Abstract
Let C be a ρ-bounded, ρ-closed, convexsubset of a modular function space . We investigate the problem of constructingcommon fixed points for asymptotic pointwise nonexpansive semigroups of mappings
, i.e. a family such that
,
, and
, where
, for every
.
MSC: 47H09, 46B20, 47H10, 47E10.
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1 Introduction
In 2008, Kirk and Xu [1] studied existence of fixed points of asymptotic pointwise nonexpansivemappings , i.e.
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equa_HTML.gif)
where , for all
. Their main result (Theorem 3.5) states thatevery asymptotic pointwise nonexpansive self-mapping of a nonempty, closed, boundedand convex subset C of a uniformly convex Banach space X has afixed point. As pointed out by Kirk and Xu, asymptotic pointwise mappings seem to bea natural generalization of nonexpansive mappings. The conditions on
can be for instance expressed in terms of thederivatives of iterations of T for differentiable T. In 2009,these results were generalized by Hussain and Khamsi to metric spaces [2]. In 2011, Khamsi and Kozlowski [3] extended their result proving the existence of fixed points of asymptoticpointwise ρ-nonexpansive mappings acting in modular function spaces.The existence of common fixed points of semigroups of nonexpansive (in a modularsense) mappings acting in modular function spaces was first established by Kozlowskiin [4] and then extended to the semigroups of asymptotic pointwise nonexpansivemappings by the authors in [5]. The proof of this important theorem is of the existential nature anddoes not describe any algorithm for constructing a common fixed point of anasymptotic pointwise ρ-nonexpansive semigroup. The current paper aimsat filling this gap. The results of this paper generalize the convergence ofgeneralized Mann processes to common fixed points of semigroups of nonexpansivesemigroups studied in the recent paper by Bin Dehaish and Kozlowski [6].
Let us recall that modular function spaces are a natural generalization of bothfunction and sequence variants of many spaces like Lebesgue, Orlicz,Musielak-Orlicz, Lorentz, Orlicz-Lorentz, Calderon-Lozanovskii spaces and manyothers, important from an applications perspective; see the book by Kozlowski [7] for an extensive list of examples and special cases. There exists anextensive literature on the topic of the fixed point theory in modular functionspaces; see e.g.[3, 7–18] and the references therein. It is also worthwhile mentioning a growinginterest in applications of the methods of the fixed point theory to semigroups ofnonlinear mappings and applications to the area of differential and integralequations (see e.g.[10, 19, 20]).
It is well known that the fixed point construction iteration processes forgeneralized nonexpansive mappings have been successfully used to develop efficientand powerful numerical methods for solving various nonlinear equations andvariational problems, often of great importance for applications in various areas ofpure and applied science. There exists an extensive literature on the subject ofiterative fixed point construction processes for asymptotically nonexpansivemappings in Hilbert, Banach, and metric spaces; see e.g.[2, 21–37] and the references therein. Kozlowski proved convergence to a fixed pointof some iterative algorithms of asymptotic pointwise nonexpansive mappings in Banachspaces [38] and the existence of common fixed points of semigroups of pointwiseLipschitzian mappings in Banach spaces [39]. Recently, the weak and strong convergence of such processes to commonfixed points of semigroups of mappings in Banach spaces was demonstrated byKozlowski and Sims [40] and by Kozlowski in [41].
We would like to emphasize that all convergence theorems proved in this paper defineconstructive algorithms that can be actually implemented. When dealing with specificapplications of these theorems, one should take into consideration how additionalproperties of the mappings, sets, and modulars involved can influence the actualimplementation of the algorithms defined in this paper.
2 Preliminaries
Let us introduce basic notions related to modular function spaces and relatednotation which will be used in this paper. For further details we refer the readerto preliminary sections of the recent articles [3, 6, 16] or to the survey article [17]; see also [7, 42, 43] for the standard framework of modular function spaces.
Let Ω be a nonempty set and Σ be a nontrivial σ-algebra ofsubsets of Ω. Let be a δ-ring of subsets of Ω, such that
for any
and
. Let us assume that there exists an increasingsequence of sets
such that
. By ℰ we denote the linear space of all simplefunctions with supports from
. By
we will denote the space of all extended measurablefunctions, i.e. all functions
such that there exists a sequence
,
and
for all
. By
we denote the characteristic function of the setA.
