−Tn)​x∗−(αn​kn​I−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 :=αn1+αn2,\displaystyle:=\frac{\alpha_{n}}{1+\alpha_{n}^{2}},

we obtain

‖(1+αn2)​(x∗−un+1)+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−Tn)​un+1)‖\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)
≥(1+αn2)​‖x∗−un+1‖.\displaystyle\geq\left(1+\alpha_{n}^{2}\right)\left\|x^{*}-u_{n+1}\right\|.

From (3.3) and (3.6) it follows that

‖x∗−un‖\displaystyle\left\|x^{*}-u_{n}\right\| (3.7)
≥?‖(1+αn2)​(x∗−un+1)+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−Tn)​un+1)‖+\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\|+
+[1−(1+kn)​αn]​αn​‖x∗−un‖−(1+kn)​αn3​‖Tn​un−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\|+
−αn​‖Tn​un−Tn​un+1‖\displaystyle-\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|
≥(1+αn2)​‖x∗−un+1‖+[1−(1+kn)​αn]​αn​‖x∗−un‖+\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\|+
−(1+kn)​αn3​‖Tn​un−un‖−αn​‖Tn​un−Tn​un+1‖\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+αn2)​‖x∗−un+1‖\displaystyle\left(1+\alpha_{n}^{2}\right)\left\|x^{*}-u_{n+1}\right\| (3.8)
≤{1−[1−(1+kn)​αn]​αn}​‖x∗−un‖+(1+kn)​αn3​‖Tn​un−un‖+\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\|+
+αn​‖Tn​un−Tn​un+1‖\displaystyle+\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|

Also, we know that

‖un−Tn​un‖\displaystyle\left\|u_{n}-T^{n}u_{n}\right\| ≤‖Tn​un−Tn​x∗‖+‖x∗−un‖\displaystyle\leq\left\|T^{n}u_{n}-T^{n}x^{*}\right\|+\left\|x^{*}-u_{n}\right\| (3.9)
≤L​‖x∗−un‖+‖x∗−un‖\displaystyle\leq L\left\|x^{*}-u_{n}\right\|+\left\|x^{*}-u_{n}\right\|
=(L+1)​‖x∗−un‖\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

‖Tn​un+1−Tn​un‖\displaystyle\left\|T^{n}u_{n+1}-T^{n}u_{n}\right\| ≤L​‖un+1−un‖\displaystyle\leq L\left\|u_{n+1}-u_{n}\right\| (3.10)
=αn​L​‖un−Tn​un‖\displaystyle=\alpha_{n}L\left\|u_{n}-T^{n}u_{n}\right\|
≤αn​L​(L+1)​‖x∗−un‖\displaystyle\leq\alpha_{n}L(L+1)\left\|x^{*}-u_{n}\right\|

From (3.8), (3.9) and (3.10), by using (1+αn2)−1≤1,∀n∈ℕ\left(1+\alpha_{n}^{2}\right)^{-1}\leq 1,\forall n\in\mathbb{N}, we get

‖x∗−un+1‖\displaystyle\left\|x^{*}-u_{n+1}\right\| (3.11)
≤{1−[1−(1+kn)​αn]​αn}​‖x∗−un‖+\displaystyle\leq\left\{1-\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\right\}\left\|x^{*}-u_{n}\right\|+
+(1+kn)​αn3​(L+1)​‖x∗−un‖+αn2​L​(L+1)​‖x∗−un‖\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 limn→∞αn=0\lim_{n\rightarrow\infty}\alpha_{n}=0 implies the existence of n0∈ℕn_{0}\in\mathbb{N}, such that

αn2≤118​(1+L),∀n≥n0\alpha_{n}^{2}\leq\frac{1}{18(1+L)},\forall n\geq n_{0} (3.12)

Condition (3.12) assures the following inequalities, ∀n≥n0\forall n\geq n_{0},

αn\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)
αn2\displaystyle\alpha_{n}^{2} ≤13​(1+L)​(1+kn)\displaystyle\leq\frac{1}{3(1+L)\left(1+k_{n}\right)}
αn\displaystyle\alpha_{n} ≤13\displaystyle\leq\frac{1}{3}

Using (3.11) and (3.13) we observe that

{1−[1−(1+kn)​αn]​αn}+(1+kn)​αn3​(L+1)+αn2​L​(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)
=1−αn+αn​[((1+kn)+L​(1+L))​αn+(1+kn)​(L+1)​αn2]\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]
≤1−αn+αn​(13+13)=1−13​αn\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

