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Research article

Viscosity-type method for solving pseudomonotone equilibrium problems in a real Hilbert space with applications

  • The aim of this article is to introduce a new algorithm by integrating a viscosity-type method with the subgradient extragradient algorithm to solve the equilibrium problems involving pseudomonotone and Lipschitz-type continuous bifunction in a real Hilbert space. A strong convergence theorem is proved by the use of certain mild conditions on the bifunction as well as some restrictions on the iterative control parameters. Applications of the main results are also presented to address variational inequalities and fixed-point problems. The computational behaviour of the proposed algorithm on various test problems is described in comparison to other existing algorithms.

    Citation: Habib ur Rehman, Poom Kumam, Kanokwan Sitthithakerngkiet. Viscosity-type method for solving pseudomonotone equilibrium problems in a real Hilbert space with applications[J]. AIMS Mathematics, 2021, 6(2): 1538-1560. doi: 10.3934/math.2021093

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  • The aim of this article is to introduce a new algorithm by integrating a viscosity-type method with the subgradient extragradient algorithm to solve the equilibrium problems involving pseudomonotone and Lipschitz-type continuous bifunction in a real Hilbert space. A strong convergence theorem is proved by the use of certain mild conditions on the bifunction as well as some restrictions on the iterative control parameters. Applications of the main results are also presented to address variational inequalities and fixed-point problems. The computational behaviour of the proposed algorithm on various test problems is described in comparison to other existing algorithms.


    The theory of fixed points takes an important place in the transition from classical analysis to modern analysis. One of the most remarkable works on fixed point of functions was done by Banach [9]. Various generalizations of Banach fixed point result were made by numerous mathematicians [1,2,3,6,12,13,14,17,19,20,25,26]. One of the generalizations of the metric space is the quasi metric space that was introduced by Wilson [30]. The commutativity condition does not hold in general in quasi metric spaces. Several authors used these concepts to prove some fixed point theorems, see [7,10,15,21]. On the other hand Bakhtin [8] and Czerwik [11] generalized the triangle inequality by multiplying the right hand side of triangle inequality in metric spaces by a parameter b1 and defined b-metric spaces, for more results, see [4,23,24]. Fixed point results for multivalued mappings generalizes the results for single-valued mappings. Many interesting results have been proved in the setting of multivalued mappings, for example, see [5,26,29]. Kamran et al. [12] introduced a new concept of generalized b metric spaces, named as extended b-metric spaces, see also [22]. They replaced the parameter b1 in triangle inequality by the control function θ:X×X[1,). Mlaiki et al. [16] replaced the triangle inequality in b-metric spaces by using control function in a different style and introduced controlled metric type spaces. Abdeljawad et al. [1] generalized the idea of controlled metric type spaces and introduced double controlled metric type spaces. They replaced the control function ξ(x,y) in triangle inequality by two functions ξ(x,y) and (x,y), see also [27,28]. In this paper, the concept of double controlled quasi metric type spaces has been discussed. Fixed point results and several examples are established. First of all, we discuss the previous concepts that will be useful to understand the paper.

    Now, we define double controlled metric type space.

    Definition 1.1.[1] Given non-comparable functions ξ,:X×X[1,). If :X×X[0,) satisfies:

    (q1) (ω,v)=0 if and only if ω=v,

    (q2) (ω,v)=(v,ω),

    (q3) (ω,v)ξ(ω,e)(ω,e)+(e,v)(e,v),

    for all ω,v,eX. Then, is called a double controlled metric with the functions ξ, and the pair (X,) is called double controlled metric type space with the functions ξ,.

    The classical result to obtain fixed point of a mapping in double controlled metric type space is given below.

    Theorem 1.2. [1] Let (X,) be a complete double controlled metric type space with the functions ξ, :X×X [1,) and let T:XX be a given mapping. Suppose that there exists k(0,1) such that

    (T(x),T(y)k(x,y), for all x,yX.

    For ω0X, choose ωg=Tgω0. Assume that

    supm1limiξ(ωi+1,ωi+2)ξ(ωi,ωi+1)(ωi+1,ωm)<1k.

