Journal of Mathematics

Volume 2016 (2016), Article ID 9641706, 8 pages

http://dx.doi.org/10.1155/2016/9641706

## Convergence Rate of Some Two-Step Iterative Schemes in Banach Spaces

^{1}Department of Statistics and Mathematical Sciences, Kwara State University, Malete, Nigeria^{2}Department of Mathematics, University of Ilorin, Ilorin, Nigeria

Received 11 July 2016; Accepted 6 September 2016

Academic Editor: Ji Gao

Copyright © 2016 O. T. Wahab et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This article proves some theorems to approximate fixed point of Zamfirescu operators on normed spaces for some two-step iterative schemes, namely, Picard-Mann iteration, Ishikawa iteration, S-iteration, and Thianwan iteration, with their errors. We compare the aforementioned iterations using numerical approach; the results show that S-iteration converges faster than other iterations followed by Picard-Mann iteration, while Ishikawa iteration is the least in terms of convergence rate. These results also suggest the best among two-step iterative fixed point schemes in the literature.

#### 1. Introduction

Fixed point theory takes a large amount of literature, since it provides useful tools to solve many problems that have applications in different fields like engineering, economics, chemistry, game theory, and so forth. However, to find fixed points is not an easy task; that is why we use iterative methods for computing them. By time, many iterative methods have been developed and it is impossible to cover them all.

In the last four decades, numerous papers were published on the iterative approximation of fixed points of self- and non-self-contractive type operators in metric spaces, Hilbert spaces, or several classes of Banach spaces, while, for strict contractive type operators, the Picard iteration can be used to approximate the unique fixed point, for operators satisfying slightly weaker contractive type conditions, instead of Picard iteration, which does not generally converge; it was necessary to consider other fixed point iteration procedures. The Krasnoselskij iteration, the Mann iteration, and the Ishikawa iteration are certainly the most studied of these fixed point iteration procedures. Other iterations which have been studied also are Implicit Mann, Implicit Ishikawa, Thianwan, S-iteration, and hybrid Picard-Mann iterations. Recently, Wahab and Rauf [1] obtained some results on a faster implicit hybrid Kirk-multistep schemes for contractive type operators, to mention but a few.

Our aim in this paper is to establish the convergence and convergence rate of some two-step iterative schemes with errors using Zamfirescu operator in Banach spaces.

#### 2. Preliminaries

The Picard iteration process is defined by the sequence and the concept of Picard iteration process with error is defined as follows: where satisfy .

In [2], Banach proved the convergence of (1) with the aid of the following contractive mapping: where , is a metric space, , and . Banach’s theorem is given as follows.

Theorem 1. *Let be a metric space and be a contraction map on . Then has a unique fixed point .*

When condition (3) is weaker, Picard iteration (1) will no longer converge to a fixed point. So, other iterations such as Mann iteration, Picard-Mann iteration, Ishikawa iteration, S-iteration, and Thianwan iteration would be considered.

Mann [3] defined an iteration process by the sequence, for , of a closed subset of normed space: where is a sequence in .

The concept of Mann iteration with error was discussed in [4] and it is given by the sequence where is a sequence in and satisfy .

Ishikawa [5] defined another iteration process given by the sequencewhere and are sequences in . The Ishikawa iteration is a double Mann iteration and has better approximation than Mann iteration (5). The Ishikawa iteration with errors is given aswhere and satisfy and . See [4].

In an attempt to reduce computational cost, Agarwal et al. [6] defined another iteration called S-iteration which is independent of Mann and Ishikawa iterations and it is defined by the sequence where and are sequences in .

The S-iteration with errors can be given aswhere and satisfy and .

In [7], Thianwan defined a new iteration process given by the sequence where and are sequences in .

The Thianwan iteration with errors is given by where and satisfy and .

Khan [8] gave a different perspective to iteration procedure. He introduced the following hybrid Picard-Mann iterative scheme for a single nonexpansive mapping which is defined as where is a sequence in with . While the Picard-Mann iteration process with errors is given as where and satisfy and .