‖x∗−un+1‖≤(1−13​αn)​‖x∗−un‖,∀n≥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

Ψn\displaystyle\Psi_{n} :=‖x∗−un‖,∀n≥n0\displaystyle:=\left\|x^{*}-u_{n}\right\|,\forall n\geq n_{0} (3.16)
λn\displaystyle\lambda_{n} :=13​αn,∀n≥n0\displaystyle:=\frac{1}{3}\alpha_{n},\forall n\geq n_{0}
σn\displaystyle\sigma_{n} :=0,∀n∈ℕ\displaystyle:=0,\forall n\in\mathbb{N}

we get

limn→∞Ψn=limn→∞‖x∗−un‖=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+αn2)​(x∗−un+1)\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right) (3.18)
a′\displaystyle a^{\prime} =(1+αn2)​(x∗−un+1)+\displaystyle=\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−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 =[1−(1+kn)​αn]​αn​(x∗−un)\displaystyle=\left[1-\left(1+k_{n}\right)\alpha_{n}\right]\alpha_{n}\left(x^{*}-u_{n}\right)
c\displaystyle c =(1+kn)​αn3​(Tn​un−un)\displaystyle=\left(1+k_{n}\right)\alpha_{n}^{3}\left(T^{n}u_{n}-u_{n}\right)
d\displaystyle d =αn​(Tn​un+1−Tn​un)\displaystyle=\alpha_{n}\left(T^{n}u_{n+1}-T^{n}u_{n}\right)

to obtain

‖a′+b+c+d‖\displaystyle\left\|a^{\prime}+b+c+d\right\| (3.19)
=∥x∗−un∥=∥(1+αn2)(x∗−un+1)+\displaystyle=\left\|x^{*}-u_{n}\right\|=\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+
+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−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)+
+[1−(1+kn)​αn]​αn​(x∗−un)+(1+kn)​αn3​(Tn​un−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)+
+αn(Tnun−Tnun+1)∥\displaystyle+\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)\|
≥?‖(1+αn2)​(x∗−un+1)+αn​((αn​kn​I−Tn)​x∗−(αn​kn​I−Tn)​un+1)‖+\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\|+
+[1−(1+kn)​αn]​αn​‖x∗−un‖−(1+kn)​αn3​‖Tn​un−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\|+
−αn​‖Tn​un−Tn​un+1‖\displaystyle-\alpha_{n}\left\|T^{n}u_{n}-T^{n}u_{n+1}\right\|
=‖a′‖+‖b‖−‖c‖−‖d‖\displaystyle=\left\|a^{\prime}\right\|+\|b\|-\|c\|-\|d\|

We shall now prove (3.19) using the following relations

‖a′+b+c+d‖+‖c‖+‖d‖\displaystyle\left\|a^{\prime}+b+c+d\right\|+\|c\|+\|d\| (3.20)
≥‖a′+b+c+d‖+‖c+d‖\displaystyle\geq\left\|a^{\prime}+b+c+d\right\|+\|c+d\|
≥?‖a′‖+‖b‖\displaystyle\stackrel{{\scriptstyle?}}\left\|a^{\prime}\right\|+\|b\|
≥‖a‖+‖b‖.\displaystyle\geq\|a\|+\|b\|.

The last inequality from (3.20) is given by (2.1), that is ‖a′‖≥‖a‖\left\|a^{\prime}\right\|\geq\|a\|. We further prove that

‖a′+b+c+d‖+‖c+d‖≥?‖a′‖+‖b‖.\left\|a^{\prime}+b+c+d\right\|+\|c+d\|\stackrel{{\scriptstyle?}}\left\|a^{\prime}\right\|+\|b\|. (3.21)

By using

x∗−un+1=x∗−un+αn​(un−Tn​un),x^{*}-u_{n+1}=x^{*}-u_{n}+\alpha_{n}\left(u_{n}-T^{n}u_{n}\right), (3.22)

