Abstract
This paper is intended for a general mathematical audience. The examples show how the study of existence of fixed points of averaged mappings \(T_{\lambda }= (1-\lambda )I+ \lambda T\), where \(0<\lambda <1\) and I is the identity operator, can help in the study of existence of fixed points of mappings T.
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1 Known results
Let X be a Banach space with norm \(\Vert \cdot \Vert \), and C be a nonempty closed convex subset of X. Let \(T:C\rightarrow C\) be a map. We denote the set of fixed points of T by \({\text {Fix}}(T)\).
Definition 1.1
[1] Let T be a self-mapping on a nonempty convex subset C of a Banach space. By an averaged mapping we mean one of the form \(T_{\lambda }= (1-\lambda )I+ \lambda T:C\rightarrow C\), where \(0<\lambda <1\), I is the identity operator.
Generally,
-
(1)
T and \(T_{\lambda }\), both have the same fixed point set (\({\text {Fix}}(T_{\lambda })={\text {Fix}}(T)\)).
Indeed, if \(p\in {\text {Fix}}(T)\), then \(T_{\lambda }p=(1-\lambda )p+\lambda Tp=p\). If \(p\in {\text {Fix}}(T_{\lambda })\), then \(p=T_{\lambda }p=(1-\lambda )p+\lambda Tp\), so \(Tp=p\).
-
(2)
An averaged mapping of the averaged mapping is an averaged mapping.
Let \(\lambda _1,\lambda _2\in (0,1)\). If \(T_{\lambda _1}=(1-\lambda _1)I+\lambda _1T\), then \((T_{\lambda _1})_{\lambda _2}=(1-\lambda _2)I+\lambda _2T_{\lambda _1}=(1-\lambda _2)I+ \lambda _2[(1-\lambda _1)I+\lambda _1T)]=(1-\lambda _1\lambda _2)I+\lambda _1\lambda _2T\).
-
(3)
In the class of nonexpansive mappings \(F:C\rightarrow C\) (satisfying \(\Vert Fx-Fy\Vert \leqslant \Vert x-y\Vert \) for all \(x,y\in C\)), if \(S=(1-\lambda )T+\lambda V\) for some \(\lambda \in (0,1)\) and if T is averaged and V is nonexpansive, then S is averaged.
Indeed, if T is averaged, we have a number \(\alpha \in (0,1)\) and a nonexpansive mapping \(U:C\rightarrow C\) such that \(T=(1-\alpha )I+\alpha U\). Put \(\gamma =\lambda + \alpha (1-\lambda )\in (0,1)\) and \(W=\frac{(1-\lambda )\alpha }{\gamma }U+\frac{\lambda }{\gamma }V\). Since W is a convex combination of two nonexpansive mappings, W remains nonexpansive. Now, we can express S as
$$\begin{aligned} S= & {} (1-\lambda )T+\lambda V= (1-\lambda )((1-\alpha )I+\alpha U)+\lambda V=\\= & {} (1-\lambda )(1-\alpha )I+(1-\lambda )U+\lambda V=(1-\gamma )I+\gamma W. \end{aligned}$$So S is averaged.
-
(4)
In the class of nonexpansive mappings \(F:C \rightarrow C\), if S and T are both averaged, then the composite ST is averaged.
If S is averaged, we have a number \(\alpha \in (0,1)\) and a nonexpansive mapping \(U:C\rightarrow C\) such that \(S=(1-\alpha )I+\alpha U\). Then \(ST=(1-\alpha )T+\alpha UT\). Since UT is again nonexpansive, by (2), ST is averaged.
-
(5)
Let H be a real Hilbert space with inner product \(\langle \cdot ,\cdot \rangle \) and C given a closed convex subset of H. Projection \(P_C:H\rightarrow C\) is the unique point in C such that \(\Vert x - P_Cx\Vert =\inf \{\Vert x-y\Vert :y\in C\}\). In particular, \(P_C\) is nonexpansive and can be written as the mean average of a nonexpansive mapping and the identity: \(P_C=\frac{1}{2}(I+S)\).
Mapping \(T:H\rightarrow H\) such that \(\langle x-y,Tx -Ty\rangle \geqslant \Vert Tx - Ty\Vert ^2\) for all \(x,y\in H\) is firmly nonexpansive (a projection is firmly nonexpansive). It is easily seen that T is firmly nonexpansive if and only if T can be expressed as \(\frac{1}{2}(I+V)\), where V is nonexpansive, see [14].
Thus, a firmly nonexpansive mapping (in particular, a projection) is an averaged map with \(\lambda = \frac{1}{2}\), see (2).
