Abstract
An intermixed algorithm for two strict pseudo-contractions in Hilbert spaces have been presented. It is shown that the suggested algorithms converge strongly to the fixed points of two strict pseudo-contractions, independently. As a special case, we can find the common fixed points of two strict pseudo-contractions in Hilbert spaces.
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1 Introduction
Let C be a nonempty closed convex subset of a real Hilbert space H with its inner product \(\langle\cdot, \cdot\rangle\) and norm \(\| \cdot\|\).
Definition 1.1
A map** \(T:C\to C\) is said to be nonexpansive if
for all \(x,y\in C\).
We use \(\operatorname{Fix}(T)\) to denote the set of fixed points of T.
Definition 1.2
A map** \(T:C\to C\) is said to be strictly pseudo-contractive if there exists a constant \(0\leq\lambda<1\) such that
Remark 1.3
It is well known that the class of strictly pseudo-contractive map**s properly includes the class of nonexpansive map**s.
Iterative construction of fixed points of nonlinear map**s has a long history and is still an active field in the nonlinear functional analysis. Let C be a nonempty closed convex subset of a real Hilbert space. Let \(T:C\to C\) be a nonlinear map**. Let \(\{\alpha _{n}\}\) be a real number sequence in \((0,1)\). For arbitrarily fixed \(x_{0}\in C\), define a sequence \(\{x_{n}\}\) in the following manner:
Iteration (1.1) is said to be a Mann iteration [1]; it has been studied extensively in the literature. If T is a nonexpansive map** with \(\operatorname{Fix}(T)\ne\emptyset\) and \(\{\alpha_{n}\}\) satisfies the condition \(\sum_{n=0}^{\infty}\alpha_{n}(1-\alpha_{n})=\infty\), then the sequence \(\{x_{n}\}\) generated by Mann’s algorithm converges weakly to a fixed point of T [2]. Now, it is well known that Mann’s algorithm fails, in general, to converge strongly in the setting of infinite-dimensional Hilbert spaces [3]. Iterative methods for nonexpansive map**s have been investigated extensively in the literature; see [2–27] and the references therein. However, iterative methods for strictly pseudo-contractive map**s are far less developed than those for nonexpansive map**s though Browder and Petryshyn [4] initiated their work in 1967. However, strictly pseudo-contractive map**s have more powerful applications than nonexpansive map**s, for example, to solve inverse problems (see Scherzer [21]). Therefore it is interesting to develop the algorithms for finding the fixed points of strictly pseudo-contractive map**s. Now, we know that Mann’s algorithm is not good enough for approximating fixed points of (even if Lipschitz continuous) pseudo-contractions. Thus, we have to find other type of iterative algorithms; see [28–35]. The first such an attempt was done by Ishikawa [7] who introduced the following Ishikawa algorithm:
where \(\{\alpha_{n}\}\) and \(\{\beta_{n}\}\) are sequences in the interval \([0,1]\), T is a (nonlinear) self-map** of C, and the initial guess \(x_{0}\in C\) is selected arbitrarily. (Ishikawa’s algorithm can be viewed as a double-step (or two-level) Mann’s algorithm.) Ishikawa proved that his algorithm converges in norm to a fixed point of a Lipschitz pseudo-contraction T if \(\{\alpha_{n}\}\) and \(\{\beta_{n}\}\) satisfy certain conditions and if T is compact.
On the other hand, iterative methods for approximating the common fixed points of a finite (or an infinite) family of nonlinear map**s have been considered by many authors. For the related work, we refer the reader to [22–26, 32, 33]. Above discussion suggests the following question.
Question 1.4
Could we construct an iterative algorithm such that it converges strongly to the fixed points of a finite family of strict pseudo-contractions?
It is our purpose in this paper to construct redundant intermixed algorithms for two strict pseudo-contractions. It is shown that the suggested algorithms converge strongly to the fixed points of two strict pseudo-contractions, independently. As a special case, we can find the common fixed points of two strict pseudo-contractions in Hilbert spaces.
