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# Common fixed point theorems for generalized \mathcal{J}\mathcal{H}-operator classes and invariant approximations

*Journal of Inequalities and Applications*
**volume 2011**, Article number: 67 (2011)

## Abstract

In this article, we introduce two new different classes of noncommuting selfmaps. The first class is more general than \mathcal{J}\mathcal{H}-operator class of Hussain et al. (Common fixed points for \mathcal{J}\mathcal{H}-operators and occasionally weakly biased pairs under relaxed conditions. Nonlinear Anal. **74**(6), 2133-2140, 2011) and occasionally weakly compatible class. We establish the existence of common fixed point theorems for these classes. Several invariant approximation results are obtained as applications. Our results unify, extend, and complement several well-known results.

**2000 Mathematical Subject Classification:** 47H09; 47H10.

## 1. Introduction

The fixed point theorem, generally known as the Banach contraction principle, appeared in explicit form in Banach's thesis in 1922 [1], where it was used to establish the existence of a solution for an integral equation. Since its simplicity and usefulness, it has become a very popular tool in solving existence problems in many branches of mathematical analysis. Banach contraction principle has been extended in many different directions. Many authors established fixed point theorems involving more general contractive conditions.

In 1976, Jungck [2] extend the Banach contraction principle to a common fixed point theorem for commuting maps. Sessa [3] defined the notion of weakly commuting maps and established a common fixed point for this maps. Jungck [4] coined the term compatible mappings to generalize the concept of weak commutativity and showed that weakly commuting maps are compatible but the converse is not true. Afterward, many authors studied about common fixed point theorems for noncommuting maps (see [5–14]).

In 1996, Al-Thagafi [15] established some theorems on invariant approximations for commuting maps. Shahzad [16], Al-Thagafi and Shahzad [17, 18], Hussain and Jungck [19], Hussain [20], Hussain and Rhoades [21], Jungck and Hussain [22], O'Regan and Hussain [23], and Pathak and Hussain [24] extended the result of Al-Thagafi [15] and Ciric [25] for pointwise *R*-subweakly commuting maps, compatible maps, *C*_{
q
} -commuting maps, and Banach operator pairs. Pathak and Hussain [26] introduced two new classes of noncommuting selfmaps, so-called \mathcal{P}-operator and \mathcal{P}-suboperator pair class. Recently, Hussain et al. [27] introduced \mathcal{J}\mathcal{H}-operator and occasionally weakly *g*-biased class which are more general than above classes and established common fixed point theorems for these class.

In this article shall introduce two new classes of noncommuting selfmaps. First class, generalized \mathcal{J}\mathcal{H}-operator class, contains \mathcal{J}\mathcal{H}-operator classes of Hussain et al. [27] and occasionally weakly compatible classes. Second class is the so-called generalized \mathcal{J}\mathcal{H}-suboperator class. We will be present some common fixed point theorems for these classes and the existence of the common fixed points for best approximation. Our results improve, extend, and complement all the results in literature.

## 2. Preliminaries

Let *M* be a subset of a norm space *X*. We shall use *cl*(*A*) and *wcl*(*A*) to denote the closure and the weak closure of a set *A*, respectively, and *d*(*x*, *A*) to denote inf{||*x*-*y*|| : *y* ∈ *A*} where *x* ∈ *X* and *A* ⊆ *X*. Let *f* and *T* be selfmaps of *M*. A point *x* ∈ *M* is called a *fixed point* of *f* if *fx* = *x*. The set of all fixed points of *f* is denoted by *F*(*f*). A point *x* ∈ *M* is called a *coincidence point* of *f* and *T* if *fx* = *Tx*. We shall call *w* = *fx* = *Tx* a *point of coincidence* of *f* and *T*. A point *x* ∈ *M* is called a *common fixed point* of *f* and *T* if *x* = *fx* = *Tx*. Let *C*(*f*, *T*), *PC*(*f*, *T*), and *F*(*f*, *T*) denote the sets of all coincidence points, points of coincidence, and common fixed points, respectively, of the pair (*f*, *T*).