-
(5)
Let H be a real Hilbert space. If \(S,T:H\rightarrow H\) are both averaged (in the class of nonexpansive mappings) and if S and T have a common fixed point, then \( {\text {Fix}}(S)\cap {\text {Fix}}(T)={\text {Fix}}(ST)\), details in [24].
In Banach spaces when T is nonexpansive mapping, \(T_{\lambda }\) may have much more felicitous asymptotic behavior than the original mapping.
Krasnosielskij [19] was the first to notice this regularizing effect (for \(\lambda =\frac{1}{2}\)). He proved
Theorem 1.2
Let C be a nonempty compact convex subset of a uniformly convex Banach space X and \(T:C\rightarrow C\) a nonexpansive mapping. Then the iterates \(\{T^n_{\frac{1}{2}}\}\) (i.e. the sequence \(x_{n+1}=\frac{1}{2}(x_n+Tx_n)\), \(x_1\in C\) be an arbitrary, \(n=1,2,3,\ldots \) ) converges strongly to a fixed point of T.
Since the existence of a fixed point is guaranteed by Schauder’s theorem, the novelty of this result lays in this asymptotic statement.
Shortly thereafter, Schaefer [23] proved that \(\{T^n_{\lambda }\}\) converges strongly for all \(0<\lambda <1\), and in 1966, Edelstein [13] noted that the uniform convexity of X can be relaxed to strict convexity.
In 1966, Browder and Petryshyn [9] separated out the notion of asymptotic regularity: T is said to be asymptotically regular on C if for each \(x\in C\), \(T^{n+1}x-T^nx \rightarrow 0\) as \(n\rightarrow \infty \). Obviously, every Banach contraction or Kannan mapping is an asymptotically regular mapping having exactly one fixed point [7, 15]. For more information, see [1, 10, 12, 21, 22].
In 1976, Ishikawa [17] proved the truly astonishing result with no restrictions on the geometry of the Banach space!
Theorem 1.3
If C is nonempty bounded closed convex subset of a Banach space X and \(T:C\rightarrow C\) is nonexpansive, then the mapping \(T_{\lambda }\) is asymptotically regular for each \(0<\lambda <1\).
Kirk [18] has extended Ishikawa’s method to metric spaces with convexity structure. For the rate of asymptotic regularity, see Bruck [11] and reference therein.
2 Examples
The search for fixed points for mappings \(T: C\rightarrow C\) that do not fit into the “known patterns” is difficult.
Instead of the mapping T, we can, for example, look for the fixed points of mapping \(T^n\), \(n=2,3,\ldots \), nth iterate of T. Sometimes it is easier, see the following example.
Example 2.1
Let \(\Vert \cdot \Vert \) be the usual norm on \(\mathbb {R}\). Define \(Tx=0\) if \(x\leqslant 1\) and \(Tx=1\) if \(x>1\). Then T is discontinuous, T does not satisfy the Kannan type condition, i.e. there is no constant \(K\in [0,\infty )\) such that for all \(x,y\in X\) satisfying \(\Vert Tx-Ty\Vert \leqslant K\cdot \{\Vert x-Tx\Vert +\Vert y-Ty\Vert \}\), take \(x=0\) and \(y=1+\varepsilon \), \(\varepsilon >0\).
On the other hand, \(T^2x\equiv 0\), so \(T^2\) is a contraction and \(p=0\) is the unique fixed point of T.
When the examination of successive iterations fails, we can look for fixed points of the averaged mapping instead of the T mapping. As we already know, the averaged mappings may have more favorable properties. In particular, averaged mapping \(T_{\lambda }\) (for \(\lambda \in (0,1)\)) has a fixed point if and only if mapping T has it, see (1).
Example 2.2
Let \(X=\mathbb {C}\) with the norm \(\Vert z\Vert =\vert z\vert \) for complex numbers, \(C=\{z\in \mathbb {C}:\vert z\vert \leqslant 1\}\), and \(T:C\rightarrow C\) be a mapping defined by \(Tz=iz\) (T is an anticlockwise rotation of \(\frac{\pi }{4}\) about the origin of coordinates). Then T is nonexpansive (\(\vert T((0,0))-T((1,0))\vert =1 =\vert (0,0)-(1,0)\vert \)) with the origin as the only fixed point and T is not asymptotically regular. Indeed, let \(z\in \mathbb {C}\setminus \{0\}\). Then,
However, \(T_{\frac{1}{2}}=\frac{1}{2}(z+iz)\), \(z\in \mathbb {C}\), is a contraction, for \(z_1,z_2\in C\) we have,
Thus, \({\text {Fix}}(T_{\frac{1}{2}})=\{0\}={\text {Fix}}(T)\) and \(T^n_{\frac{1}{2}}z\rightarrow 0\) for all \(z\in C\) (so \(T_{\frac{1}{2}}\) is asymptotically regular).