    In the addition, assume that, for every ωX, we have

    limgξ(ω,ωg), and limg(ω g,ω) exists and are finite.

    Then T has a unique fixed point ωX.

    Now, we are introducing the concept of double controlled quasi metric type space and controlled quasi metric type space.

    Definition 1.3. Given non-comparable functions ξ,:X×X[1,). If :X×X[0,) satisfies

    (q1) (ω,v)=0 if and only if ω=v,

    (q2) (ω,v) ξ(ω,e)(ω,e)+(e,v)(e,v),

    for all ω,v,eX. Then, is called a double controlled quasi metric type with the functions ξ, and (X,) is called a double controlled quasi metric type space. If (e,v)=ξ(e,v) then (X,) is called a controlled quasi metric type space.

    Remark 1.4. Any quasi metric space or any double controlled metric type space is also a double controlled quasi metric type space but, the converse is not true in general, see examples (1.5, 2.4, 2.12 and 2.15).

    Example 1.5. Let X={0,1,2}. Define :X×X[0, ) by (0,1)=4, (0,2)=1, (1,0)=3=(1,2), (2,0)=0, (2,1)=2, (0,0)=(1,1)=(2,2)=0.

    Define ξ,:G×G[1, ) as ξ(0,1)= ξ(1,0)= ξ(1,2)=1, ξ(0,2)=54, ξ(2,0)=109, ξ(2,1)=2019, ξ(0,0)= ξ(1,1)= ξ(2,2)=1, (0,1)=(1,0)=(0,2)=(1,2)=1, (2,0)=32, (2,1)=118, (0,0)=(1,1)=(2,2)=1. It is clear that is double controlled quasi metric type with the functions ξ, . Let w=0, e=2, v=1, we have

    (0,1)=4>3=(0,2)+(2,1).

    So is not a quasi metric. Also, it is not a controlled quasi metric type. Indeed,

    (0,1)=4>134=ξ(0,2)(0,2)+ξ(2,1)(2,1).

    Moreover, it is not double controlled metric type space because, we have

    (0,1)=ξ(0,2)(0,2)+(2,1)(2,1)=5516(1,0).

    The convergence of a sequence in double controlled quasi metric type space is defined as:

    Definition 1.6. Let (X,) be a double controlled quasi metric type space with two functions. A sequence {ut} is convergent to some u in X if and only if limt(ut,u)=limt(u,ut)=0.

    Now, we discuss left Cauchy, right Cauchy and dual Cauchy sequences in double controlled quasi metric type space.

    Definition 1.7. Let (X,) be a double controlled quasi metric type space with two functions.

    (i) The sequence {ut} is a left Cauchy if and only if for every ε>0 such that (um,ut)<ε, for all t>m>tε, where tε is some integer or limt,m(um,ut)=0.

    (ii) The sequence {ut} is a right Cauchy if and only if for every ε>0 such that (um,ut)<ε, for all m>t>tε, where tε is some integer.

    (iii) The sequence {ut} is a dual Cauchy if and only if it is both left as well as right Cauchy.

    Now, we define left complete, right complete and dual complete double controlled quasi metric type spaces.

    Definition 1.8. Let (X,) be a double controlled quasi metric type space. Then (X,) is left complete, right complete and dual complete if and only if each left-Cauchy, right Cauchy and dual Cauchy sequence in X is convergent respectively.

    Note that every dual complete double controlled quasi metric type space is left complete but the converse is not true in general, so it is better to prove results in left complete double controlled quasi metric type space instead of dual complete.

    Best approximation in a set and proximinal set are defined as:

    Definition 1.9. Let (,) be a double controlled quasi metric type space. Let A be a non-empty set and l. An element y0A is called a best approximation in A if

    (l,A)=(l,y0), where (l,A)=infyA(l,y)and (A,l)=(y0,l), where (A,l)=infyA(y,l).

    If each l has a best approximation in A, then A is know as proximinal set. P() is equal to the set of all proximinal subsets of .

    Double controlled Hausdorff quasi metric type space is defined as:

    Definition 1.10. The function H:P(E)×P(E)[0,), defined by

    H(C,F)=max{supaC(a,F), supbF(C,b)}

    is called double controlled quasi Hausdorff metric type on P(E). Also (P(E),H) is known as double controlled Hausdorff quasi metric type space.

