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Research article

Communicable disease model in view of fractional calculus

  • Received: 10 January 2023 Revised: 15 February 2023 Accepted: 19 February 2023 Published: 24 February 2023
  • MSC : 34A08

  • The COVID-19 pandemic still gains the attention of many researchers worldwide. Over the past few months, China faced a new wave of this pandemic which increases the risk of its spread to the rest of the world. Therefore, there has become an urgent demand to know the expected behavior of this pandemic in the coming period. In this regard, there are many mathematical models from which we may obtain accurate predictions about the behavior of this pandemic. Such a target may be achieved via updating the mathematical models taking into account the memory effect in the fractional calculus. This paper generalizes the power-law growth model of the COVID-19. The generalized model is investigated using two different definitions in the fractional calculus, mainly, the Caputo fractional derivative and the conformable derivative. The solution of the first-model is determined in a closed series form and the convergence is addressed. At a specific condition, the series transforms to an exact form. In addition, the solution of the second-model is evaluated exactly. The results are applied on eight European countries to predict the behavior/variation of the infected cases. Moreover, some remarks are given about the validity of the results reported in the literature.

    Citation: Weam G. Alharbi, Abdullah F. Shater, Abdelhalim Ebaid, Carlo Cattani, Mounirah Areshi, Mohammed M. Jalal, Mohammed K. Alharbi. Communicable disease model in view of fractional calculus[J]. AIMS Mathematics, 2023, 8(5): 10033-10048. doi: 10.3934/math.2023508

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  • The COVID-19 pandemic still gains the attention of many researchers worldwide. Over the past few months, China faced a new wave of this pandemic which increases the risk of its spread to the rest of the world. Therefore, there has become an urgent demand to know the expected behavior of this pandemic in the coming period. In this regard, there are many mathematical models from which we may obtain accurate predictions about the behavior of this pandemic. Such a target may be achieved via updating the mathematical models taking into account the memory effect in the fractional calculus. This paper generalizes the power-law growth model of the COVID-19. The generalized model is investigated using two different definitions in the fractional calculus, mainly, the Caputo fractional derivative and the conformable derivative. The solution of the first-model is determined in a closed series form and the convergence is addressed. At a specific condition, the series transforms to an exact form. In addition, the solution of the second-model is evaluated exactly. The results are applied on eight European countries to predict the behavior/variation of the infected cases. Moreover, some remarks are given about the validity of the results reported in the literature.



    This paper considers a fractional coupled system on an infinite interval involving the Erdélyi-Kober derivative:

    {Dγ,δ1βu(x)+F(x,u(x),v(x))=0,x(0,+),Dγ,δ2βv(x)+G(x,u(x),v(x))=0,x(0,+),limx0xβ(2+γ)Iδ1+γ,2δ1u(x)=0,limxxβ(1+γ)Iδ1+γ,2δ1u(x)=0,limx0xβ(2+γ)Iδ2+γ,2δ2v(x)=0,limxxβ(1+γ)Iδ2+γ,2δ2v(x)=0, (1.1)

    where δ1,δ2(1,2], γ(2,1), and β>0. Dγ,δ1β, Dγ,δ2β are Erdélyi-Kober fractional derivatives (EKFDs for short), and Iδ1+γ,2δ1,Iδ2+γ,2δ2 are the Erdélyi-Kober fractional integrals. F,G are continuous functions. We discuss the existence of positive solutions for (1.1).

    During the past several decades, fractional equations have been studied widely; see [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36] for instance. From the literature, we can see that there are many fractional derivatives used in differential equations. Among these various definitions, the widely used ones are the Riemann-Liouville and Caputo fractional derivatives, in many works. To generalize the Riemann-Liouville fractional derivative, Erdélyi-Kober defined a new fractional derivative, and we call it the Erdélyi-Kober fractional derivative. Moreover, the Erdélyi-Kober operator is very useful; we can refer to [6,9,14,15,16,17] and the references therein. The Erdélyi-Kober operator is a fractional integration operation which was given by Arthur Erdélyi and Hermann Kober in 1940 [23]. Some of these definitions and results were given in Samko et al. [3], Kiryakova [19], and McBride [20].

    Nowadays, the theory of fractional operators in the Erdélyi-Kober frame has attracted much interest from researchers. The study of fractional systems is also very important, as these systems appear in various applications, especially in biological sciences. Recently, some problems of Erdélyi-Kober type fractional differential equations on infinite intervals received widespread attention from many scholars; see [8,21,22] for example.

    Recently, in [8], the authors investigated the following equation:

    {(Dϑ,σθu)(x)+F(u(x))=0,0x<,limt0xθ(2σ)Iσ+ϑ,2σu(x)=0,limt+xθ(2σ)Iσ+ϑ,2σu(x)=0,

    where σ(1,2), ϑ(1,2), θ>0, and F is a given continuous function, Dϑ,σθ denotes the EKFD, and Iσ+ϑ,2σ denotes the Erdélyi-Kober fractional integral. The authors studied the existence and nonexistence of positive solutions for this problem by utilizing a fixed point result which uses the strongly positive-like operators and eigenvalue criteria.