Example 2.3
Let \(X=\mathbb {R}\) with the usual norm \(\Vert x\Vert =\vert x\vert \). Define a mapping \(T:\mathbb {R}\rightarrow \mathbb {R}\) by \(Tx=-3x+2\). For \(x=0\) and \(y=1\), we have \(\vert T0-T1\vert =3>1= \vert 0-1\vert \), so T is not a nonexpansive mapping. But then \(T_{\frac{1}{2}}x=\frac{1}{2}(x+Tx) = 1-x\), \(x\in \mathbb {R}\), is isometry and \(T_{\frac{1}{2}}\) does not satisfy Kannan’s condition (\(\vert T_{\frac{1}{2}}0-T_{\frac{1}{2}}1\vert =1\) and \(\{\vert 0-T_{\frac{1}{2}}0\vert +\vert 1-T_{\frac{1}{2}}1\vert \}=2\)). But averaged mapping \((T_{\frac{1}{2}})_{\frac{1}{2}}=T_{\frac{1}{4}}x=(1-\frac{1}{4})x+ \frac{1}{4}Tx=\frac{1}{2}\) for all \(x\in \mathbb {R}\), and \(T_{\frac{1}{4}}\) is a contraction (see [4, Example 1 (2)]). Thus, \({\text {Fix}}(T_{\frac{1}{4}})={\text {Fix}}(T)\). Unfortunately, this procedure is not the rule. We will show this in the next example.
Example 2.4
Let \(X=[1,\infty )\) with the usual norm \(\Vert x\Vert =\vert x\vert \). Define a mapping \(T:X\rightarrow X\) by \(Tx=2x-1\). For \(x=1\) and \(y=2\), we have \(\vert T1-T2\vert =2>1= \vert 1-2\vert \), so T is not a nonexpansive mapping. Similarly for mappings \(T_{\lambda }=(1-\lambda )I+\lambda T\), \(0< \lambda <1\), we have \(\vert T_{\lambda }1-T_{\lambda }2\vert =1+\lambda >1 =\vert 1-2\vert \).
Example 2.5
Let \(X=l_{\infty }\) be a normed space of all bounded sequences \(x=(x_1,x_2,\ldots )\) of real numbers with the norm \(\Vert x\Vert =\sup \limits _{i}\vert x_i\vert \). Let \(C=\{x\in l_{\infty }:\Vert x \Vert \leqslant 1\}\). Define \(T:C\rightarrow C\) by \(Tx=(0,x_1^2,x_2^2,x_3^2,\ldots )\) for \(x=(x_1,x_2,x_3,\ldots )\) in C. Obviously, \({\text {Fix}}(T)=\{0\}\), where \(0=(0,0,0,\ldots )\).
If \(a=(\frac{3}{4},\frac{3}{4},\frac{3}{4},\ldots )\) and \(b=(\frac{1}{2},\frac{1}{2},\frac{1}{2},\ldots )\), then \(\Vert a-b\Vert =\Vert (\frac{1}{4},\frac{1}{4},\frac{1}{4},\ldots )\Vert =\frac{1}{4}\), and
Let \(T_{\lambda }x=(1-\lambda )x+\lambda Tx\), \(0<\lambda <1\). Then
Hence, T and averaged mappings \(T_{\lambda }\) for all \(0<\lambda <1\) are not nonexpansive.
We will illustrate the procedure described above with another examples.
Example 2.6
[15, Example 2.12] Let \(X=[0,\frac{\pi }{2}]\) with the usual norm \(\Vert x\Vert =\vert x\vert \) and \(T:X\rightarrow X\) be given by \(Tx=\cos x\). Then T is not a contraction. Suppose there exists \(M<1\) such that \(\vert \frac{\cos x - \cos y}{x - y}\vert \leqslant M\) for all \(x\ne y\). Letting \(y\rightarrow x\), we get \(\vert \sin x\vert \leqslant M\) for all \(x,y\in [0,\frac{\pi }{2}]\), which is false. Of course, T as a continuous mapping, by Schauder’s Theorem has a fixed point. This can be obtained by weaker means. Consider the mapping \(T_{\frac{1}{2}}x=\frac{1}{2}(x+\cos x)\), \(x\in X\). Let \(x,y\in X\) and \(x<y\). By Mean Value Theorem,
for some \(x<\xi <y\), so \(T_{\frac{1}{2}}\) is a contraction and \({\text {Fix}}(T)={\text {Fix}}(T_{\frac{1}{2}})=\{p\}\), \(p\approx 0.739\).