Definition 2.1[7]
Let be a nontrivial, convex and even function. We saythat ρ is a regular convex function pseudomodular if
-
(i)
;
-
(ii)
ρ is monotone, i.e.
for all
implies
, where
;
-
(iii)
ρ is orthogonally subadditive, i.e.
for any
such that
,
;
-
(iv)
ρ has the Fatou property, i.e.
for all
implies
, where
;
-
(v)
ρ is order continuous in ℰ, i.e.
and
implies
.
Similarly to the case of measure spaces, we say that a set is ρ-null if
for every
. We say that a property holds ρ-almosteverywhere if the exceptional set is ρ-null. As usual we identify anypair of measurable sets whose symmetric difference is ρ-null as wellas any pair of measurable functions differing only on a ρ-null set.With this in mind we define
, where each element is actually an equivalence classof functions equal ρ-a.e. rather than an individual function.
Definition 2.2[7]
We say that a regular function pseudomodular ρ is a regular convexfunction modular if implies
ρ-a.e. The class of all nonzero regular convex function modularsdefined on Ω will be denoted by ℜ.
Let ρ be a convex function modular. A modular function space is thevector space . In the vector space
, the following formula:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equb_HTML.gif)
defines a norm, frequently called the Luxemburg norm.
The following notions will be used throughout the paper.
Definition 2.4[8]
Let .
-
(a)
We say that
is ρ-convergent to f and write
if and only if
.
-
(b)
A sequence
, where
, is called ρ-Cauchy if
as
.
-
(c)
We say that
is ρ-complete if and only if any ρ-Cauchy sequence in
is ρ-convergent.
-
(d)
A set
is called ρ-closed if for any sequence of
, the convergence
implies that f belongs to B.
-
(e)
A set
is called ρ-bounded if
.
Since ρ fails in general the triangle identity, many of the knownproperties of limit may not extend to ρ-convergence. For example,ρ-convergence does not necessarily imply theρ-Cauchy condition. However, it is important to remember that theρ-limit is unique when it exists. The following proposition bringstogether a few facts, which will be often used in the proofs of our results.
Proposition 2.1[7]
Let.
-
(i)
isρ-complete.
-
(ii)
ρ-Balls
areρ-closed.
-
(iii)
If
for an
then there exists a subsequence
of
such that
ρ-a.e.
-
(iv)
whenever
ρ-a.e. (note: this property is equivalent to the Fatou property).
Let us recall the definition of an asymptotic pointwise nonexpansive mapping actingin a modular function space.
Definition 2.5[3]
Let and let
be nonempty and ρ-closed. A mapping
is called
-
(i)
a pointwise Lipschitzian mapping if there exists
such that
-
(ii)
an asymptotic pointwise nonexpansive if there exists a sequence of mappings
such that
and for any
.
A point is called a fixed point of T whenever
. The set of fixed points of T will bedenoted by
.
Define , where
, for any
and
. Clearly then
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ1_HTML.gif)
Definition 2.6 Define as a class of all asymptotic pointwise nonexpansivemappings T such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ2_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ3_HTML.gif)
The notion of the asymptotic pointwise nonexpansiveness will be now extended to aone-parameter family of mappings. Throughout this paper J will be thesemigroup of all nonnegative numbers, that is, with normal addition.
Definition 2.7 A one-parameter family of mappings from C into itself is said to bean asymptotic pointwise nonexpansive semigroup on C if ℱ satisfiesthe following conditions:
-
(i)
for
;
-
(ii)
for
and
;
-
(iii)
for each
,
is an asymptotic pointwise nonexpansive mapping, i.e. there exists a function
such that
(2.4)
such that for every
;
-
(iv)
for each
, the mapping
is ρ-continuous.
For each let
denote the set of its fixed points. Define then theset of all common fixed points set for mappings from ℱ as the followingintersection:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Eque_HTML.gif)
The common fixed points are frequently interpreted as the stationary points of thesemigroup ℱ. Note that without loss of generality we may assume for any
and
and
.