The map *T* is called *contraction* [resp. *f-contraction*] on *M* if ||*Tx*-*Ty*|| ≤ *k*||*x*-*y*|| [resp. ||*Tx* - *Ty*|| ≤ *k*||*fx* - *fy*||] for all *x*, *y* ∈ *M* and for some *k* ∈ [0, 1). The map *T* is called *nonexpansive* [resp. *f-nonexpansive*] on *M* if ||*Tx* - *Ty*|| ≤ ||*x* - *y*|| [resp. ||*Tx* - *Ty*|| ≤ ||*fx* - *fy*||] for all *x*, *y* ∈ *M*. The pair (*f*, *T*) is called:

**(i):** *commuting* if *Tfx* = *fTx* for all *x* ∈ *M*;

**(ii):** *R-weakly commuting*[8] if for all *x* ∈ *M*, there exists *R >* 0 such that

If *R* = 1, then the maps are called *weakly commuting*;

**(iii):** *compatible*[28] if \underset{n\to \infty}{lim}\parallel Tf{x}_{n}-fT{x}_{n}\parallel \phantom{\rule{0.3em}{0ex}}=0 when {*x*_{
n
} } is a sequence such that

for some *t* ∈ *M*;

**(iv):** *weakly compatible*[29] if *Tfx* = *fTx* for all *x* ∈ *C*(*f*, *T*);

**(v):** *occasionally weakly compatible*[18, 30] if *fTx* = *Tfx* for some *x* ∈ *C*(*f*, *T*);

**(vi):** *Banach operator pair*[31] if *f*(*F*(*T*)) ⊆ *F*(*T*);

**(vii):**\mathcal{P}-*operator*[26] if ||*u* - *Tu*|| ≤ diam (*C*(*f*, *T*)) for some *u* ∈ *C*(*f*, *T*);

**(viii):**\mathcal{J}\mathcal{H}-*operator*[27] if there exist a point *w* = *fx* = *Tx* in *PC*(*f*, *T*) such that

The set *M* is called *convex* if *kx* + (1 - *k*)*y* ∈ *M* for all *x*, *y* ∈ *M* and all *k* ∈ [0, 1]; and *q-starshaped* with *q* ∈ *M* if the segment [*q*, *x*] = {*kx* + (1 - *k*)*q* : *k* ∈ [0, 1]} joining *q* to *x* is contained to *M*. The map *f* : *M* → *M* is called *affine* if *M* is convex and *f*(*kx* + (1 - *k*)*y*) = *kfx* + (1 - *k*)*fy* for all *x*, *y* ∈ *M* and all *k* ∈ [0, 1]; and *q-affine* if *M* is *q*-starshaped and *f*(*kx* + (1 - *k*)*q*) = *kfx* + (1 - *k*)*fq* for all *x*, *y* ∈ *M* and all *k* ∈ [0, 1].

A map *T* : *M* → *X* is said to be *semicompact* if a sequence {*x*_{
n
} } in *M* such that (*x*_{
n
} - *Tx*_{
n
} ) → 0 has a subsequence {*x*_{
j
} } in *M* such that *x*_{
j
} → *z* for some *z* ∈ *M*. Clearly if *cl*(*T*(*M*)) is compact, then *T*(*M*) is complete, *T*(*M*) is bounded, and *T* is semicompact. The map *T* : *M* → *X* is said to be *weakly semicompact* if a sequence {*x*_{
n
} } in *M* such that (*x*_{
n
} - *Tx*_{
n
} ) → 0 has a subsequence {*x*_{
j
} } in *M* such that *x*_{
j
} → *z* weakly for some *z* ∈ *M*. The map *T* : *M* → *X* is said to be *demiclosed* at 0 if, for every sequence {*x*_{
n
} } in *M* converging weakly to *x* and {*Tx*_{
n
} } converges to 0 ∈ *X*, then *Tx* = 0.

## 3. Generalized \mathcal{J}\mathcal{H}-operator classes

We begin this section by introduce a new noncommuting class.

**Definition 3.1**. Let *f* and *T* be selfmaps of a normed space *X*. The order pair (*f*, *T*) is called a *generalized*\mathcal{J}\mathcal{H}-*operator with order n* if there exists a point *w* = *fx* = *Tx* in *PC*(*f*, *T*) such that

for some *n* ∈ ℕ.

