We establish some common fixed and common coincidence point theorems for expansive type mappings in the setting of b 2 metric space. Our results extend some known results in metric spaces to b 2 metric space. The re search is meaningful and I recommend it to be published when the followings have been improved.
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Before stating our main results, some necessary definitions might be introduced
as follows.
Definition 2.1. [
1) d ( x , y ) = 0 if and only if x = y .
2) d ( x , y ) = d ( y , x ) .
3) d ( x , y ) ≤ s [ d ( x , y ) + d ( y , z ) ] .
In this case, the pair ( X , d ) is called a b metric space.
Definition 2.2. [
1) For every pair of distinct points x , y ∈ X , there exists a point z ∈ X such that d ( x , y , z ) ≠ 0 .
2)If at least two of three points x , y , z are the same, then d ( x , y , z ) = 0 .
3) The symmetry:
d ( x , y , z ) = d ( x , z , y ) = d ( y , x , z ) = d ( y , z , x ) = d ( z , x , y ) = d ( z , y , x )
for all x , y , z ∈ X .
4) The rectangle inequality: d ( x , y , z ) ≤ d ( x , y , a ) + d ( y , z , a ) + d ( z , x , a ) for all x , y , z , a ∈ X .
Then d is called a 2 metric on X and ( X , d ) is called a 2 metric space.
Definition 2.3. [
1) For every pair of distinct points x , y ∈ X , there exists a point z ∈ X such that d ( x , y , z ) ≠ 0 .
2) If at least two of three points x , y , z are the same, then d ( x , y , z ) = 0 .
3) The symmetry:
d ( x , y , z ) = d ( x , z , y ) = d ( y , x , z ) = d ( y , z , x ) = d ( z , x , y ) = d ( z , y , x )
for all x , y , z ∈ X .
4) The rectangle inequality: d ( x , y , z ) ≤ s [ d ( x , y , a ) + d ( y , z , a ) + d ( z , x , a ) ] for all x , y , z , a ∈ X .
Then d is called a b 2 metric on X and ( X , d ) is called a b 2 metric space with parameter s. Obviously, for s = 1 , b 2 metric reduces to 2 metric.
Definition 2.4. [
1) A sequence { x n } is said to be b2-convergent to x ∈ X , written as lim n → ∞ x n = x , if all a ∈ X , lim n → ∞ d ( x n , x , a ) = 0 .
2) { x n } is Cauchy sequence if and only if d ( x n , x m , a ) → 0 , when n , m → ∞ . for all a ∈ X .
3) ( X , d ) is said to be b2-complete if every b2-Cauchy sequence is a b2- convergent sequence.
Definition 2.5. [
Definition 2.6. [
Definition 2.7. [
Proposition 2.8. [
Theorem 3.1. Let ( X , d ) be a b 2 metric space. Suppose mappings f , g : X → X are onto and satisfy
d ( f x , f y , a ) ≥ α d ( g x , f x , a ) + β d ( g y , f y , a ) + γ d ( g x , g y , a ) (1)
for all x , y , a ∈ X and x ≠ y , where α , β ∈ ℝ , γ > 0 . Suppose the following hypotheses:
1) f X or g X is complete,
2) α + β + γ > 2 ,
3) g X ⊂ f X .
Then f and g have a coincidence point.
Proof. From 2), we get α + γ > 0 or β + γ > 0 . Indeed, if we suppose α + γ ≤ 0 and β + γ ≤ 0 , we have α + β + 2 γ ≤ 0 . Since γ ≥ 0 , we have α + β + γ ≤ 0 . That is a contradiction.
Let x 0 ∈ X , since g X ⊂ f X , we take x 1 ∈ X such that f x 1 = g x 0 . Again, we can take x 2 ∈ X such that f x 2 = g x 1 . Continuing in the same way, we construct two sequences { x n } and { y n } in X such that y n = f x n + 1 = g x n for all n ∈ ℕ .
If g x m − 1 = g x m for some m ∈ ℕ , then f x m = g x m . Thus x m is a coincidence point of f and g .
Now, assume that x n − 1 ≠ x n for all n ∈ ℕ .
Step 1: It is shown that lim n → ∞ d ( y n + 1 , y n + 2 , a ) = 0 .