2 Preliminaries
Let C be a nonempty closed convex subset of H. The (nearest point or metric) projection from H onto C is defined as follows: for each point \(x\in H\), \(P_{C}x\) is the unique point in C with the property:
Note that \(P_{C}\) is characterized by the inequality:
Consequently, \(P_{C}\) is nonexpansive.
In order to prove our main results, we need the following well-known lemmas.
Lemma 2.1
([28])
Let C be a nonempty closed convex subset of a real Hilbert space H. Let \(T:C\to C\) be a λ-strictly pseudo-contractive map**. Then \(I-T\) is demi-closed at 0, i.e., if \(x_{n} \rightharpoonup x\in C\) and \(x_{n}-Tx_{n}\to0\), then \(x=Tx\).
Lemma 2.2
([18])
Let \(\{x_{n}\}\) and \(\{y_{n}\} \) be bounded sequences in a Banach space E and \(\{\beta_{n}\}\) be a sequence in \([0,1]\) with \(0<\liminf_{n\rightarrow\infty}\beta_{n}\leq \limsup_{n\rightarrow \infty}\beta_{n}<1\). Suppose that \(x_{n+1}=(1-\beta_{n})x_{n}+\beta_{n}z_{n}\) for all \(n\geq0\) and \(\limsup_{n\rightarrow \infty}(\|z_{n+1}-z_{n}\|-\|x_{n+1}-x_{n}\|)\leq0\). Then \(\lim_{n\rightarrow\infty}\|z_{n}-x_{n}\|=0\).
Lemma 2.3
([17])
Assume \(\{ a_{n}\}\) is a sequence of nonnegative real numbers such that \(a_{n+1}\leq (1-\gamma_{n})a_{n}+\gamma_{n}\delta_{n}\), \(n\geq0\) where \(\{\gamma_{n}\}\) is a sequence in \((0,1)\) and \(\{\delta_{n}\}\) is a sequence in R such that
-
(i)
\(\sum_{n=0}^{\infty}\gamma_{n}=\infty\);
-
(ii)
\(\limsup_{n\rightarrow\infty}\delta_{n}\leq0\) or \(\sum_{n=0}^{\infty}|\delta_{n}\gamma_{n}|<\infty\).
Then \(\lim_{n\rightarrow\infty}a_{n}=0\).
3 Main results
Let C be a nonempty closed convex subset of a real Hilbert space H. Let \(T:C\to C\) be a λ-strict pseudo-contraction. Let \(f:C\to H\) be a \(\rho_{1}\)-contraction and \(g:C\to H\) be a \(\rho _{2}\)-contraction. (A map** \(f:C\to H\) is said to be contractive if \(\| f(x)-f(y)\|\le\rho\|x-y\|\) for some \(\rho\in[0,1)\) and for all \(x, y\in C\).) Let \(k\in(0,1-\lambda)\) be a constant.
Now we propose the following redundant intermixed algorithm for two strict pseudo-contractions S and T.
Algorithm 3.1
For arbitrarily given \(x_{0}\in C\), \(y_{0}\in C\), let the sequences \(\{x_{n}\}\) and \(\{y_{n}\}\) be generated iteratively by
where \(\{\alpha_{n}\}\) and \(\{\beta_{n}\}\) are two real number sequences in \((0,1)\).
Remark 3.2
Note that the definition of the sequence \(\{x_{n}\}\) is involved in the sequence \(\{y_{n}\}\) and the definition of the sequence \(\{y_{n}\}\) is also involved in the sequence \(\{x_{n}\}\). So, this algorithm is said to be the redundant intermixed algorithm. We can use this algorithm to find the fixed points of S and T, independently.
Theorem 3.3
Suppose that \(\operatorname{Fix}(S)\ne\emptyset\) and \(\operatorname{Fix}(T)\neq \emptyset\). Assume the following conditions are satisfied:
-
(C1)
\(\lim_{n\to\infty}\alpha_{n}=0\) and \(\sum_{n=0}^{\infty}\alpha_{n}=\infty\);
-
(C2)
\(\beta_{n}\in[\xi_{1}, \xi_{2}]\subset(0,1)\) for all \(n\ge0\).