Denoting and
for
, we note that without loss of generality we canassume that ℱ is asymptotically nonexpansive if
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ5_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ6_HTML.gif)
Define . Note that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ7_HTML.gif)
The above notation will be consistently used throughout this paper.
Definition 2.8 By we will denote the class of all asymptotic pointwisenonexpansive semigroups on C such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ8_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ9_HTML.gif)
Note that we do not assume that all functions are bounded by a common constant. Therefore, we donot assume that ℱ is uniformly Lipschitzian.
Definition 2.9 We will say that a semigroup is ρ continuous if
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equf_HTML.gif)
for any and
.
The concept ρ-type is a powerful technical tool which is used in theproofs of many fixed point results. The definition of a ρ-type isbased on a given sequence. In this work, we generalize this definition to be adaptedto one-parameter family of mappings.
Definition 2.10 Let be convex and ρ-bounded.
-
(1)
A function
is called a ρ-type (or shortly a type) if there exists a one-parameter family
of elements of K such that for any
we have
-
(2)
Let τ be a ρ-type. A sequence
is called a minimizing sequence of τ if
Note that τ is convex provided ρ is convex.
Let us recall the modular equivalents of uniform convexity introduced in [3].
Definition 2.11 Let . We define the following uniform convexity (UC) typeproperties of the function modular ρ:
-
(i)
Let
,
. Define
Let
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equj_HTML.gif)
and if
. We say that ρ satisfies (UC) if forevery
,
,
. Note that for every
,
, for
small enough.
-
(ii)
We say that ρ satisfies (UUC1) if there exists
, for every
, and
such that
We will need the following result, being a modular equivalent of a norm property inuniformly convex Banach spaces; see e.g.[26].
Lemma 2.1[6]
Letbe (UUC1) and let
be bounded away from 0 and 1. If there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ10_HTML.gif)
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ11_HTML.gif)
then
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equl_HTML.gif)
The following property plays in the theory of modular function space a role similarto the reflexivity in Banach spaces; see e.g.[9].
Definition 2.12 We say that has property
if and only if every nonincreasing sequence
of nonempty, ρ-bounded,ρ-closed, and convex subsets of
has nonempty intersection.
Similarly to the Banach space case, the modular uniform convexity implies property.
Theorem 2.1[3]
Letbe (UUC1) then
has property
.
The next lemma is a generalization of the minimizing sequence property for typesdefined by sequences in Lemma 4.3 in [16] to the one-parameter semigroup case.
Lemma 2.2[5]
Assumeis (UUC1). LetCbe a nonempty, ρ-bounded,ρ-closed, and convex subset of
. Letτbe a type defined by a one-parameter family
inC.
-
(i)
If
, then
.
-
(ii)
Any minimizing sequence
ofτisρ-convergent. Moreover, theρ-limit of
is independent of the minimizing sequence.
Using Lemma 2.2, the authors proved the following common fixed point result forasymptotic pointwise nonexpansive semigroups.
Theorem 2.2[5]
Assumeis (UUC1). LetCbe aρ-closed, ρ-bounded, convex,and nonempty subset. Let
be an asymptotic pointwise nonexpansive semigroup onC. Then ℱ has a common fixed point and the set
of common fixed points isρ-closed and convex.
3 The demiclosedness principle
In this section we will use the notion the uniform continuity of the function modularρ in the sense of the following definition (see e.g.[16]).
Definition 3.1 We say that is uniformly continuous if for every
and
, there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ12_HTML.gif)
provided and
.
Let us mention that the uniform continuity holds for a large class of functionmodulars. For instance, it can be proved that in Orlicz spaces over a finiteatomless measure [44] or in Orlicz sequence spaces [45] the uniform continuity of the Orlicz modular is equivalent to the-type condition. Recall that ρ satisfiesthe
-type condition if and only if there exists
such that
, for any
.
Let us recall the definition of the Opial property and the strong Opial property inmodular function spaces [16, 46].