    The following lemma plays an important role in the proof of our main result.

    Lemma 1.11. Let (X,) be a double controlled quasi metric type space. Let (P(E),H) be a double controlled Hausdorff quasi metric type space on P(E). Then, for all C,FP(E) and for each cC, there exists fcF, such that H(C,F)(c,fc) and H(F,C)(fc,c).

    Let (X,) be a double controlled quasi metric type space, u0X and T:XP(X) be multifunctions on X. Let u1Tu0 be an element such that (u0,Tu0)=(u0,u1), (Tu0,u0)=(u1,u0). Let u2Tu1 be such that (u1,Tu1)=(u1,u2), (Tu1,u1)=(u2,u1). Let u3Tu2 be such that (u2,Tu2)=(u2,u3) and so on. Thus, we construct a sequence ut of points in X such that ut+1Tut with (ut,Tut)=(ut,ut+1), (Tut,ut)=(u t+1,ut), where t=0,1,2,. We denote this iterative sequence by {XT(ut)}. We say that {XT(ut)} is a sequence in X generated by u0 under double controlled quasi metric . If is quasi b-metric then, we say that {XT(ut)} is a sequence in X generated by u0 under quasi b -metric . We can define {XT(ut)} in other metrics in a similar way.

    Now, we define double controlled rational contracion which is a generalization of many other classical contractions.

    Definition 2.1. Let (X,) be a complete double controlled quasi-metric type space with the functions α,μ :X×X [1,). A multivalued mapping T:XP(X) is called a double controlled rational contracion if the following conditions are satisfied:

    H(Tx,Ty)k(Q(x,y)), (2.1)

    for all x,yX, 0<k<1 and

    Q(x,y)=max{(x,y),(x,Tx),(x,Tx)(x,Ty)+(y,Ty)(y,Tx)(x,Ty)+(y,Tx)}.

    Also, for the terms of the sequence {XT(ut)}, we have

    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)<1k (2.2)

    and for every uX

    limtα(u,ut) and limtμ(ut,u) are finite. (2.3)

    Now, we prove that an operator T satisfying certain rational contraction condition has a fixed point in double controlled quasi metric type space.

    Theorem 2.2. Let (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and T:XP(X) be a double controlled rational contracion. Then, T has a fixed point uX.

    Proof. By Lemma 1.11 and using inequality (2.1), we have

    (ut,ut+1)H(Tut1,Tut)k(Q(ut1,ut)).
    Q(ut1,ut)max{(ut1,ut),(ut1,ut),(ut1,ut)(ut1,Tut)+(ut,ut+1)(ut,Tut1)(ut1,Tut)+(ut,Tut1)}=(ut1,ut).

    Therefore,

    (ut,ut+1)k(ut1,ut). (2.4)

    Now,

    (ut1,ut)H(Tut2,Tut1)k(Q(ut2,ut1)).
    Q(ut2,ut1)=max{(u t2,ut1),(ut2,ut1),
    (ut2,ut1)(ut2,Tut1)+(ut1,ut)(ut1,Tut2)(ut2,Tut1)+(ut1,Tut2)}.

    Therefore,

    (ut1,ut)k(ut2,ut1). (2.5)

    Using (2.5) in (2.4), we have

    (ut,ut+1)k2(ut2,ut1).

    Continuing in this way, we obtain

    (ut,ut+1)kt(u0,u1). (2.6)

    Now, by using (2.6) and by using the technique given in [1], it can easily be proved that {ut} is a left Cauchy sequence. So, for all natural numbers with t<m, we have

    limt,m(ut,um)=0. (2.7)

    Since (X,) is a left complete double controlled quasi metric type space, there exists some uX such that

    limt(ut,u)=limt(u,ut)=0. (2.8)

    By using triangle inequality and then (2.1), we have

    (u,Tu)α(u,ut+1)(u,ut+1)+μ(ut+1,Tu)(ut+1,Tu)
    α(u,ut+1)(u,ut+1)+μ(ut+1,Tu)max{(ut,u),(ut,ut+1),(ut,ut+1)(ut,Tu)+(u,Tu)(u,ut)(ut,Tu)+(u,ut)}.