Then the sequences \(\{x_{n}\}\) and \(\{y_{n}\}\) generated by (3.1) converge strongly to the fixed points \(P_{\operatorname{Fix}(T)} f(y^{*})\) and \(P_{\operatorname{Fix}(S)} g(x^{*})\) of T and S, respectively, where \(x^{*}\in \operatorname{Fix}(T)\) and \(y^{*}\in \operatorname{Fix}(S)\).
Proof
First, we give the following propositions.
Proposition 3.4
The sequences \(\{x_{n}\}\) and \(\{y_{n}\}\) are bounded.
In order to prove this proposition, we need the following result.
Proposition 3.5
The map** \(P_{C}[\alpha f +(1-k-\alpha)I+kT]\) is contractive for small enough α.
Proof
Let \(x,y\in C\). Then we have
Thus, we get
for all \(x,y\in C\) as \(k\leq(1-\alpha)(1-\lambda)\) (that is, \(\alpha \le1-\frac{k}{1-\lambda}\)). □
Next, we prove Proposition 3.4.
Proof
Since \(\operatorname{Fix}(S)\ne\emptyset\) and \(\operatorname{Fix}(T)\neq \emptyset\), we can choose \(x^{*}\in \operatorname{Fix}(T)\) and \(y^{*}\in \operatorname{Fix}(S)\). From (3.1), we have
where \(\rho=\max\{\rho_{1},\rho_{2}\}\). Similarly, we have
Hence, we obtain
By induction, we have
So, \(\{x_{n}\}\) and \(\{y_{n}\}\) are bounded. □
Proposition 3.6
\(\|x_{n}-Tx_{n}\|\to0\) and \(\|y_{n}-Sy_{n}\|\to0\).
Proof
We first estimate \(\|x_{n+1}-x_{n}\|\). Set \(u_{n}=P_{C}[\alpha _{n}f(y_{n})+(1-k-\alpha_{n})x_{n}+kTx_{n}]\), \(n\ge0\). It follows that
Since \(\alpha_{n}\to0\), we deduce that
From Lemma 2.2, we get
From (3.1), we derive
Thus,
It follows that
Similarly, we can obtain
□
By Proposition 3.5, we know that the map** \(P_{C}[\alpha f +(1-k-\alpha )I+kT]\) is contractive for small enough α. Thus, the equation \(x=P_{C}[tf(x) +(1-k-t)x+kTx]\) has a unique fixed point, denoted by \(x_{t}\), that is,
for small enough t. In order to prove Theorem 3.3, we need the following lemma.
Lemma 3.7
Suppose \(\operatorname{Fix}(T)\neq\emptyset\). Then, as \(t\to0\), the net \(\{x_{t}\}\) defined by (3.2) converges strongly to a fixed point of T.
Proof
Let \(x^{*}\in \operatorname{Fix}(T)\). From (3.2), we have
hence,
Thus, \(\{x_{t}\}\) is bounded. Again, from (3.2), we get
It follows that
Let \(\{t_{n}\}\subset(0,1)\). Assume that \(t_{n}\to0\) as \(n\to\infty\). Put \(x_{n}:=x_{t_{n}}\). We have \(\lim_{n\to\infty}\| x_{n}-Tx_{n}\|=0\). Set \(y_{t}=tf(x_{t})+(1-k-t)x_{t}+kTx_{t}\), for all t. Then we have \(x_{t}=P_{C}y_{t}\), and for any \(x^{*}\in \operatorname{Fix}(T)\),
From the property of the metric projection, we deduce
So,
Hence,
By similar arguments to [28], we find that the net \(\{x_{t}\}\) converges strongly to \(x^{*}\in \operatorname{Fix}(T)\). This completes the proof. □
Remark 3.8
From Lemma 3.7, we know that the net \(\{x_{t}\}\) defined by \(x_{t}=P_{C}[tu +(1-k-t)x_{t}+kTx_{t}]\) where \(u\in H\), converges to \(P_{\operatorname{Fix}(T)} u\). Let \(x^{*}\in \operatorname{Fix}(T)\) and \(y^{*}\in \operatorname{Fix}(S)\). If we take \(u=f(y^{*})\), then the net \(\{x_{t}\}\) defined by \(x_{t}=P_{C}[tf(y^{*}) +(1-k-t)x_{t}+kTx_{t}]\), converges to \(P_{\operatorname{Fix}(T)} f(y^{*})\).