Suppose β + γ > 0 , take x = x n + 1 , y = x n + 2 into (1). we have
d ( y n , y n + 1 , a ) ≥ α d ( y n , y n + 1 , a ) + ( β + γ ) d ( y n + 1 , y n + 2 , a ) (2)
Then
( 1 + α ) d ( y n , y n + 1 , a ) ≥ ( β + γ ) d ( y n + 1 , y n + 2 , a ) , for all n ∈ ℕ (3)
Since β + γ > 0 , ( 1 − α ) d ( y n , y n + 1 , a ) ≥ 0 . If 1 − α < 0 , then d ( y n , y n + 1 , a ) = 0 . If 1 − α = 0 , then d ( y n + 1 , y n + 2 , a ) = 0 . Therefore { y n } is constant sequence when 1 ≤ α . Suppose 1 − α > 0 , then 0 < 1 − α β + γ < 1 and
d ( y n + 1 , y n + 2 , a ) ≤ 1 − α β + γ d ( y n , y n + 1 , a ) (4)
Suppose α + γ > 0 , take x = x n + 2 , y = x n + 1 into (1). We have
d ( y n , y n + 1 , a ) ≥ ( α + γ ) d ( y n + 1 , y n + 2 , a ) + β d ( y n , y n + 1 , a ) (5)
Then
( 1 − β ) d ( y n , y n + 1 , a ) ≥ ( α + γ ) d ( y n + 1 , y n + 2 , a ) , for all n ∈ ℕ (6)
Similarly, since α + γ > 0 , suppose 1 − β > 0 , then 0 < 1 − β α + γ < 1 and
d ( y n + 1 , y n + 2 , a ) ≤ 1 − β α + γ d ( y n , y n + 1 , a ) (7)
Let h = max { 1 − α β + γ , 1 − β α + γ } , we know 0 < h < 1 , applying (4) and (7), we get
d ( y n + 1 , y n + 2 , a ) ≤ h d ( y n , y n + 1 , a ) ≤ ⋯ ≤ h n + 1 d ( y 0 , y 1 , a ) , n = 0,1,2, ⋯ (8)
then lim n → ∞ d ( y n + 1 , y n + 2 , a ) = 0 .
Step 2: As { d ( y n , y n + 1 , a ) } is decreasing, if d ( y n − 1 , y n , a ) = 0 , then d ( y n , y n + 1 , a ) = 0 . Since part 2 of Definition 2.3, d ( y 0 , y 1 , y 0 ) = 0 , we have d ( y n , y n + 1 , y 0 ) = 0 for all n ∈ ℕ .
Since d ( y m − 1 , y m , y m ) = 0 , we have
d ( y n , y n + 1 , y m ) = 0 (9)
for all n ≥ m − 1 . For 0 ≤ n < m − 1 , we have m − 1 ≥ n + 1 , and from(9) we have
d ( y m − 1 , y m , y n + 1 ) = d ( y m − 1 , y m , y n ) = 0 (10)
It implies that
d ( y n , y n + 1 , y m ) ≤ s d ( y n , y n + 1 , y m − 1 ) + s d ( y n + 1 , y m , y m − 1 ) + s d ( y m , y n , y m − 1 ) = s d ( y n , y n + 1 , y m − 1 ) (11)
Since d ( y n , y n + 1 , y n + 1 ) = 0 , from the above inequality, we have
d ( y n , y n + 1 , y m ) ≤ s m − n − 1 d ( y n , y n + 1 , y n + 1 ) = 0 (12)
for all 0 ≤ n ≤ m − 1 . From (9) and (12), we have
d ( y n , y n + 1 , y m ) = 0 (13)
for all n , m ∈ ℕ . Now, for all i , j , k ∈ ℕ with i < j , we have
d ( y j − 1 , y j , y i ) = d ( y j − 1 , y j , y k ) = 0 (14)
From (14) and triangular inequality, Therefore
d ( y i , y j , y k ) ≤ s [ d ( y i , y j , y j − 1 ) + d ( y j , y k , y j − 1 ) + d ( y k , y i , y j − 1 ) ] = s d ( y i , y j − 1 , y k ) ≤ ⋯ ≤ s j − i d ( y i , y i , y k ) = 0 (15)
This proves that for all i , j , k ∈ ℕ ,
d ( y i , y j , y k ) = 0 (16)
Step 3: It is proved that the sequence { y n } is a b2-Cauchy sequence. Let m , n ∈ ℕ with m > n . We claim that, there exists n 0 ∈ ℕ , such that
d ( y n , y m , a ) < ε (17)
for all m > n ≥ n 0 , a ∈ X . This is done by induction on m.
Let n ≥ n 0 and m = n + 1 . Then we get
d ( y n , y m , a ) = d ( y n , y n + 1 , a ) < d ( y n − 1 , y n , a ) < ε (18)
Then (17) holds for m = n + 1 .
Assume now that (17) holds for some m ≥ n + 1 . We will show that (17) holds for m + 1 . Take x = x n , y = x m + 1
d ( y n − 1 , y m , a ) = d ( f x n , f x m + 1 , a ) ≥ α d ( g x n , f x n , a ) + β d ( g x m + 1 , f x m + 1 , a ) + γ d ( g x n , g x m + 1 , a ) ≥ α d ( y n , y n − 1 , a ) + β d ( y m + 1 , y m , a ) + γ d ( y n , y m + 1 , a ) (19)
Then
s [ d ( y n − 1 , y n , y m ) + d ( y n − 1 , y n , a ) + d ( y n , y m , a ) ] ≥ α d ( y n , y n − 1 , a ) + β d ( y m + 1 , y m , a ) + γ d ( y n , y m + 1 , a ) (20)
We get
γ d ( y n , y m + 1 , a ) ≤ s [ d ( y n − 1 , y n , y m ) + d ( y n − 1 , y n , a ) + d ( y n , y m , a ) ] − α d ( y n , y n − 1 , a ) − β d ( y m + 1 , y m , a ) ≤ 2 s ε − ( α + β ) ε (21)
Then
d ( y n , y m + 1 , a ) ≤ 2 s ε − ( α + β ) ε γ = 2 s − α − β γ ε < ε (22)
Thus we have proved that (17) holds for m + 1 . From (17), we know { y n } is a Cauchy sequence in ( X , d ) .