    Using (2.3), (2.7) and (2.8), we get (u,Tu)0. That is, uTu. Thus u is a fixed point of T.

    If we take single-valued mapping instead of multivalued mapping, then we obtain the following result.

    Theorem 2.3. Let (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and T:XX be a mapping such that:

    (Tx,Ty)k(Q(x,y)),

    for all x,yX, 0<k<1 and

    Q(x,y)=max{(x,y),(x,Tx),(x,Tx)(x,Ty)+(y,Ty)(y,Tx)(x,Ty)+(y,Tx)},

    Suppose that, for every uX and for the Picard sequence {ut}

    limtα(u,ut), limtμ(ut,u) are finite and
    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)<1k.

    Then, T has a fixed point uX.

    We present the following example to illustrate Theorem 2.3.

    Example 2.4. Let X={0,1,2,3}. Define :X×X[0, ) by: (0,1)=1, (0,2)=4, (0,3)=5, (1,0)=0, (1,2)=10, (1,3)=1, (2,0)=7, (2,1)=3, (2,3)=5, (3,0)=3, (3,1)=6, (3,2)=2, (0,0)=(1,1)=(2,2)=(3,3)=0. Define α,μ:X×X[1, ) as: α(0,1)=2, α(1,2)= α(0,2)= α(1,0)=α(2,0)=α(3,1)=α(2,3)=α(0,3)=1, α(1,3)=2, α(2,1)=73, α(3,0)=43, α(3,2)=32, α(0,0)=α(1,1)=α(2,2)=α(3,3)=1, μ(1,2)=μ(2,1)=μ(2,0)=μ(3,0)=μ(0,3)=1, μ(1,0)=32, μ(0,1)=2, μ(1,3)=3, μ(3,1)=1, μ(3,2)=4, μ(2,3)=1, μ(0,2)=52, μ(0,0)=μ(1,1)=μ(2,2)=μ(3,3)=1. Clearly (X,) is a double controlled quasi metric type space, but it is not a controlled quasi metric type space. Indeed,

    (1,2)=10>4=α(1,0)(1,0)+α(0,2)(0,2).

    Also, it is not a double controlled metric type space. Take T0=T1={0}, T2=T3={1} and k=13. We observe that

    (Tx,Ty)k(Q(x,y)), for all x,yX.

    Let u0=2, we have u1=Tu0=T2=1, u2=Tu1=0, u3=Tu2=0,

    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)=2<3=1k.

    Also, for every uX, we have

    limtα(u,ut)< and limtμ(ut,u)<.

    All hypotheses of Theorem 2.3 are satisfied and u=0 is a fixed point.

    As every quasi b-metric space is double controlled quasi metric type space but the converse is not true in general, so we obtain a new result in quasi b-metric space as a corollary of Theorem 2.3.

    Theorem 2.5. Let (X,) be a left complete quasi b -metric space and T:XX be a mapping. Suppose that there exists k(0,1) such that

    (Tx,Ty)k(Q(x,y))

    whenever,

    Q(x,y)=max{(x,y),(x,Tx),(x,Tx)(x,Ty)+(y,Ty)(y,Tx)(x,Ty)+(y,Tx)},

    for all x,yX. Assume that 0<bk<1. Then, T has a fixed point uX.

    Remark 2.6. In the Example 2.3, note that is quasi b -metric with b=103, but we can not apply Theorem 2.5 for any b=103 and k=13, because bk1.

    Quasi metric version of Theorem 2.2 is given below:

    Theorem 2.7. Let (X,) be a left complete quasi metric space and T:XP(X) be a multivalued mapping. Suppose that there exists 0<k<1 such that

    H(Tx,Ty)k(max{(x,y),(x,Tx),(x,Tx)(x,Ty)+(y,Ty)(y,Tx)(x,Ty)+(y,Tx)})

    for all x,yX. Then T has a fixed point uX.