Finally, we prove that \(x_{n}\to P_{\operatorname{Fix}(T)} f(y^{*})\) and \(y_{n}\to P_{\operatorname{Fix}(S)}g(x^{*})\), where \(x^{*}\in \operatorname{Fix}(T)\) and \(y^{*}\in \operatorname{Fix}(S)\). We note the following fact. If the sequence \(\{w_{n}\}\) is bounded and \(\| w_{n}-Tw_{n}\|\to0\), we easily deduce that
Set \(v_{n}=P_{C}[\alpha_{n}g(x_{n})+(1-k-\alpha_{n})y_{n}+kSy_{n}]\) for all \(n\ge0\). Thus, we deduce that the sequences \(\{u_{n}\}\) and \(\{v_{n}\}\) satisfy: (1) \(\{u_{n}\}\) and \(\{ v_{n}\}\) are bounded; (2) \(\|u_{n}-Tu_{n}\|\to0\) and \(\|v_{n}-Sv_{n}\|\to0\). Therefore,
and
Next, we estimate \(\|u_{n}-P_{\operatorname{Fix}(T)} f(y^{*})\|\). Set \(\tilde{u}_{n}=\alpha _{n}f(y_{n})+(1-k-\alpha_{n})x_{n}+kTx_{n}\) and \(\tilde{v}_{n}=\alpha _{n}g(x_{n})+(1-k-\alpha_{n})y_{n}+kSy_{n}\) for all n. We have
It follows that
Thus,
Similarly, we also have
Therefore,
We can check that all assumptions of Lemma 2.3 are satisfied. Therefore, \(x_{n}\to P_{\operatorname{Fix}(T)} f(y^{*})\) and \(y_{n}\to P_{\operatorname{Fix}(S)} g(x^{*})\). This completes the proof. □
Algorithm 3.9
For arbitrarily given \(x_{0}\in C\), let the sequence \(\{x_{n}\}\) be generated iteratively by
where \(\{\alpha_{n}\}\) and \(\{\beta_{n}\}\) are two real number sequences in \((0,1)\).
Theorem 3.10
Suppose \(\operatorname{Fix}(T)\neq \emptyset\). Assume the following conditions are satisfied:
-
(C1)
\(\lim_{n\to\infty}\alpha_{n}=0\) and \(\sum_{n=0}^{\infty}\alpha_{n}=\infty\);
-
(C2)
\(\beta_{n}\in[\xi_{1}, \xi_{2}]\subset(0,1)\) for all \(n\ge0\).
Then the sequence \(\{x_{n}\}\) generated by (3.3) converge strongly to the fixed points \(P_{\operatorname{Fix}(T)}(0)\), which is the minimum norm element in \(\operatorname{Fix}(T)\).
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Acknowledgements
The authors are grateful to the three reviewers for their valuable comments and suggestions. Zhangsong Yao was supported by the Scientific Research Project of Nan**g **aozhuang University (2015NXY46).
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Yao, Z., Kang, S.M. & Li, HJ. An intermixed algorithm for strict pseudo-contractions in Hilbert spaces. Fixed Point Theory Appl 2015, 206 (2015). https://doi.org/10.1186/s13663-015-0454-7
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DOI: https://doi.org/10.1186/s13663-015-0454-7