Example 2.7
Let \(X=[1,\infty )\) with the usual norm \(\Vert x\Vert =\vert x\vert \) and \(T:X\rightarrow X\) be given by \(Tx=\frac{1}{x^2}\). Since \(\vert T1-T(1+\frac{1}{10})\vert = \frac{21}{121}\approx 0.17 > 0.1 =\vert 1-(1+\frac{1}{10})\vert \), T is not a nonexpansive mapping. Consider the mapping \(T_{\frac{1}{2}}x=\frac{1}{2}(x+\frac{1}{x^2})\), \(x\geqslant 1\). By Mean Value Theorem,
for some \(x<\xi <y\), so \(T_{\frac{1}{2}}\) is a contraction and \({\text {Fix}}(T)={\text {Fix}}(T_{\frac{1}{2}})=\{1\}\).
Remark 2.8
Let F be a self-mapping on a nonempty convex subset C of a Banach space X. By a backwards mapping for F we mean the mapping \(T_F:D\rightarrow X\) (\(D\subset X\)) such that \(F=(1-\lambda )I+\lambda T_F\) for some \(0<\lambda <1\), i.e., \(T_F=(1-\frac{1}{\lambda })I+\frac{1}{\lambda }F\).
Generally,
-
(6)
F and \(T_F\)(\(=\)the backwards mapping for F) both have the same fixed point set, \({\text {Fix}}(F)={\text {Fix}}(T_F)\).
Indeed, if \(p\in {\text {Fix}}(F)\), then \(T_Fp=(1-\frac{1}{\lambda })p+\frac{1}{\lambda }Fp=p\). If \(p\in {\text {Fix}}(T_F)\), then \(T_Fp=(1-\frac{1}{\lambda })p+\frac{1}{\lambda }Fp=p\), so \(Fp=p\).
-
(7)
A backwards mapping of the backwards mapping is a backwards mapping.
Let \(\lambda _1,\lambda _2\in (0,1)\) and \(F:C\rightarrow C\). If \(T_F=(1-\frac{1}{\lambda _1})I+\frac{1}{\lambda _1}F\), then \(G_{T_F}=(1-\frac{1}{\lambda _2})I+\frac{1}{\lambda _2}T_F=(1-\frac{1}{\lambda _2})I+\frac{1}{\lambda _2}[(1-\frac{1}{\lambda _1})I+\frac{1}{\lambda _1}F]=(1-\frac{1}{\lambda _1\lambda _2})I+\frac{1}{\lambda _1\lambda _2}F\).
3 New results
The following two problems are significant in the study of fixed points:
-
(1)
What conditions on the structure of the ambient space X and / or on the properties of T must be added to assure that mapping T has at least one fixed point?
-
(2)
How one can locate and approximate such a fixed point?
The literature on the subject is very extensive and multi-threaded. In recently published papers by Berinde [2, 3], Berinde and Pǎcurar [5, 6] drew attention to the so-called enriched nonexpansive (contractive, Kannan, Chatterjea) mappings. In the next theorems, we will give some extension of the Berinde and Pǎcurar result for enriched contractions [4] and some earlier results from paper [16].
Let \(b\in [0,\infty )\) and let \(\mathcal {S}\) denote the class of those functions \(\alpha :[0,\infty )\rightarrow [0,b+1)\) which satisfy the simple condition: \(\alpha (t_n)\rightarrow b+1 \Rightarrow t_n\rightarrow 0\). (We do not assume that \(\alpha \) is continuous in any sense.) For example, \(\alpha _1 (t)=(b+1)e^{-t}\), \(\alpha _2(t)=\frac{b+1}{t+1}\), \(t>0\).
Theorem 3.1
Let \((X,\Vert \cdot \Vert )\) be a Banach space. Suppose \(T:X\rightarrow X\) is a mapping for which exists \(0\leqslant b,K<\infty \), \(\alpha \in \mathcal {S}\), such that for each \(x,y\in X\),
and \(T_{\frac{1}{b+1}}\) is an asymptotically regular mapping. If T is continuous, then T has a unique fixed point \(z\in X\) and for each \(x_0\in X\) a sequence given by \(x_{n+1}=T_{\frac{1}{b+1}}x_n\), \(n=0,1,\ldots \), converges to z as \(n\rightarrow \infty \).
Proof
Let us consider two cases.
- Case 1.:
-
When \(b=0\), then we can apply [16, Theorem 2.1].