    Now, we extend the sequence {XT(ut)} for two mappings. Let (X,) be a double controlled quasi metric type space, u0X and S,T:XP(X) be the multivalued mappings on X. Let u1Su0 such that (u0,Su0)=(u0,u1) and (Su0,u0)=(u1,u0). Now, for u1X, there exist u2Tu1 such that (u1,Tu1)=(u1,u2) and (Tu1,u1)=(u2,u1). Continuing this process, we construct a sequence ut of points in X such that u2t+1Su2t, and u2t+2Tu2t+1 with (u2t,Su2t)=(u2t,u2+1), (Su2t,u2t)=(u2t+1,u2t) and (u2t+1,Tu2t+1)=(u2t+1,u2t+2), (Tu2t+1,u2t+1)=(u2t+2,u2t+1). We denote this iterative sequence by {TS(ut)} and say that {TS(ut)} is a sequence in X generated by u0.

    Now, we introduce double controlled Reich type contraction.

    Definition 2.8. Let X be a non empty set, (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and S,T:XP(X) be a multivalued mappings. Suppose that the following conditions are satisfied:

    H(Sx,Ty)c((x,y))+k((x,Sx)+(y,Ty)), (2.9)
    H(Tx,Sy)c((x,y))+k((x,Tx)+(y,Sy)), (2.10)

    for each x,yX, 0<c+2k<1. For u0X, choose ut{TS(ut)}, we have

    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui+1,um)<1kc+k. (2.11)

    Then the pair (S,T) is called a double controlled Reich type contraction.

    The following results extend the results of Reich [18].

    Theorem 2.9. Let S,T:XP(X) be the multivalued mappings, (X,) be a left complete double controlled quasi metric type space and (S,T) be a pair of double controlled Reich type contraction. Suppose that, for all uX

    limtα(u,ut) is finite and limtμ(ut,u)<1k. (2.12)

    Then, S and T have a common fixed point ˙z in X.

    Proof. Consider the sequence {TS(ut)}. Now, by Lemma 1.11, we have

    (u2t,u2t+1)H(Tu2t1,Su2t) (2.13)

    By using the condition (2.10), we get

    (u2t,u2t+1)c((u2t1,u2t)+k((u2t1,Tu2t1)+(u2t,Su2t)c((u2t1,u2t)+k((u2t1,u2t)+(u2t,u2t+1))
    (u2t,u2t+1)η((u2t1,u2t)), (2.14)

    where η=c+k1k. Now, by Lemma 1.11, we have

    (u2t1,u2t)H(Su2t2,Tu2t1).

    So, by using the condition (2.9), we have

    (u2t1,u2t)c(u2t2,u2t1)+k((u2t2,Su2t2)+(u2t1,Tu2t1))c(u2t2,u2t1)+k((u2t2,u2t1)+(u2t1,u2 t))
    (u2t1,u2t)c+k1k((u2t2,u2t1))=η((u2t2,u2t1)). (2.15)

    Using (2.14) in (2.15), we have

    (u2t,u2t+1)η2(u2t2,u2t1). (2.16)

    Now, by Lemma 1.11 we have

    (u2t2,u2t1)H(Tu2t3,Su2t2).

    Using the condition (2.10), we have

    (u2t2,u2t1)c(u2t 3,u2t2)+k((u2t3,u2t2)+(u2t2,u2t1))

    implies

    (u2t2,u2t1)η((u2t3,u2t2)). (2.17)

    From (2.16) and (2.17), we have

    η2((u2t2,u2t1))η3((u2t3,u2t2)). (2.18)

    Using (2.18) in (2.14), we have

    (u2t,u2t+1)η3((u2t3,u2t2)).

    Continuing in this way, we get

    (u2t,u2t+1)η2t((u0,u1)). (2.19)

    Similarly, by Lemma 1.11, we have

    (u2t1,u2t)η2t1((u0,u1)).