we obtain

‖a′‖\displaystyle\left\|a^{\prime}\right\| =∥(1+αn2)(x∗−un+1)+\displaystyle=\|\left(1+\alpha_{n}^{2}\right)\left(x^{*}-u_{n+1}\right)+ (3.23)
+αn((αnknI−Tn)x∗−(αnknI−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)\|
=∥(1+αn2)(x∗−un+1)+knαn2(x∗−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)
−αn(Tnx∗−Tnun+1)∥\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}\right)\|
=∥(1+kn)αn2(αnun−αnTnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{2}\left(\alpha_{n}u_{n}-\alpha_{n}T^{n}u_{n}\right)+
+(1+kn)​αn2​(x∗−un)+(x∗−un+1)+\displaystyle+\left(1+k_{n}\right)\alpha_{n}^{2}\left(x^{*}-u_{n}\right)+\left(x^{*}-u_{n+1}\right)+
−αn(Tnx∗−Tnun+1)∥\displaystyle-\alpha_{n}\left(T^{n}x^{*}-T^{n}u_{n+1}\right)\|
=∥(1+kn)αn3(un−Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
−αn​(Tn​x∗−Tn​un+1)+(1+kn)​αn2​(x∗−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)+
+(x∗−un)+αn(un−Tnun)∥\displaystyle+\left(x^{*}-u_{n}\right)+\alpha_{n}\left(u_{n}-T^{n}u_{n}\right)\|
=∥(1+kn)αn3(un−Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
−αn​(Tn​x∗−Tn​un+1−Tn​x∗+Tn​un)+(1+kn)​αn2​(x∗−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)+
+(x∗−un)+αn(un−Tnun)+αn(−Tnx∗+Tnun)∥\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)\|
=∥(1+kn)αn3(un−Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+
−αn​(−Tn​un+1+Tn​un)+(1+kn)​αn2​(x∗−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)+
+(x∗−un)+αn(un−x∗)∥.\displaystyle+\left(x^{*}-u_{n}\right)+\alpha_{n}\left(u_{n}-x^{*}\right)\|.

The last equality is true because Tn​x∗=x∗T^{n}x^{*}=x^{*}. Finally, we have

‖a′‖\displaystyle\left\|a^{\prime}\right\| =∥(1+kn)αn3(un−Tnun)+\displaystyle=\|\left(1+k_{n}\right)\alpha_{n}^{3}\left(u_{n}-T^{n}u_{n}\right)+ (3.24)
−αn​(Tn​un−Tn​un+1)+\displaystyle-\alpha_{n}\left(T^{n}u_{n}-T^{n}u_{n+1}\right)+
+(1−αn+(1+kn)αn2)(x∗−un)∥\displaystyle+\left(1-\alpha_{n}+\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left(x^{*}-u_{n}\right)\|
≤‖−((1+kn)​αn3​(un−Tn​un)+αn​(Tn​un−Tn​un+1))‖+\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\|+
+(1−αn+(1+kn)​αn2)​‖x∗−un‖\displaystyle+\left(1-\alpha_{n}+\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left\|x^{*}-u_{n}\right\|
=‖x∗−un‖+\displaystyle=\left\|x^{*}-u_{n}\right\|+
+‖−((1+kn)​αn3​(un−Tn​un)+αn​(Tn​un−Tn​un+1))‖+\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\|+
−(1−(1+kn)​αn2)​‖x∗−un‖\displaystyle-\left(1-\left(1+k_{n}\right)\alpha_{n}^{2}\right)\left\|x^{*}-u_{n}\right\|
=‖a′+b+c+d‖+‖c+d‖−‖b‖.\displaystyle=\left\|a^{\prime}+b+c+d\right\|+\|c+d\|-\|b\|.

The last equality is true because we already know that

‖x∗−un‖=‖a′+b+c+d‖.\left\|x^{*}-u_{n}\right\|=\left\|a^{\prime}+b+c+d\right\|. (3.25)

Remark 3.1. If limn→∞αn≠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 (αn)n\left(\alpha_{n}\right)_{n} and (βn)n\left(\beta_{n}\right)_{n} satisfying (1.6). Let TT be an asymptotically pseudocontractive and uniformly Lipschitzian with L≥1L\geq 1 self-map of BB. Let x∗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 x∗x^{*},
(ii) the modified Ishikawa iteration (1.5) strongly converges to x∗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,∀n∈ℕk_{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 (αn)n\left(\alpha_{n}\right)_{n} and (βn)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: ∑(kn−1)<+∞,∑αn2<+∞\sum\left(k_{n}-1\right)<+\infty,\sum\alpha_{n}^{2}<+\infty and ∑βn<+∞\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:

⟨Tn​xn+1−x∗,j​(xn+1−x∗)⟩≤kn​‖xn+1−x∗‖2−ϕ​(‖xn+1−x∗‖),\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, x∗x^{*} is a fixed point and ϕ:[0,∞)→[0,∞)\phi:[0,\infty)\rightarrow[0,\infty) is a strictly increasing function with ϕ​(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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