- Case 2.:
-
Let \(b>0\) and let us denote \(\lambda = \frac{1}{b+1}\in (0,1)\). By (8), we get
$$\begin{aligned} \Vert \left( \frac{1-\lambda }{\lambda }\right) (x-y)+Tx-Ty \Vert \leqslant \alpha (\Vert x-y \Vert )\cdot \Vert x-y \Vert +K\cdot \{\Vert x-Tx \Vert + \Vert y-Ty \Vert \}, \end{aligned}$$which can be written in an equivalent form as
$$\begin{aligned} \Vert [(1-\lambda )x+\lambda Tx]-[(1-\lambda )y+\lambda Ty] \Vert\leqslant & {} \lambda \cdot \alpha (\Vert x-y \Vert )\cdot \Vert x-y \Vert \\&+K\cdot \{\Vert x-[(1-\lambda )x+\lambda Tx] \Vert \\&+ \Vert y-[(1-\lambda )y+\lambda Ty] \Vert , \end{aligned}$$and
$$\begin{aligned} \Vert T_{\lambda }x-T_{\lambda }y \Vert \leqslant \lambda \cdot \alpha (\Vert x-y \Vert )\cdot \Vert x-y \Vert +K\cdot \{\Vert x-T_{\lambda }x \Vert + \Vert y-T_{\lambda }y \Vert \}. \,\,(9) \end{aligned}$$Fix \(x_0\in X\) and let \(x_{n+1}=T_{\lambda }x_n\), \(n=0,1,2,\ldots \) If \(x_{n+1}=x_n\) for some \(n\in \mathbb {N}\cup \{0\}\), then \((1-\lambda )x_n+\lambda Tx_n=x_n\), so \(Tx_n=x_n\) is the fixed point of T. Suppose \(x_{n+1}\ne x_n\) for all \(n\geqslant 0\). Assume that \(\{x_n\}\) is not a Cauchy sequence. Then \(\limsup \limits _{n,m\rightarrow \infty }\Vert x_n-x_m\Vert >0\). In view of triangle inequality and (9), we get
$$\begin{aligned} \Vert x_n- x_m\Vert\leqslant & {} \Vert x_n-x_{n+1}\Vert +\Vert x_{n+1}-x_{m+1}\Vert +\Vert x_{m+1}-x_m\Vert \\\leqslant & {} \lambda \cdot \alpha (\Vert x_n-x_m\Vert )\Vert x_n-x_m\Vert +(K+1)\{\Vert x_n-x_{n+1}\Vert +\Vert x_m-x_{m+1}\Vert \}. \end{aligned}$$Under the assumption \(\limsup \limits _{n,m\rightarrow \infty }\Vert x_n-x_m\Vert >0\), asymptotic regularity of \(\{x_n\}\), the inequality
$$\begin{aligned} \dfrac{\Vert x_n-x_m\Vert }{\Vert x_n-x_{n+1}\Vert +\Vert x_m-x_{m+1}\Vert }\leqslant \dfrac{K+1}{1-\lambda \cdot \alpha (\Vert x_n-x_m\Vert )} \end{aligned}$$now implies
$$\begin{aligned} \limsup _{n,m\rightarrow \infty }\dfrac{K+1}{1-\lambda \cdot \alpha (\Vert x_n-x_m\Vert )}=+\infty , \end{aligned}$$from which
$$\begin{aligned} \limsup _{n,m\rightarrow \infty }\alpha (\Vert x_n-x_m\Vert )=\frac{1}{\lambda }=b+1. \end{aligned}$$But since \(\alpha \in \mathcal {S}\) this implies \(\limsup \limits _{n,m\rightarrow \infty }\Vert x_n-x_m\Vert =0\), which is a contradiction. Thus, \(\{x_n\}\) is a Cauchy sequence. Because X is a Banach space \(\lim \limits _{n\rightarrow \infty }x_n=z\in X\).
Using the continuity of \(T_{\lambda }\), we immediately obtain \(z=T_{\lambda }z\), so by (1), \(Tz=z\).
Assume that \(u\ne z\) is another fixed point of \(T_{\lambda }\). Then by (9),
a contradiction. Hence, z is the unique fixed point of T.
It is clear that for each \(x_0\in X\), the sequence \(\{x_{n+1}=T_{\lambda }x_n\}\) converges to z. \(\square \)
The next result is inspired by theorem of Boyd and Wong [8]. A mapping T satisfying \(\Vert Tx-Ty\Vert \leqslant \varphi (\Vert x-y\Vert )\), \(\varphi (t)<t\) for each \(t>0\), may not posses a fixed point unless some additional condition is assumed on \(\varphi \). Boyd and Wong [8] assumed \(\varphi \) to be upper semi-continuous from the right.