It is obvious that a \mathcal{J}\mathcal{H}-operator pair (*f*, *T*) is generalized \mathcal{J}\mathcal{H}-operator with order *n*. But the converse is not true in general, see Example 3.2.

**Example 3.2**. Let *X* = ℝ with usual norm and *M* = [0, ∞). Define *f*, *T* : *M* → *M* by

Then *C*(*f*, *T*) = {0, 2} and *PC*(*f*, *T*) = {3, 5}. Obvious (*f*, *T*) is a generalized \mathcal{J}\mathcal{H}-operator with order *n* ≥ 2 but not a \mathcal{J}\mathcal{H}-operator and so not a occasionally weakly compatible and not weakly compatible. Moreover, note that *F*(*T*) = {1} and *f* 1 = 2 ∉ *F*(*T*) which implies that (*f*, *T*) is not a Banach operator pair.

**Theorem 3.3**. *Let f and T be selfmaps of a nonempty subset M of a normed space X and* (*f*, *T*) *be a generalized*\mathcal{J}\mathcal{H}-*operator with order n on M. If f and T satisfying the following condition:*

*for all x*, *y* ∈ *M and* 0 ≤ *k <* 1, *then f and T have a unique common fixed point*.

*Proof*. By the notation of generalized \mathcal{J}\mathcal{H}-operator, we get that there exists a point *w* ∈ *M* such that *w* = *fx* = *Tx* and

for some *n* ∈ ℕ. Suppose there exists another point *y* ∈ *M* for which *z* = *fy* = *Ty*. Then from (3.2), we get

Since 0 ≤ *k <* 1, the inequality (3.4) implies that ||*Tx* - *Ty*|| = 0, which, in turn implies that *w* = *fx* = *Tx* = *z*. Therefore, there exists a unique element *w* in *M* such that *w* = *fx* = *Tx*. So diam(*PC*(*f*, *T*)) = 0. Using (3.3), we have

Thus *w* = *x*, that is *x* is a unique common fixed point of *f* and *T*. □

**Definition 3.4**. Let *M* be a *q*-starshaped subset of a normed space *X* and *f*, *T* selfmaps of a normed space *M*. The order pair (*f*, *T*) is called a *generalized*\mathcal{J}\mathcal{H}-*suboperator with order n* if for each *k* ∈ [0, 1], (*f*, *T*_{
k
} ) is a generalized \mathcal{J}\mathcal{H}-operator with order *n* that is, for *k* ∈ [0, 1] there exists a point *w* = *fx* = *T*_{
k
}*x* in *PC*(*f*, *T*_{
k
} ) such that

for some *n* ∈ ℕ, where *T*_{
k
} is selfmap of *M* such that *T*_{
k
}*x* = *kTx* + (1 - *k*)*q* for all *x* ∈ *M*.

Clearly, a generalized \mathcal{J}\mathcal{H}-suboperator with order *n* is generalized \mathcal{J}\mathcal{H}-operator with order *n* but the converse is not true in general, see Example 3.5.

**Example 3.5**. Let *X* = ℝ with usual norm and *M* = [0, ∞). Define *f*, *T* : *M* → *M* (see Example 3.2). Then *M* is *q*-starshaped for *q* = 0 and *C*(*f*, *T*) = {0, 2}, C\left(f,{T}_{k}\right)=\left\{\frac{2}{k}\right\}, and PC\left(f,{T}_{k}\right)=\left\{\frac{4}{k}\right\} for *k* ∈ (0, 1). Obvious (*f*, *T*) is a generalized \mathcal{J}\mathcal{H}-operator with *n* = 2 but not a generalized \mathcal{J}\mathcal{H}-suboperator for every *n* ∈ ℕ as

for each *k* ∈ (0, 1).