Definition 3.2 We say that satisfies the ρ-a.e. Opial property iffor every
which is ρ-a.e. convergent to 0 suchthat there exists a
for which
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ13_HTML.gif)
the following inequality holds for any not equal to 0:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ14_HTML.gif)
Definition 3.3 We say that satisfies the ρ-a.e. strong Opialproperty if for every
which is ρ-a.e. convergent to 0 suchthat there exists a
for which
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ15_HTML.gif)
the following equality holds for any :
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ16_HTML.gif)
Remark 3.1 Note that the ρ-a.e. strong Opial property impliesthe ρ-a.e. Opial property [46].
Remark 3.2 Also, note that, by virtue of Theorem 2.1 in [46], every convex, orthogonally additive function modular ρ hasthe ρ-a.e. strong Opial property. Let us recall that ρis called orthogonally additive if whenever
. Therefore, all Orlicz and Musielak-Orlicz spacesmust have the strong Opial property.
Note that the Opial property in the norm sense does not necessarily hold for severalclassical Banach function spaces. For instance the norm Opial property does not holdfor spaces for
, while the modular strong Opial property holds in
for all
.
Lemma 3.1[4]
Let. Assume that
has theρ-a.e. strong Opial property. Let
be a nonempty, stronglyρ-bounded, andρ-a.e. compact convex set. Thenanyρ-type defined in C attains its minimum inC.
To begin our discussion of the demiclosedness principle, let us quote the followingversion of this theorem applied to the asymptotic pointwise nonexpansive mappings [[6], Theorem 4.1].
Theorem 3.1 (Demiclosedness principle)
Let. Assume that
-
(1)
ρis (UUC1),
-
(2)
ρhas the strong Opial property,
-
(3)
ρhas the
property and is uniformly continuous.
Letbe nonempty, convex, stronglyρ-bounded, andρ-closed, and let
. Let
, and
. If
ρ-a.e. and
, then
.
We will need a version of the above theorem without assuming that. This will require a different proof, which issketched below.
Theorem 3.2Let. Assume that
-
(1)
ρis (UUC1),
-
(2)
ρhas the strong Opial property,
-
(3)
ρhas the
property and is uniformly continuous.
Letbe nonempty, convex, stronglyρ-bounded, andρ-closed. Let
be an asymptotic pointwise nonexpansive mapping such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equm_HTML.gif)
andfor any
. We will assume the functions
are bounded onC, i.e. Tis uniformlyρ-Lipschitzian mapping. Let
, and
. If
ρ-a.e. and
, then
. In particular, we have
.
Proof Define the ρ-type function
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equn_HTML.gif)
Note that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equo_HTML.gif)
Indeed, we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equp_HTML.gif)
which implies
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equq_HTML.gif)
for any , where
. Hence
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equr_HTML.gif)
Since ρ has the property, we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equs_HTML.gif)
Since ρ is uniformly continuous, we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equt_HTML.gif)
for any and
. In particular, we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equu_HTML.gif)
In other words, we have , for any
. Since ρ has the Opial property, it iseasy to prove that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equv_HTML.gif)
Since T is an asymptotic pointwise nonexpansive mapping, we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equw_HTML.gif)
Since ρ is (UUC1), then arguing similarly to the proof ofTheorem 4.1 in [6], we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equx_HTML.gif)
Since T is ρ-continuous, we get , i.e.
. □
As a corollary to this result, we get the following important result.
Corollary 3.1Let. Assume that
-
(1)
ρis (UUC1),
-
(2)
ρhas the strong Opial property,
-
(3)
ρhas the
property and is uniformly continuous.
Letbe nonempty, convex, stronglyρ-bounded, andρ-closed. Let
be asymptotic pointwise nonexpansive mappings such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equy_HTML.gif)
for any, with
, and
, for any
. We will assume the functions
and
are bounded onC. Let
, and
. If
ρ-a.e. and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equz_HTML.gif)
then. In particular, we have
.
The above results lead us to the following version of the demiclosedness principlefor semigroup of mappings.
Theorem 3.3 (Demiclosedness principle)
Let. Assume that
-
(1)
ρis (UUC1),
-
(2)
ρhas the strong Opial property,
-
(3)
ρhas the
property and is uniformly continuous.
Letbe nonempty, convex, stronglyρ-bounded, andρ-closed, and let
be continuous. Let
, and
. Assume
ρ-a.e. If there exist
such that
is irrational and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equaa_HTML.gif)
then.