Let \(b\in [0,\infty )\) and let \(\mathcal {U}\) denote the class of all mappings \(\varphi :[0,\infty )\rightarrow [0,\infty )\) satisfying
-
(a)
\(\varphi (t)<t(b+1)\) for all \(t>0\),
-
(b)
\(\varphi \) is upper semi-continuous, that is \(t_n\rightarrow t\geqslant 0 \Rightarrow \limsup \limits _{n\rightarrow \infty }\varphi (t_n)\leqslant \varphi (t)\).
Theorem 3.2
Let \((X,\Vert \cdot \Vert )\) be a Banach space. Suppose \(T:X\rightarrow X\) is a mapping for which exists \(0\leqslant b,K<\infty \), \(\varphi \in \mathcal {U}\) such that for each \(x,y\in X\),
and \(T_{\frac{1}{b+1}}\) is an asymptotically regular mapping. If T is continuous, then T has a unique fixed point \(z\in X\) and for each \(x_0\in X\) a sequence given by \(x_{n+1}=T_{\frac{1}{b+1}}x_n\), \(n=0,1,2,\ldots \), converges to z as \(n\rightarrow \infty \).
Proof
Let us consider two cases.
- Case 1.:
-
When \(b=0\), then we can apply [16, Theorem 2.2].
- Case 2.:
-
Let \(b>0\) and let us denote \(\lambda = \frac{1}{b+1}\in (0,1)\). Analogically as in the proof of theorem 2.6, we get
$$\begin{aligned} \Vert T_{\lambda }x-T_{\lambda }y \Vert \leqslant \lambda \cdot \varphi (\Vert x-y \Vert )+K\cdot \{\Vert x-T_{\lambda }x \Vert + \Vert y-T_{\lambda }y \Vert \}.\, \,(11) \end{aligned}$$Let \(x_0\in X\) and let \(x_{n+1}=T_{\lambda }x_n\), \(n=0,1,2,\ldots \) If \(x_{n+1}=x_n\) for some \(n\in \mathbb {N}\cup \{0\}\), then \((1-\lambda )x_n+\lambda Tx_n=x_n\), so \(Tx_n=x_n\) is the fixed point of T. Suppose \(x_{n+1}\ne x_n\) for all \(n\geqslant 0\). Assume that \(\{x_n\}\) is not a Cauchy sequence. Then there exists \(\varepsilon >0\) and integers \(n_i,m_i\in \mathbb {N}\) such that \(m_i>n_i\geqslant i\) and
$$\begin{aligned} \Vert x_{n_i}-x_{m_i}\Vert \geqslant \varepsilon ~~\hbox {for}~~i=1,2,\ldots \end{aligned}$$Also, choosing \(m_i\) small as possible, if may be assumed that
$$\begin{aligned} \Vert x_{n_i}-x_{m_i-1}\Vert <\varepsilon . \end{aligned}$$Hence, for each \(i\in \mathbb {N}\), we have
$$\begin{aligned} \varepsilon \leqslant \Vert x_{n_i}-x_{m_i}\Vert \leqslant&~\Vert x_{n_i}-x_{m_i-1}\Vert +\Vert x_{m_i-1}-x_{m_i}\Vert<\\ <&~\varepsilon + \Vert x_{m_i-1}-x_{m_i}\Vert , \end{aligned}$$and it follows from asymptotic regularity that
$$\begin{aligned} \lim _{i\rightarrow \infty }\Vert x_{n_i}-x_{m_i}\Vert =\varepsilon . \end{aligned}$$Next observe, by (11), that
$$\begin{aligned} \Vert x_{n_i}-x_{m_i}\Vert \leqslant&~ \Vert x_{n_i}-x_{n_i+1}\Vert +\Vert x_{n_i+1}-x_{m_i+1}\Vert +\Vert x_{m_i+1}-x_{m_i}\Vert \leqslant \\ \leqslant&~\lambda \cdot \varphi (\Vert x_{n_i}-x_{m_i}\Vert )+\\&~+(K+1)\{\Vert x_{n_i}-x_{n_i+1}\Vert +\Vert x_{m_i}-x_{m_i+1}\Vert \}. \end{aligned}$$Letting \(i\rightarrow \infty \), using asymptotic regularity and upper semi-continuity of \(\varphi \), we obtain
$$\begin{aligned} 0<\varepsilon =\lim _{i\rightarrow \infty }\Vert x_{n_i}-x_{m_i}\Vert \leqslant \limsup _{i\rightarrow \infty }\lambda \cdot \varphi (\Vert x_{n_i}-x_{m_i}\Vert )\leqslant \lambda \cdot \varphi (\varepsilon )<\varepsilon , \end{aligned}$$which is a contradiction. Hence, \(\{x_n\}\) is a Cauchy sequence. The conclusion of the proof follows as in the proof of Theorem 3.1. \(\square \)
Example 3.3
[16, Example 2.3] If \(b=0\) and \(Tx=\dfrac{x}{x+1}\) if \(x\geqslant 0\), then T does not satisfy the Banach theorem, take \(x=0\) and \(y=\varepsilon \), where \(\varepsilon > 0\) and \(\varepsilon \downarrow 0\). On the other hand, T satisfy \(\vert Tx -Ty\vert \leqslant \varphi (\vert x-y\vert ))\) with \(\varphi (t)=\dfrac{t}{t+1}\). Obviously, the mapping T is asymptotically regular and has a unique fixed point \(p=0\).