**Theorem 3.6**. *Let f and T be selfmaps on a q-starshaped subset M of a normed space X. Assume that f is q-affine*, (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}-*suboperator with order n*_{0}, *and for all x*, *y* ∈ *M*,

*Then F*(*f*, *T*) ≠ ∅ *if one of the following conditions holds:*

**(a):** *cl*(*T*(*M*)) *is compact and f and T are continuous;*

**(b):** *wcl*(*T*(*M*)) *is weakly compact, f is weakly continuous and* (*f* - *T*) *is demiclosed at* 0;

**(c):** *T*(*M*) *is bounded, T is semicompact and f and T are continuous;*

**(d):** *T*(*M*) *is bounded, T is weakly semicompact, f is weakly continuous and* (*f* - *T*) *is demiclosed at* 0.

*Proof*. Let {*k*_{
n
} } ⊆ (0, 1) such that *k*_{
n
} → 1 as *n* → ∞. For *n* ∈ ℕ, we define *T*_{
n
} : *M* → *M* by *T*_{
n
}*x* = *k*_{
n
}*Tx* + (1 - *k*_{
n
} )*q* for all *x* ∈ *M*. Since (*f*, *T* ) is a generalized \mathcal{J}\mathcal{H}-suboperator with order *n*_{0}, (*f*, *T*_{
n
} ) is a generalized \mathcal{J}\mathcal{H}-operator order *n*_{0} for all *n* ∈ ℕ. Using inequality (3.7) it follows that

for all *x*, *y* ∈ *M*. By Theorem 3.3, there exists *x*_{
n
} ∈ *M* such that *x*_{
n
} = *fx*_{
n
} = *T*_{
n
}*x*_{
n
} for every *n* ∈ ℕ.

(a): As *cl*(*T*(*M*)) is compact, there exists a subsequence {*Tx*_{
m
} } of {*Tx*_{
n
} } such that \underset{m\to \infty}{lim}T{x}_{m}=y for some *y* ∈ *M*. By the definition of *T*_{
m
} , we get

Since *f* and *T* are continuous, *y* = *fy* = *Ty* that is *y* ∈ *F*(*f*, *T*) and then *F*(*f*, *T*) ≠ ∅.

(b): From weakly compact of *wcl*(*T*(*M*)) there exist a subsequence {*x*_{
m
} } of {*x*_{
n
} } in *M* converging weakly to *y* ∈ *M* as *m* → ∞. Since *f* is weakly continuous, *fy* = *y* that is \underset{m\to \infty}{lim}\left(f{x}_{m}-T{x}_{m}\right)=0. It follows from (*f* - *T*) is demiclosed at 0 and \underset{m\to \infty}{lim}\left(f{x}_{m}-T{x}_{m}\right)=0 that *fy* - *Ty* = 0. Therefore, *y* = *fy* = *Ty* that is *F*(*f*, *T*) ≠ ∅.

(c): Since *T*(*M*) is bounded, *k*_{
n
} → 1, and

for all *n* ∈ ℕ, we get \underset{m\to \infty}{lim}\left({x}_{n}-T{x}_{n}\right)=0. As *T* is semicompact, there exist a subsequence {*x*_{
m
} } of {*x*_{
n
} } in *M* such that \underset{m\to \infty}{lim}{x}_{m}=y for some *y* ∈ *M*. By definition of *T*_{
m
} , we get

By the continuous of both *f* and *T*, we have *y* = *fy* = *Ty*. Therefore *F*(*f*, *T*) ≠ ∅.

(d): Similarly case (c), we have \underset{m\to \infty}{lim}\left({x}_{n}-T{x}_{n}\right)=0. Since *T* is weakly semicompact, there exist a subsequence {*x*_{
m
} } of {*x*_{
n
} } in *M* such that converging weakly to *y* ∈ *M* as *m* → ∞. By weak continuity of *f*, we get *fy* = *y*. It follows from \underset{m\to \infty}{lim}\left(f{x}_{m}-T{x}_{m}\right)=\underset{m\to \infty}{lim}\left({x}_{m}-T{x}_{m}\right)=0, *x*_{
m
} converging weakly to *y*, and *f* - *T* is demiclosed at 0 that (*f* - *T*)(*y*) = 0 which implies that *fy* = *Ty*. Therefore *y* = *fy* = *Ty* and hence *y* ∈ *F*(*f*, *T*).

