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Approximating fixed points for continuous functions on an arbitrary interval
Journal of Inequalities and Applications volume 2013, Article number: 214 (2013)
Abstract
In this research article, we introduce a new iterative method for solving a fixed point problem of continuous functions on an arbitrary interval. We then prove the convergence theorem of the proposed algorithm. We finally give numerical examples to compare the result with Mann, Ishikawa and Noor iterations. Our main results extend the corresponding results in the literature.
MSC:47H09, 47H10.
1 Introduction
Let C be a closed interval on the real line and let be a continuous function. A point is called a fixed point of f if .
One classical way to approximate a fixed point of a nonlinear mapping was introduced in 1953 by Mann [1] as follows: a sequence defined by and
for all , where is a sequence in . Such an iteration process is known as Mann iteration. In 1991, Borwein and Borwein [2] proved the convergence theorem for a continuous function on the closed and bounded interval in the real line by using iteration (1.1).
Another classical iteration process was introduced by Ishikawa [3] as follows: a sequence defined by and
for all , where and are sequences in . Such an iterative method is known as Ishikawa iteration. In 2006, Qing and Qihou [4] proved the convergence theorem of the sequence generated by iteration (1.2) for a continuous function on the closed interval in the real line (see also [5]).
In 2000, Noor [6] defined the following iterative scheme by and
for all , where , and are sequences in . Such an iterative method is known as Noor iteration. Phuengrattana and Suantai [7] considered the convergence of Noor iteration for continuous functions on an arbitrary interval in the real line.
In this paper, motivated by the previous ones, we introduce a new modified iteration process for solving a fixed point problem for continuous functions on an arbitrary interval in the real line. Numerical examples are also presented to compare the result with Mann, Ishikawa and Noor iterations.
2 Main results
We begin this section by proving the following crucial lemmas.
Lemma 2.1 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , , , and be sequences in with and . Let be a sequence generated iteratively by and
where , , and .
If , then a is a fixed point of f.
Proof Let and suppose . Then is bounded. So, is bounded by the continuity of f. So are , , and . Moreover, since and . We also have since and .
From (2.1) we obtain
Let , and . Then we observe
So, from (2.2) we obtain
It is easy to see that since and . Similarly, we have since and . This shows that is a divergent sequence since and . This contradicts the convergence of . Hence and a is a fixed point of f. □
Lemma 2.2 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , , , and be sequences in with and . Let be a sequence generated iteratively by and
where , , and .
If is bounded, then is convergent.
Proof Suppose is not convergent. Let and . Then . We first show that if , then . Suppose . Without loss of generality, we suppose . Since f is continuous, there exists δ with such that for ,
Since is bounded and f is continuous, is bounded. Hence , , and are all bounded. Using
we can easily show that , and . Thus there exists a positive integer N such that for all ,
Since , there exists such that . Let , then . For , there exist two cases as follows.
-
(i)
, then using (2.3). So, we have .
-
(ii)
, then and by (2.3). So, we obtain , , . Hence
(2.4)
We observe that
From (2.2), (2.4) and (2.5), we have
Thus . From (i) and (ii), we have . Similarly, we get that , , …. Thus we have for all . So, , which is a contradiction with . Thus .
We next consider the following two cases.
-
(i)
There exists such that . Then . It follows that
and
Hence, we obtain
Similarly, we obtain . So, we conclude that . Since there exists , . This shows that , which is a contradiction.
-
(ii)
For all or . Since and , there exists such that for . So, it is always that for ; or it is always that for . If for , then , which is a contradiction with . If for , then , which is a contradiction with . Thus we conclude that . This completes the proof. □
We are now ready to prove the main results of this paper.
Theorem 2.3 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , , , and be sequences in with and . Let be a sequence generated iteratively by and
where , , and .
If is bounded, then converges to a fixed point of f.
Proof Let be a bounded sequence. Then, by Lemma (2.2), is a convergent sequence. Hence, by Lemma (2.1), it converges to a fixed point of f. □
As a direct consequence of Theorem 2.3, we obtain the following result.
Theorem 2.4 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , , , and be sequences in with and . Let be a sequence generated iteratively by and
where , , and .
Then converges to a fixed point of f if and only if is bounded.
Corollary 2.5 Let be a continuous function. Let , , , and be sequences in with and . Let be a sequence generated iteratively by and
where , , and .
Then converges to a fixed point of f.
If we take , then we obtain the following result.
Corollary 2.6 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , and be sequences in . Let be a sequence generated iteratively by and
where , , and .
Then converges to a fixed point of f if and only if is bounded.
If we take and , then we obtain the following result.
Corollary 2.7 Let C be a closed interval on the real line (can be unbounded) and let be a continuous function. Let , and be sequences in . Let be a sequence generated iteratively by and
where , , and .
Then converges to a fixed point of f if and only if is bounded.
Remark 2.8 Corollary 2.7 extends the main result obtained in [8] from the modified Ishikawa iteration to the modified Noor iteration.
3 Numerical examples
In this section, we give numerical examples to demonstrate the convergence of the algorithm defined in this paper. For convenience, we call the iteration (2.1) the CP-iteration.
Example 3.1 Let be defined by . Then f is a continuous function. Use the initial point and the control conditions , , , and .
Example 3.2 Let be defined by . Then f is a continuous function. Use the initial point and the control conditions , , , and .
Remark 3.3 From Table 1, Figure 1, Table 2 and Figure 2, we observe that the sequence generated by the CP-iteration converges to a fixed point faster than that of Mann, Ishikawa and Noor iterations.
References
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Qing Y, Cho SY, Qin X: Convergence of Ishikawa iteration with error terms on an arbitrary interval. Commun. Korean Math. Soc. 2011, 26: 229–235. 10.4134/CKMS.2011.26.2.229
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Phuengrattana W, Suantai S: On the rate of convergence of Mann Ishikawa, Noor and SP iterations for continuous functions on an arbitrary interval. J. Comput. Appl. Math. 2011, 235: 3006–3014. 10.1016/j.cam.2010.12.022
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Acknowledgements
The authors wish to thank Professor Suthep Suantai for the valuable guidance and suggestion. The first author was supported by the Thailand Research Fund, the Commission on Higher Education, and University of Phayao under Grant No. MRG5580016.
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PC and NP contributed equally. All authors read and approved the final manuscript.
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Cholamjiak, P., Pholasa, N. Approximating fixed points for continuous functions on an arbitrary interval. J Inequal Appl 2013, 214 (2013). https://doi.org/10.1186/1029-242X-2013-214
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DOI: https://doi.org/10.1186/1029-242X-2013-214