If f X is complete, there exists u ∈ f X and p ∈ X such that y n = g x n = f x n + 1 → u = f p .
If β + γ > 0 , let x = x n + 1 , y = p into (1), We have
d ( y n , u , a ) ≥ α d ( y n , y n + 1 , a ) + β d ( g p , u , a ) + γ d ( y n + 1 , g p , a ) ≥ α d ( y n , y n + 1 , a ) + β d ( g p , u , a ) + γ [ d ( u , g p , a ) s − d ( y n + 1 , u , a ) − d ( y n + 1 , u , g p ) ] (23)
Therefore
( β + γ s ) d ( u , g p , a ) ≤ d ( y n , u , a ) + | α | d ( y n , y n + 1 , a ) + γ d ( y n + 1 , u , a ) + γ d ( y n + 1 , u , g p ) (24)
We take a natural number n 1 such that
d ( y n , u , a ) ≤ β + γ s 4 ε , | α | d ( y n , y n + 1 , a ) ≤ β + γ s 4 ε ,
d ( y n + 1 , u , a ) ≤ β + γ s 4 ε , d ( y n + 1 , u , g p ) ≤ β + γ s 4 ε
for n ≥ n 1 . Thus, we obtain d ( u , g p , a ) ≤ ε . Therefore f p = u = g p .
If α + γ > 0 , let x = p , y = x n + 1 into (1), We get
d ( y n , u , a ) ≥ α d ( g p , u , a ) + β d ( y n , y n + 1 , a ) + γ d ( y n + 1 , g p , a ) (25)
Therefore
( α + γ s ) d ( u , g p , a ) ≤ d ( y n , u , a ) + | β | d ( y n , y n + 1 , a ) + γ d ( y n + 1 , u , a ) + γ d ( y n + 1 , u , g p ) (26)
We take a natural number n 2 such that
d ( y n , u , a ) ≤ α + γ s 4 ε , | β | d ( y n , y n + 1 , a ) ≤ α + γ s 4 ε ,
d ( y n + 1 , u , a ) ≤ α + γ s 4 ε , d ( y n + 1 , u , g p ) ≤ α + γ s 4 ε
for n ≥ n 2 . Thus, we obtain d ( u , g p , a ) ≤ ε . Therefore f p = u = g p .
In short, no matter what the situation is, u is always the point of coincidence of f and g, p is the coincidence point of f and g.
If g X is complete, there exists u ∈ g X ⊂ f X and p , q ∈ X , such that y n = g x n → u = g q = f p . The rest proof is the same as that f X is complete.
Theorem 3.2. Let ( X , d ) be a b 2 metric space. Let f , g be mappings satisfying f X ⊃ g X and (1), for all α , β ∈ ℝ , γ > 1 . If 1). f X or g X is complete, 2). α + β + γ > 2 , 3). f and g is weakly compatible. Then f and g have a common fixed point.
Proof. According to Theorem 3.1, there exists u , p ∈ X such that u = f p = g p . Suppose there also exists v , z ∈ X such that v = f z = g z , choose x = p , y = z into (1), we get
d ( u , v , a ) = d ( f p , f z , a ) ≥ γ d ( g p , g z , a ) = γ d ( u , v , a ) (27)
Therefore, there exists d ( u , v , a ) = 0 , then u = v . f and g have the point of coincidence u . According to Proposition 2.8, u is the unique common fixed point of f and g .
Corollary 3.3. Let ( X , d ) be a complete b 2 metric space. Let f be surjective mapping satisfying d ( f x , f y , a ) ≥ α d ( x , f x , a ) + β d ( y , f y , a ) + γ d ( x , y , a ) , for all x , y , a ∈ X , x ≠ y , where with α , β ∈ ℝ , γ > 0 and α + β + γ > 2 , then f has a fixed point, if γ > 0 , then f has a unique fixed point.
Proof. Follows from Theorem 3.1, by taking g = 1 x , identify map, then we get the result.
In this paper, a known existence theorems of common fixed points for two mappings satisfying expansive conditions in b 2 metric space were generalized and improved. Based on the research, a new method to discuss the existence problems of common fixed points for mappings with this type expansive condition was taken out. And the results show that the proposed method is better than the former ones.
This project is supported by NSFC (grant No. 11761072, 11261062) and Research Fund for the Doctoral Program of Higher Education of China (grant No. 20114407120011).
Cui, J.X., Zhao, J.W. and Zhong, L.N. (2017) Unique Com- mon Fixed Point in b2 Metric Spaces. Open Access Library Journal, 4: e3896. https://doi.org/10.4236/oalib.1103896