□

**Remark 3.7**. We can replace assumption of *f* being *q*-affine by *q* ∈ *F*(*f*) and *f*(*M*) = *M* in Theorem 3.6.

If *f* is identity mapping in Theorem 3.6, then we get the following corollary.

**Corollary 3.8**. *Let T be selfmaps on a q-starshaped subset M of a normed space X. Assume that for all x*, *y* ∈ *M*,

*Then F*(*T*) ≠ ∅ *if one of the following conditions holds:*

(a): *cl*(*T*(*M*)) *is compact and T is continuous;*

(b): *wcl*(*T*(*M*)) *is weakly compact and* (*I* - *T*) *is demiclosed at* 0, *where I is identity on M;*

(c): *T*(*M*) *is bounded, T is semicompact and T is continuous;*

(d): *T*(*M*) *is bounded, T is weakly semicompact and* (*I* - *T*) *is demiclosed at* 0, *where I is identity on M*.

## 4. Invariant approximations

In 1999, invariant approximations for noncommuting maps were considered by Shahzad [32]. As *M* is a subset of a normed space *X* and *p* ∈ *X*, let

and

The set *B*_{
M
} (*p*) is called the set of best approximants to *p* ∈ *X* out of *M*. Let {\mathcal{C}}_{0} denote the class of closed convex subsets *M* of *X* containing 0. It is known that *B*_{
M
} (*p*) is closed, convex, and contained in {M}_{p}\in {\mathcal{C}}_{0}.

**Theorem 4.1**. *Let M be a subset of a normed space X, f and T be selfmaps of X with T*(∂*M* ∩ *M*) ⊆ *M, p* ∈ *F*(*f*, *T*), *B*_{
M
} (*p*) *be a closed q-starshaped. Assume that f*(*B*_{
M
} (*p*)) = *B*_{
M
} (*p*), *q* ∈ *F* (*f* ), (*f*, *T* ) *is a generalized*\mathcal{J}\mathcal{H}-*suboperator with order n*_{0}*on B*_{
M
} (*p*), *and for all x*, *y* ∈ *B*_{
M
} (*p*) ∪ {*p*},

*If cl*(*T*(*B*_{
M
} (*p*))) *is compact, f and T are continuous on B*_{
M
} (*p*), *then F* (*f*, *T* )∩*B*_{
M
}(*p*) ≠ ∅.

*Proof*. Let *x* ∈ *B*_{
M
} (*p*). It follows from ||*kx* + (1 - *k*)*p* - *p*)|| = *k*||*x* - *p*|| *< d*(*p*, *M*) for all *k* ∈ (0, 1) that {*kx*+(1 - *k*)*p* : *k* ∈ (0, 1)}∩*M* ≠ ∅ which implies that *x* ∈ ∂*M* ∩ *M*. So *B*_{
M
} (*p*) ⊆ ∂*M* ∩ *M* and hence *T*(*B*_{
M
} (*p*)) ⊆ *T* (∂*M* ∩ *M* ). As *T* (∂*M* ∩ *M* ) ⊆ *M* that *T*(*B*_{
M
} (*p*)) ⊆ *M*. Now the result follows from Theorem 3.6 (*a*) with *M* = *B*_{
M
} (*p*). Therefore, *F*(*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅. □

**Theorem 4.2**. *Let M be a subset of a normed space X, f and T be selfmaps of X with T*(∂*M* ∩ *M*) ⊆ *M, p* ∈ *F*(*f*, *T*), {C}_{M}^{f}\left(p\right)*be a closed q-starshaped. Assume that*f\left({C}_{M}^{f}\left(p\right)\right)={C}_{M}^{f}\left(p\right), *q* ∈ *F* (*f* ), (*f*, *T* ) *is a generalized*\mathcal{J}\mathcal{H} -*suboperator with order n*_{0}*on*{C}_{M}^{f}\left(p\right), *and for all*x,y\in {C}_{M}^{f}\left(p\right)\cup \left\{p\right\},

*If*cl\left(T\left({C}_{M}^{f}\left(p\right)\right)\right)*is compact, f and T are continuous on*{C}_{M}^{f}\left(p\right), *then F* (*f*, *T*)∩*B*_{
M
} (*p*) ≠ ∅.