Now we note that T does not satisfy the Kannan type condition, i.e. there is no constant \(K\in [0,\infty )\) such that for all \(x,y\geqslant 0\) satisfying \(\vert Tx-Ty\vert \leqslant K\cdot \{\vert x-Tx\vert +\vert y-Ty\vert \}\), take \(x=0\) and \(y=\varepsilon \), where \(\varepsilon \) is sufficiently small. Therefore we cannot apply [15, Theorem 2.6] or [7, Theorem 2.1].
In the next result, we show that the assumption of the existence of constant \(K\in [0,\infty )\) can be relaxed to weaker concept using certain functions, see [20].
Let \(\mathcal {B}\) denote the class of functions \(\beta : [0,\infty ) \rightarrow [0,\infty )\) satisfying the following condition, \(\limsup \limits _{t\rightarrow 0}\beta (t) <\infty \).
Using the approach of Theorem 3.2, we prove the following result:
Theorem 3.4
Let \((X,\Vert \cdot \Vert )\) be a Banach space. Suppose \(T:X\rightarrow X\) is a mapping for which exists \(0\leqslant b<\infty \), \(\varphi \in \mathcal {U}\), \(\beta _1,\beta _2\in \mathcal {B}\) such that for each \(x,y\in X\),
and \(T_{\frac{1}{b+1}}\) is an asymptotically regular mapping. If T is continuous, then T has a unique fixed point \(z\in X\) and for each \(x_0\in X\) a sequence given by \(x_{n+1}=T_{\frac{1}{b+1}}x_n\), \(n=0,1,2,\ldots \), converges to z as \(n\rightarrow \infty \).
Proof
Let us consider two cases.
- Case 1.:
-
When \(b=0\), then we can apply [20, Theorem 3, for \({{\varvec{a}}}={{\varvec{b}}}=0\) and \(\ell _1=\ell _2=1\)].
- Case 2.:
-
Let \(b>0\) and let us denote \(\lambda = \frac{1}{b+1}\in (0,1)\). Analogically as in the proof of Theorem 3.1, we get
$$\begin{aligned} \Vert T_{\lambda }x-T_{\lambda }y \Vert\leqslant & {} \\\leqslant & {} \lambda \cdot \varphi (\Vert x-y \Vert )+ \beta _1(\Vert x-T_{\lambda }x \Vert )\cdot \Vert x-T_{\lambda }x \Vert \\&+ \beta _2(\Vert y-T_{\lambda }y \Vert )\cdot \Vert y-T_{\lambda }y \Vert . \, \qquad \qquad \qquad \qquad \qquad \qquad \,(13) \end{aligned}$$Let \(x_0\in X\) and let \(x_{n+1}=T_{\lambda }x_n\), \(n=0,1,2,\ldots \) If \(x_{n+1}=x_n\) for some \(n\in \mathbb {N}\cup \{0\}\), then \((1-\lambda )x_n+\lambda Tx_n=x_n\), so \(Tx_n=x_n\) is the fixed point of T. Suppose \(x_{n+1}\ne x_n\) for all \(n\geqslant 0\). Assume that \(\{x_n\}\) is not a Cauchy sequence. Then there exists \(\varepsilon >0\) and integers \(n_i,m_i\in \mathbb {N}\) such that \(m_i>n_i\geqslant i\) and
$$\begin{aligned} \Vert x_{n_i}-x_{m_i}\Vert \geqslant \varepsilon ~~\hbox {for}~~i=1,2,\ldots \end{aligned}$$We may choose \(m_i\) small as possible such that
$$\begin{aligned} \Vert x_{n_i}-x_{m_i-1}\Vert <\varepsilon . \end{aligned}$$In view of triangle inequality, we have for each \(i\in \mathbb {N}\) that
$$\begin{aligned} \varepsilon \leqslant \Vert x_{n_i}-x_{m_i}\Vert \leqslant \Vert x_{n_i}-x_{m_i-1}\Vert +\Vert x_{m_i-1}-x_{m_i}\Vert < \varepsilon + \Vert x_{m_i-1}-x_{m_i}\Vert . \end{aligned}$$It follows from asymptotic regularity that
$$\begin{aligned} \lim _{i\rightarrow \infty }\Vert x_{n_i}-x_{m_i}\Vert =\varepsilon . \end{aligned}$$Next observe from (13), we have for each i that