Proof In view of Corollary 3.1, we know that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equab_HTML.gif)
Let us denote . Note that for any
we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equac_HTML.gif)
Combining the above we get
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equad_HTML.gif)
Since is irrational, then the set
is dense in
[47]. Since ℱ is continuous and ρ is uniformly continuous,we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equae_HTML.gif)
Hence as desired. □
4 Convergence of generalized Krasnosel’skii-Mann iteration processes
Let us start with the precise definition of the generalized Krasnosel’skii-Manniteration process for semigroups of nonlinear mappings.
Definition 4.1 Let ,
and
. The generalized Krasnosel’skii-Mann iterationprocess
generated by the semigroup ℱ, the sequences
and
, is defined by the following iterative formula:
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ17_HTML.gif)
and
-
(1)
is bounded away from 0 and 1,
-
(2)
,
-
(3)
for every
.
Definition 4.2 We say that a generalized Krasnosel’skii-Mann iterationprocess is well defined if
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ18_HTML.gif)
Arguing exactly like in the proof of Lemma 5.2 in [6] (see also Lemma 22.20 in [40]), we get the following result.
Lemma 4.1Letbe (UUC1). Let
be aρ-closed, ρ-bounded, and convexset. Let
,
, and let
be a sequence generated by a generalized Krasnosel’skii-Mannprocess
. Then there exists an
such that
.
We will prove now a generic version of the convergence theorem for the sequences which are generated by the Krasnosel’skii-Manniteration process and are at the same time approximate fixed point sequences.
Theorem 4.1Let. Assume that
-
(1)
ρis (UUC1),
-
(2)
ρhas the strong Opial property,
-
(3)
ρhas the
property and is uniformly continuous.
Letbe nonempty, ρ-a.e. compact,convex, stronglyρ-bounded, andρ-closed, and let
. Assume that
is a well defined Krasnosel’skii-Mann iteration process.If for the sequence
generated by
we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ19_HTML.gif)
whereare such that
is irrational, then
convergesρ-a.e. to a common fixed point
.
Proof Observe that by Theorem 2.2 the set of fixed points is nonempty, convex and ρ-closed.Consider
, two ρ-a.e. cluster points of
. There exits then
,
subsequences of
such that
ρ-a.e., and
ρ-a.e. It follows from Theorem 3.3 that
and
. By Lemma 4.1, there exist
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ20_HTML.gif)
We claim that . Assume to the contrary that
. Then by the Opial property we have
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ21_HTML.gif)
The contradiction implies that . Therefore,
has at most one ρ-a.e. cluster point.Since C is ρ-a.e. compact it follows that the sequence
has exactly one ρ-a.e. cluster point
, which means that
ρ-a.e. Applying the demiclosedness principle again, we get
. By the same argument, we get
(observe that the construction of w did notdepend on the selection of
). From the density of
in
, we conclude that
for any
, as claimed. □
Let us apply the above result to some more specific situations. First we need toprove a series of axillary results. Let us start with the following elementarylemma.
Lemma 4.2[31]
Suppose
is a bounded sequence of real numbers and
is a double index sequence of real numbers which satisfy
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equaf_HTML.gif)
for each. Then
converges to an
.
The following result provides an important technique which will be used in thispaper.
Lemma 4.3Letbe (UUC1). Let
be aρ-closed, ρ-bounded, and convexset. Let
. Assume thatwis a common fixed point of ℱ. Let us denote by
a sequence generated by the generalized Krasnosel’skii-Mannprocess
. Then there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ22_HTML.gif)
Proof Let . Since
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equag_HTML.gif)
it follows that for every ,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ23_HTML.gif)
Denote for every
and
. Observe that by the assumptions on the sequence
,
. By Lemma 4.2, there exists an
such that
, as claimed. □
Lemma 4.4Letbe (UUC1). Let
be aρ-closed, ρ-bounded, and convexset, and
. Let
be a generalized Krasnosel’skii-Mann iteration process.Then
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ24_HTML.gif)
and
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ25_HTML.gif)
Proof By Theorem 2.2, ℱ has at least one common fixed point. In view of Lemma 4.3, there exists
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ26_HTML.gif)
Note that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ27_HTML.gif)
and that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ28_HTML.gif)
Set ,
, and note that
by (4.10), and
by (4.11). Observe also that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ29_HTML.gif)
Hence, it follows from Lemma 2.1 that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ30_HTML.gif)
which by the construction of the sequence is equivalent to
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ31_HTML.gif)
as claimed. □
Lemma 4.5Letbe (UUC1) and have the
property. Let
be aρ-closed, ρ-bounded, and convexset and let
. Denote by
the sequence generated by a well defined generalizedKrasnosel’skii-Mann process
. Let
be such that for every
there exists a strictly increasing sequence of natural numbers
satisfying the following conditions:
-
(a)
as
,
-
(b)
, where
.