*Proof*. Let x\in {C}_{M}^{f}\left(p\right). By definition of {C}_{M}^{f}\left(p\right) and f\left({C}_{M}^{f}\left(p\right)\right)={C}_{M}^{f}\left(p\right), we have {C}_{M}^{f}\left(p\right)\subseteq {B}_{M}\left(p\right). Using the same argument in the proof of Theorem 4.1 shows that there exists *x* ∈ ∂*M* ∩ *M*. It follows from *T*(∂*M* ∩ *M*) ⊆ *f*(*M*) ∩ *M* that *Tx* ∈ *f*(*M*). Therefore, we can find a point *z* ∈ *M* such that *Tx* = *fz*. Thus z\in {C}_{M}^{f}\left(p\right) which implies that T\left({C}_{M}^{f}\left(p\right)\right)\subseteq f\left({C}_{M}^{f}\left(p\right)\right)={C}_{M}^{f}\left(p\right). Now the result follows from Theorem 3.6 (*a*) with M={B}_{M}^{f}\left(p\right). Therefore, we have *F* (*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅. □

**Theorem 4.3**. *Let M be a subset of a normed space X, f and T be selfmaps of X with T*(∂*M* ∩ *M*) ⊆ *M, p* ∈ *F*(*f*, *T*), *B*_{
M
} (*p*) *be a weakly closed and q-starshaped. Assume that f*(*B*_{
M
} (*p*)) = *B*_{
M
} (*p*), *q* ∈ *F* (*f*), (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}-*suboperator with order n*_{0}*on B*_{
M
} (*p*), *and for all x*, *y* ∈ *B*_{
M
} (*p*) ∪ {*p*},

*If wcl*(*T*(*B*_{
M
}(*p*))) *is weakly compact, f is weakly continuous on B*_{
M
}(*p*) *and* (*f* - *T*) *is demiclosed at* 0, *then F*(*f*, *T*) ∩ *B*_{
M
}(*p*) ≠ ∅.

*Proof*. We use an argument similar to that in Theorem 4.1 and apply Theorem 3.6 (*b*) instead of Theorem 3.6 (*a*). □

**Theorem 4.4**. *Let M be a subset of a normed space X, f and T be selfmaps of X with T*(∂*M* ∩ *M*) ⊆ *M, p* ∈ *F*(*f*, *T*), {C}_{M}^{f}\left(p\right)*be a weakly closed and q-starshaped. Assume that*f\left({C}_{M}^{f}\left(p\right)\right)={C}_{M}^{f}\left(p\right), *q* ∈ *F* (*f*), (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}-*suboperator with order n*_{0}*on*{C}_{M}^{f}\left(p\right), *and for all*x,y\in {C}_{M}^{f}\left(p\right)\cup \left\{p\right\},

*If*wcl\left(T\left({C}_{M}^{f}\left(p\right)\right)\right)*is weakly compact, f is weakly continuous on*{C}_{M}^{f}\left(p\right)*and* (*f* - *T*) *is* *demiclosed at* 0, *then F*(*f*, *T*) ∩ *B*_{
M
}(*p*) ≠ ∅.

*Proof*. We use an argument similar to that in Theorem 4.2 and apply Theorem 3.6 (*b*) instead of Theorem 3.6 (*a*). □

**Theorem 4.5**. *Let M be a subset of a normed space X, f and T be selfmaps of X, p* ∈ *F*(*f*, *T*), M\in {\mathcal{C}}_{0}*with T* (*M*_{
p
} ) ⊆ *f*(*M* ) ⊆ *M. Assume that* ||*fx* - *p*|| = ||*x* - *p*|| *for all x* ∈ *M and for all x*, *y* ∈ *M*_{
p
} ∪ {*p*},

*If cl*(*f*(*M*_{
p
} )) *is compact, then B*_{
M
} (*p*) *is nonempty, closed, and convex and T* (*B*_{
M
} (*p*)) ⊆ *f*(*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). *If in addition, for all x*, *y* ∈ *BM* (*p*),

*then F*(*f*) ∩ *B*_{
M
} (*p*) ≠ ∅ *and F*(*T*) ∩ *B*_{
M
} (*p*) ≠ ∅. *Moreover, F*(*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅ *if for some q* ∈ *B*_{
M
} (*p*), *f is q-affine and* (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}*suboperator with order n on B*_{
M
} (*p*).