$$\begin{aligned} \Vert x_{n_i}- x_{m_i}\Vert \leqslant&\Vert x_{n_i}- x_{n_i+1}\Vert + \Vert x_{m_i}- x_{m_i+1}\Vert + \Vert x_{n_i+1}- x_{m_i+1}\Vert \leqslant \\ \leqslant&\Vert x_{n_i}- x_{n_i+1}\Vert + \Vert x_{m_i}- x_{m_i+1}\Vert + \\&+\lambda \cdot \varphi (\Vert x_{n_i}- x_{m_i}\Vert ) + \\&+ \beta _1(\Vert x_{n_i}-x_{n_i+1}\Vert )\cdot \Vert x_{n_i}- x_{n_i+1}\Vert ) + \\&+ \beta _2(\Vert x_{m_i}- x_{m_i+1}\Vert )\cdot \Vert x_{m_i}- x_{m_i+1}\Vert . \end{aligned}$$It follows from \(\mathcal {B}\) that there exist \(i_1\ge i_0 \in \mathbb {N}\) and \(L>0\) such that
$$\begin{aligned} \beta _1(\Vert x_{n_i}-x_{n_i+1}\Vert )\leqslant L,\quad \text{ and }\quad \beta _2(\Vert x_{m_i}-x_{m_i+1}\Vert )\leqslant L \end{aligned}$$for all \(i\ge i_1\). Thus, by above inequality, we have for all \(i\ge i_1\),
$$\begin{aligned} \Vert x_{n_i}-x_{m_i}\Vert \leqslant&\Vert x_{n_i}-x_{n_i+1}\Vert + \Vert x_{m_i}-x_{m_i+1}\Vert + \\&+ \lambda \cdot \varphi (\Vert x_{n_i}-x_{m_i}\Vert ) + \\&+ L\{\Vert x_{n_i}-x_{n_i+1}\Vert + \Vert x_{m_i}-x_{m_i+1}\Vert \}. \end{aligned}$$Making \(i\rightarrow \infty \), using asymptotic regularity and upper semi-continuity of \(\varphi \), we get
$$\begin{aligned} 0<\varepsilon =\lim _{i\rightarrow \infty }\Vert x_{n_i}-x_{m_i}\Vert \leqslant \limsup _{i\rightarrow \infty }\lambda \cdot \varphi (\Vert x_{n_i}-x_{m_i}\Vert )\leqslant \lambda \cdot \varphi (\varepsilon )<\varepsilon , \end{aligned}$$which is a contradiction. Hence, \(\{x_n\}\) is a Cauchy sequence. The conclusion of the proof follows as in the proof of Theorem 3.1. \(\Box \) Now notice that mappings satisfying the condition
$$\begin{aligned} \vert Tx-Ty\vert <\vert x-y\vert +K\cdot \{\vert x-Tx\vert + \vert y-Ty\vert \}~\hbox {for all}~x,y\in X, x\ne y, \end{aligned}$$
where \(0\leqslant K<\infty \), may not have a fixed point.
Example 3.5
[16, Example 2.5] Let \(X=\mathbb {R}\) be with the usual norm and let \(Tx=\ln (1+e^x)\), \(x\in \mathbb {R}\). Then T is asymptotically regular (\(T^nx=\ln (n+e^x)\), \(n=1,2,\ldots \)) and fixed point free (\(Tx>x\) for all \(x\in \mathbb {R}\)). It is easy to see that T satisfies
Now we note, that T does not satisfy the Kannan type condition, i.e. there is no constant \(K\in [0,\infty )\) such that for all \(x,y\geqslant 0\) satisfying \(\vert Tx-Ty\vert \leqslant K\cdot \{\vert x-Tx\vert +\vert y-Ty\vert \}\): for sufficiently large values \(n\in \mathbb {N}\), we have
Added in proof. Recently published paper Popescu, O., A new class of contractive mappings, Acta Math. Hungar. https://doi.org/10.1007/s10474-021-01154-6, contains additional information about enriched contractions, enriched Kannan mappings and enriched Chatterjea mappings.
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Górnicki, J., Bisht, R.K. Around averaged mappings. J. Fixed Point Theory Appl. 23, 48 (2021). https://doi.org/10.1007/s11784-021-00884-y
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DOI: https://doi.org/10.1007/s11784-021-00884-y