    Now, we can write inequality (2.19) as

    (ut,ut+1)ηt((u0,u1)). (2.20)

    Now, by using (2.20) and by using the technique given in [1], it can easily be proved that {ut} is a left Cauchy sequence. So, for all natural numbers with t<m, we have

    limt,m(ut,um)=0. (2.21)

    Since (X,) is a left complete double controlled quasi metric type space. So {ut}˙zX, that is

    limt(ut,˙z)=limt(˙z,ut)=0. (2.22)

    Now, we show that ˙z is a common fixed point. We claim that (˙z,T˙z)=0. On contrary suppose (˙z,T˙z)>0. Now by Lemma 1.11, we have

    (u2t+1,T˙z)H(Su2t,T˙z).
    (u2t+1,T˙z)c((u2t,˙z))+k[(u2t,u2t+1)+(˙z,T˙z))] (2.23)

    Taking limt on both sides of inequality (2.23), we get

    limt(u2t+1,T˙z)climt(u2t,˙z)+klimt[(u2t,u2t+1)+(˙z,T˙z))]

    By using inequalities (2.21) and (2.22), we get

    limt(u2t+1,T˙z)k((˙z,T˙z)) (2.24)

    Now,

    (˙z,T˙z)α(˙z,u2t+1)(˙z,u2t+1)+μ(u2t+1,T˙z)(u2t+1,T˙z)

    Taking limt and by using inequalities (2.12), (2.22) and (2.24), we get

    (˙z,T˙z)<(˙z,T˙z).

    It is a contradiction, therefore

    (˙z,T˙z)=0.

    Thus, ˙zT˙z. Now, suppose (˙z,S˙z)>0. By Lemma 1.11, we have

    (u2t,S˙z)H(Tu2t1,S˙z).

    By inequality (2.10), we get

    (u2t,S˙z)c((u2t1,˙z))+k[(u2t1,u2t)+(˙z,S˙z)].

    Taking limt on both sides of above inequality, we get

    limt(u2t,S˙z)k((˙z,S˙z)). (2.25)

    Now,

    (˙z,S˙z)α(˙z,u2t)(˙z,u2t)+μ(u2t,S˙z)(u2t,S˙z).

    Taking limt and by using inequality (2.12), (2.22) and (2.25), we get

    (˙z,S˙z)<(˙z,S˙z).

    It is a contradiction. Hence, ˙zS˙z. Thus, ˙z is a common fixed point for S and T.

    Theorem 2.10 presents a result for single-valued mapping which is a consequence of the previous result.

    Theorem 2.10. Let (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and S,T:XX be the mappings such that:

    (Sx,Ty)c((x,y))+k((x,Sx)+(y,Ty))

    and

    (Tx,Sy)c((x,y))+k((x,Tx)+(y,Sy)),

    for each x,yX, 0<c+2k<1. Suppose that, for every uX and for the Picard sequence {ut}

    limtα(u,ut) is finite, limtμ(ut,u)<1k and
    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)<1kc+k.

    Then S and T have a common fixed point uX.

    Quasi b-metric version of Theorem 2.10 is given below:

    Theorem 2.11. Let (X,) be a left complete quasi b-metric type space with the functions α,μ:X×X[1,) and S,T:XX be the mappings such that:

    (Sx,Ty)c((x,y))+k((x,Sx)+(y,Ty))

    and

    (Tx,Sy)c((x,y))+k((x,Tx)+(y,Sy)),

    for each x,yX, 0<c+2k<1 and b<1kc+k. Then S and T have a common fixed point uX.

    The following example shows that how double controlled quasi metric type spaces can be used where the quasi b-metric spaces cannot.

    Example 2.12. Let X={0,12,14,1}. Define :X×X[0, ) by (0,12)=1, (0,14)=13, (14,0)=15, (12,0)=1, (14,12)=3, (14,1)=12, (1,14)=13 and (x,y)=|xy|, if otherwise. Define α,μ:X×X[1, ) as follows α(12,14)=165, α(0,14)=32,  α(14,1)=3,  α(1,14)=125 and α(x,y)=1, if otherwise. μ(0,12)=145,  μ(1,12)=3 and μ(x,y)=1, if otherwise. Clearly is double controlled quasi metric type for all x,y,zX. Let, T0={0}, T12={14}, T14={0}, T1={14},  S0=S14={0}, S12={14}, S1={0} and c=25, k=14. Now, if we take the case x=12, y=14, we have

    H(S12,T14)=H({14},{0})=(14,0)=151780=c((x,y))+k((x,Sx)+(y,Ty)). Also, satisfied for all cases x,yX. That are inequalities (2.9) and (2.10) hold. Take u0=1, then u1=Su0=0, u2=Tu1=0, u3=Su2=0.