*Proof*. Assume that *p* ∉ *M*. If *u* ∈ *M*\*M*_{
p
} , then ||*u*|| *>* 2||*p*||. Since 0 ∈ *M*, we get

Thus *α* := *d*(*p*, *M*_{
p
} ) = *d*(*p*, *M*). As *cl*(*f* (*M*_{
p
} )) is compact and the norm is continuous that there exists *z* ∈ *cl*(*f*(*M*_{
p
} )) such that *β* := *d*(*p*, *cl*(*f* (*M*_{
p
} ))) = ||*z* - *p*||. So we have

for all *y* ∈ *M*_{
p
} . Therefore, *α* = *β* and *B*_{
M
} (*p*) is nonempty closed and convex such that *f*(*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). Next step, we show that *T* (*B*_{
M
} (*p*)) ⊆ *f* (*B*_{
M
} (*p*)). Suppose that *w* ∈ *T*(*B*_{
M
} (*p*)). It follows from *T* (*B*_{
M
} (*p*)) ⊆ *T* (*M*_{
p
} ) ⊆ *f* (*M*) that there exists *w*_{1} ∈ *M*_{
p
} and *w*_{2} ∈ *M* such that *w* = *Tw*_{1} = *fw*_{2}. Using the condition (4.5), we have

Thus, *w*_{2} ∈ *B*_{
M
} (*p*) and *w*_{1} ∈ *f* (*B*_{
M
} (*p*)) which implies that *T* (*B*_{
M
} (*p*)) ⊆ *f* (*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). Now, suppose that *f* satisfies inequality (4.6) on *B*_{
M
} (*p*). Therefore, the condition (4.5) on *M*_{
p
} ∪ {*p*} implies that

for all *x*, *y* ∈ *B*_{
M
} (*p*). Since *f* (*M*_{
p
} ) is compact, *f* (*B*_{
M
} (*p*)) and *T* (*B*_{
M
} (*p*)) are compact. Moreover, *f*(*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*) and *T* (*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). It follows from Corollary 3.8 that *F*(*f*) ∩ *B*_{
M
} (*p*) ≠ ∅ and *F*(*T*) ∩ *BM* (*p*) ≠ ∅. Finally, we follow from Theorem 3.6 by replacing *M* with *B*_{
M
} (*p*). □

**Theorem 4.6**. *Let M be a subset of a normed space X, f and T be selfmaps of X, p* ∈ *F*(*f*, *T*), M\in {\mathcal{C}}_{0}*with T* (*M*_{
p
} ) ⊆ *f* (*M* ) ⊆ *M. Assume that* ||*fx* - *p*|| = ||*x* - *p*|| *for all x* ∈ *M and for all x*, *y* ∈ *M*_{
p
} ∪ {*p*},

*If cl*(*T*(*M*_{
p
} )) *is compact, then B*_{
M
} (*p*) *is nonempty, closed, convex, and T* (*B*_{
M
} (*p*)) ⊆ *f*(*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). *If in addition, for all x*, *y* ∈ *B*_{
M
} (*p*),

*then F*(*T*) ∩ *B*_{
M
} (*p*) ≠ ∅. *Moreover, F*(*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅ *if for some q* ∈ *B*_{
M
} (*p*), *f is q-affine and* (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}*suboperator with order n on B*_{
M
} (*p*).

*Proof*. We can obtain the result by using an argument similar to that in Theorem 4.5.