The most generalized operator used to approximate fixed point is the one proved by Zamfirescu [9]. The Zamfirescu operator was obtained from the Banach [2], Kannan [10], and Chatterjea [11] contractive mappings as follows. The operator is called a Kannan mapping if there exists such that Another similar definition due to Chatterjea mapping is as follows: there exists such thatBy combining (3), (14), and (15) conditions, we have the Zamfirescu operator given, for , by The equivalence of (16) is given as follows: where and .

The following Lemma is useful in the proof of our results.

Lemma 2 (see [12]). *Let be a real number such that and is a sequence of nonnegative numbers such that ; then for any sequence of positive numbers satisfying we have*

The Zamfirescu operator was used to prove the strong convergence of (1) as follows.

Theorem 3. *Let be a nonempty subset of a normed space . Let be z-operator. If , , . Then converges strongly to a fixed point of .*

*Proof. *By Lemma 2, has a unique fixed point in , say . Let ; since is a -operator, at least one of each conditions is satisfied. If holds, then is equivalent to This implies Therefore, Similarly, if holds, we obtain Let Then we have and in view of (25) it results in that inequalities (23) and (24) become Now, for and in (26), we obtain which implies By Lemma 2, we conclude that converges strongly to .

#### 3. Analytical Results

We present our main results using* z*-operators (16) on normed spaces for the two-step iterative schemes with errors defined in the last section.

Throughout, denotes the set of all positive integers, is nonempty convex subset of a complete normed space , and is a self-map.

Theorem 4. *Let be a nonempty subset of a normed space . Let be -operator (16). Let be defined with iterative process (5). If , , . Then converges strongly to a fixed point of .*

*Proof. *Let ; by using (5) and (16), we have Substituting (28) in (29), Hence, by Lemma 2 and using the fact that , , and , it results in that Therefore,

Theorem 5. *Let be a nonempty subset of a normed . Let be -operator (16). Let be defined with iterative process (13). If , , . Then converges strongly to a fixed point of .*

*Proof. *From (26), we have Also from (28), we have Now, using (13) and (28), we have This implies Also, Substituting (37) in (36), we have Using (28) in (38), we have where .

By Lemma 2 and using the fact that , and , this becomes Hence,

Theorem 6. *Let be a nonempty subset of a normed space . Let be -operator. Let be defined with iterative process (7). If , , . Then, converges strongly to a fixed point of .*

*Proof. *From (26), we have Also from (28), we have Now, by combining (7) and (28), we have This gives Also, from (7), we have This also gives By combining (47) and (45), we have Using (28) in (48), we have By letting and by Lemma 2, using the fact that , , and , we get Therefore,

Theorem 7. *Let be a nonempty subset of a normed space . Let be -operator (16). Let be defined with iterative process (9). If , , . Then converges strongly to a fixed point of .*

*Proof. *From (26), we have Also from (28), we have Applying (28) to (9), we obtain Also, from (9), we have Substituting (55) in (54), we have Inequality (56) becomes By letting and by Lemma 2, using the condition of the theorem, we have Hence,

Theorem 8. *Let be a nonempty subset of a normed space . Let be -operator (16). Let be defined with iterative process (11). If , , . Then converges strongly to a fixed point of .*

*Proof. *From (26), we have Also from (28), we have Now, applying (28) to in (11), we get This implies Also, applying (28) to in (11), we get This gives By combining (63) and (65), we have Thus, applying Lemma 2 and the condition of the theorem, we obtain Therefore,

#### 4. Numerical Results

In this section, we support our analytical results with the following two numerical examples.

*Example 9. *Let the function be defined by with fixed point . Initial guess is , .

*Example 10. *Let the function be defined by with fixed point . Initial guess is , , .

The results for Examples 9 and 10 with various iterations are presented in Tables 1 and 2, respectively.