Thenis an approximate fixed point sequence for all mappings
where
, that is,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ32_HTML.gif)
for every.
Proof Let us fix . Note that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ33_HTML.gif)
By , it suffices to prove that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ34_HTML.gif)
To this end observe that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ35_HTML.gif)
in view of the assumption (a) and by (4.9) in Lemma 4.4. Observe that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ36_HTML.gif)
Indeed,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equah_HTML.gif)
which tends to zero as because of (4.17), Lemma 4.4, the fact that theprocess is well defined, assumptions (b) and (2.9), and the boundedness of eachfunction
. This convergence gives us, via
, the required (4.20). On the other hand,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equai_HTML.gif)
which tends to zero as because of assumption (a), Lemma 4.4, (4.20),the fact that the process is well defined, and the fact that the semigroup isasymptotic pointwise nonexpansive. Since ρ has the
property,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equaj_HTML.gif)
which completes the proof of the lemma. □
The following theorem is an immediate consequence of Lemma 4.5 andTheorem 4.1.
Theorem 4.2Letbe uniformly continuous function modular satisfying (UUC1).Assume in addition thatρsatisfies
and has the strong Opial property. Let
be aρ-closed, ρ-bounded, and convexset and let
. Denote by
the sequence generated by a well defined generalizedKrasnosel’skii-Mann process
. Let
be such that
is irrational and that there exists a strictly increasing sequence of naturalnumbers
satisfying the following conditions:
-
(a)
as
,
-
(b)
, where
,
-
(c)
, where
.
Then the sequenceconvergesρ-a.e. to a common fixed point
.
Remark 4.1 Observe that a sequence satisfying assumptions of Theorem 4.2 can bealways constructed. The main difficulty is in ensuring that the correspondingprocess
is well defined.
The next result answers the question when the sequence generated by the generalizedKrasnosel’skii-Mann process will converge strongly to a common fixed point.Not surprisingly we need to add a compactness assumption.
Theorem 4.3Under the assumptions of Theorem 4.2, if in additionCis assumed to beρ-compact, then the sequencegenerated by
converges strongly to a common fixed point
, that is,
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ37_HTML.gif)
Proof It follows from Theorem 4.2 that there exists a common fixedpoint such that
converges ρ-a.e. Byρ-compactness of C there exist
and a subsequence
of
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ38_HTML.gif)
By Proposition 2.1 there exists a subsequence of
such that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ39_HTML.gif)
By the uniqueness of the ρ-a.e. limit we get . Hence
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equ40_HTML.gif)
On the other hand, the limit exists by Lemma 4.3, which implies that
![](http://media.springernature.com/full/springer-static/image/art%3A10.1186%2F1687-1812-2015-3/MediaObjects/13663_2014_821_Equak_HTML.gif)
as claimed. □
Remark 4.2 Observe that in view of the assumption, the ρ-compactness of theset C assumed in Theorem 4.3 is equivalent to the compactness in thesense of the norm defined by ρ.
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Acknowledgements
This work was funded by the Deanship of Scientific Research (DSR), King AbdulazizUniversity, Jeddah, under Grant No. (247-001-D1434). The authors,therefore, acknowledge with thanks technical and financial support of DSR.
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Bin Dehaish, B.A., Khamsi, M.A. & Kozlowski, W.M. On the convergence of iteration processes for semigroups of nonlinear mappings inmodular function spaces. Fixed Point Theory Appl 2015, 3 (2015). https://doi.org/10.1186/1687-1812-2015-3
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DOI: https://doi.org/10.1186/1687-1812-2015-3