□

**Theorem 4.7**. *Let M be a subset of a Banach space X, f and T be selfmaps of X, p* ∈ *F*(*f*, *T*), M\in {\mathcal{C}}_{0}*with T* (*Mp*) ⊆ *f* (*M* ) ⊆ *M. Assume that* ||*fx* - *p*|| = ||*x* - *p*|| *for all x* ∈ *M and for all x*, *y* ∈ *M*_{
p
} ∪ {*p*},

*If wcl*(*f*(*M*_{
p
} )) *is weakly compact and* (*f* - *T*) *is demiclosed at* 0, *then B*_{
M
} (*p*) *is nonempty, (weakly) closed, and convex and T*(*B*_{
M
} (*p*)) ⊆ *f*(*B*_{
M
} (*p*)) ⊆ *B*_{
M
} (*p*). *If, in addition, for all x*, *y* ∈ *B*_{
M
} (*p*),

*then F*(*f*) ∩ *B*_{
M
} (*p*) ≠ ∅ *and F*(*T*) ∩ *B*_{
M
} (*p*) ≠ ∅. *Moreover, F*(*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅ *if for some q* ∈ *B*_{
M
} (*p*), *f is q-affine, weakly continuous on B*_{
M
} (*p*) *and* (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}*suboperator with order n on B*_{
M
} (*p*).

*Proof*. To obtain the result, we use an argument similar to that in Theorem 4.5 and apply Theorem 3.6 (*b*) instead of Theorem 3.6(a), respectively. Finally, we use Lemma 5.5 of Singh et al. [33] with *f*(*x*) = ||*x* - *p*|| and *C* = *wcl*(*T*(*M*_{
p
} )) to show that there exists *z* ∈ *C* such that *d*(*p*, *C*) = ||*z* - *p*||. □

**Theorem 4.8**. *Let M be a subset of a Banach space X, f and T be selfmaps of X, p* ∈ *F*(*f*, *T*), M\in {\mathcal{C}}_{0}*with T* (*M*_{
p
} ) ⊆ *f* (*M* ) ⊆ *M. Assume that* ||*fx* - *p*|| = ||*x* - *p*|| *for all x* ∈ *M and for all x*, *y* ∈ *M*_{
p
} ∪ {*p*},

*If wcl*(*f*(*M*_{
p
} )) *is weakly compact and* (*f* - *T*) *is demiclosed at* 0, *then B*_{
M
} (*p*) *is nonempty, (weakly) closed, and convex and T*(*B*_{
M
} (*p*)) ⊆ *f* (*BM* (*p*)) ⊆ *B*_{
M
} (*p*). *If in addition, for all x*, *y* ∈ *B*_{
M
} (*p*),

*then F*(*T*) ∩ *B*_{
M
} (*p*) ≠ ∅. *Moreover, F*(*f*, *T*) ∩ *B*_{
M
} (*p*) ≠ ∅ *if for some q* ∈ *B*_{
M
} (*p*), *f is q-affine, weakly continuous on B*_{
M
} (*p*) *and* (*f*, *T*) *is a generalized*\mathcal{J}\mathcal{H}*suboperator with order n on B*_{
M
} (*p*).

*Proof*. We can obtain the result using an argument similar to that in Theorem 4.7. □

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## Acknowledgements

Mr. Wutiphol Sintunavarat would like to thank the Research Professional Development Project Under the Science Achievement Scholarship of Thailand (SAST) and the Faculty of Science, KMUTT for financial support during the preparation of this manuscript for Ph.D. Program at KMUTT. The second author was supported by the Commission on Higher Education, the Thailand Research Fund and the King Mongkut's University of Technology Thonburi (KMUTT) (Grant No.MRG5380044).

Moreover, we also would like to thank the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission for financial support (Grant No. 54000267). Special thanks are also due to the reviewer, who have made a number of valuable comments and suggestions which have improved the manuscript greatly.

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### Authors' contributions

WS designed and performed all the steps of proof in this research and also wrote the paper. PK participated in the design of the study and suggest many good ideas that made this paper possible and helped to draft the first manuscript. All authors read and approved the final manuscript.

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Sintunavarat, W., Kumam, P. Common fixed point theorems for generalized \mathcal{J}\mathcal{H}-operator classes and invariant approximations.
*J Inequal Appl* **2011, **67 (2011). https://doi.org/10.1186/1029-242X-2011-67

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DOI: https://doi.org/10.1186/1029-242X-2011-67

### Keywords

- common fixed point
- occasionally weakly compatible maps
- Banach operator pair
- \mathcal{P}-operator pair
- \mathcal{J}\mathcal{H}-operator pair
- generalized \mathcal{J}\mathcal{H}-operator pair
- invariant approximation