    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui+1,um)=1<1513=1kc+k.

    which shows that inequality (2.11) holds. Thus the pair (S,T) is double controlled Reich type contraction. Finally, for every uX, we obtain

    limtα(u,ut) is finite, limtμ(ut,u)1k.

    All hypotheses of Theorem 2.9 are satisfied and ˙z=0 is a common fixed point.

    Note that is quasi b-metric with b=3, but Theorem 2.11 can not be applied because b1kc+k, for all b=3. Therefore, this example shows that generalization from a quasi b-metric spaces to a double controlled quasi metric type spaces is real.

    Taking c=0 in Theorem 2.9, we get the following result of Kannan-type.

    Theorem 2.13. Let (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and S,T:XP(X) be the multivalued mappings such that:

    H(Sx,Ty)k((x,Sx)+(y,Ty))

    and

    H(Tx,Sy)k((x,Tx)+(y,Sy)),

    for each x,yX, 0<c+2k<1. Suppose that, for every uX and for the sequence {TS(ut)}, we have

    limtα(u,ut) is finite, limtμ(ut,u)<1k and
    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)<1kk.

    Then S and T have a common fixed point uX. Then, S and T have a common fixed point ˙z in X.

    Taking c=0 and S=T in Theorem 2.9, we get the following result.

    Theorem 2.14. Let (X,) be a left complete double controlled quasi-metric type space with the functions α,μ:X×X[1,) and T:XP(X) be a multivalued mapping such that:

    H(Tx,Ty)k((x,Tx)+(y,Ty))

    for each x,yX, 0<c+2k<1. Suppose that, for every uX and for the sequence {T(ut)}, we have

    limtα(u,ut) is finite, limtμ(ut,u)<1k and
    supm1limiα(ui+1,ui+2)α(ui,ui+1)μ(ui,um)<1kk.

    Then T has a fixed point.

    Now, the following example illustrates Theorem 2.14.

    Example 2.15. Let X=[0,3). Define :X×X[0, ) as

    (x,y)={0, if x=y,(xy)2+x, if xy.

    with

    α(x,y)={2, if x,y1,x+22, otherwise., μ(x,y)={1, if x,y1y+22, otherwise.

    Clearly (X,) is double controlled quasi metric type space. Choose Tx={x4} and k=25. It is clear that T is Kannan type double controlled contraction. Also, for each u X, we have

    limtα(u,ut)<, limtμ(ut,u)<1k.

    Thus, all hypotheses of Theorem 2.14 are satisfied and ˙z=0 is the fixed point.

    Quasi b-metric version of Theorem 2.14 is given below:

    Theorem 2.16. Let (X,) be a left complete quasi b -metric space and T:XP(X) be a mapping such that:

    H(Tx,Ty)k[(x,Tx)+(y,Ty)],

    for all x,yX, k[0,12) and

    b<1kk.

    Then T has a fixed point uX.

    The following remark compare, distinguish and relate the quasi b-metric with the double controlled quasi metric type spaces and illustrate the importance of double controlled quasi metric type spaces.

    Remark 2.17. In the Example 2.15, (x,y)=(xy)2+x is a quasi b-metric with b2, but we can not apply Theorem 2.16 because T is not Kannan type b-contraction. Indeed b1kk, for all b2.

    It has been shown that double controlled quasi metric is general and better than other metrics, like controlled quasi metric, controlled metric, extended quasi-b-metric, extended b-metric, quasi-b-metric and quasi metric. Also, left, right and dual completeness has been discussed. Results in dual complete spaces can be obtained as corollaries. These results may be extended to obtain results for other contractions. Double controlled quasi metric like ordered spaces can be introduced to establish new results. These results may be applied to find applications to random impulsive differential equations, dynamical systems, graph theory and integral equations.

    The authors would also like to thank the editor and the reviewers for their helpful comments and suggestions. This research was funded by the Center of Excellence in Theoretical and Computational Science (TaCS-CoE), KMUTT.

    The authors declare that they do not have any competing interests.



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