    In [9], the authors studied a fractional coupled system:

    {cDϱu(τ)=F(τ,u(τ),z(τ),cDς1z(τ),Iξz(τ)),τ[0,T]:=K,2<ϱ3,1<ς1<2,cDςz(τ)=G(τ,u(τ),cDϱ1u(τ),Iζu(τ),z(τ),τ[0,T]:=K,2<ς3,1<ϱ1<2,u(0)=ϕ1(z),u(0)=ε1z(k1),u(T)=γρϑρ(ϖ+v)Γ(ϖ)ϑ0σρv+ρ1z(σ)(ϑρσρ)1ϖdσ:=γJv,ϖρv(ϑ),z(0)=ϕ2(u),z(0)=ε2z(k2),z(T)=δvφv(θ+ω)Γ(θ)φ0σvω+υ1u(σ)(φvσv)1θdσ:=δJω,θvu(φ),

    where cDϱ,cDς1,cDς,cDϱ1 are the Liouville-Caputo fractional derivatives of order 2<ϱ,ς3, 1<ς1,ϱ1<2. Iξ,Iζ are the Riemann-Liouville fractional integrals of order 1<ξ,ζ<2. Jυ,ϖρ,Jω,θv are the Erdélyi-Kober fractional integrals of order ϖ,θ>0, with v,ω>0, ρ, ϑ(,+). F,G:K×(,+)4(,+) and ϕ1,ϕ2:C(K,(,+))(,+) are continuous functions. γ,δ,ε1,ε2 are positive real constants. The existence result was given by the Leray-Schauder alternative, and the uniqueness result was obtained due to Banach's fixed-point theorem. By the same methods, Arioua and Titraoui [18] studied system (1.1). Moreover, In [10], Arioua and Titraoui also investigated a new fractional problem involving the Erdélyi-Kober derivative. Inspired by the above articles, we use different methods to consider the fractional coupled system involving Erdélyi-Kober derivative (1.1). We employ the Guo-Krasnosel'skii fixed point theorem to discuss (1.1) in a special Banach space, and we also use the monotone iterative technique to study this system. Some existence results of positive solutions for system (1.1) are obtained, including the existence results of at least two positive solutions.

    Definition 2.1. (see [2]) Let α(,+). Cnα, nN, denotes a set of all functions f(t),t>0, with f(t)=tpf1(t) with p>α and f1Cn[0,).

    Definition 2.2. (see [1,2]) For a function uCα, the σ-order right-hand Erdélyi-Kober fractional integral is

    (Iγ,σβu)(t)=βtβ(γ+σ)Γ(σ)t0sβ(γ+1)1u(s)(tβsβ)1σds,σ,β>0,γ(,+),

    in which, Γ is the Euler gamma function.

    Definition 2.3. (see [2]) Let n1<δn,nN, and for uCα, the σ-order right-hand Erdélyi-Kober fractional derivative is

    (Dγ,σβu)(t)=nj=1(γ+j+tβddt)(Iγ+σ,nσβu)(t),

    where

    nj=1(γ+j+tβddt)(Iγ+σ,nσβu)=(γ+1+tβddt)(γ+n+tβddt)(Iγ+σ,nσβu).

    Lemma 2.1. (see [10]) Let 1<σ2, 2<γ<1, β>0, and hC2α, with 0sβ(γ+m)1h(τ)dτ<, m=1,2. The fractional problem

    {Dγ,σβu(x)+h(x)=0,x>0,limx0xβ(2+γ)Iσ+γ,2δu(x)=0,limxxβ(1+γ)Iσ+γ,2σu(x)=0,

    has a unique solution given by u(x)=0Gσ(x,s)sβ(γ+1)1h(s)ds, where

    Gσ(x,s)={βΓ(σ)[xβ(γ+1)xβ(δ+γ)(xβsβ)σ1],0<sx<,βΓ(σ)xβ(γ+1),0<xs<. (2.1)

    Lemma 2.2. (see [10]) For 1<σ2, 2<γ<1, and β>0, the function Gσ, defined in (2.1), has the following properties:

    (i) Gσ(x,s)1+xβ(1+γ)>0, for x,s>0;

    (ii) Gσ(x,s)1+xβ(1+γ)βΓ(σ), for x,s>0;

    (iii) for 0<τλxτ and s>τλ2, where λ>1,τ>0, we have

    Gσ(x,s)1+xβ(1+γ)β(σ1)τβ(1+γ)Γ(σ)λβ(1γ)(1+τβ(1+γ))=βp(τ)Γ(σ),

    where p(τ)=(σ1)τβ(1+γ)λβ(1+γ)(1+τβ(1+γ)).

    Lemma 2.3. (see [18]) Let 0<σ1,σ21 and F,GC2α with

    0sβ(γ+m)1F(s,u(s),v(s))ds<,m=1,2,
    0sβ(γ+m)1G(s,u(s),v(s))ds<,m=1,2.

    Then, (1.1) has a unique solution given by

    u(x)=0Gσ1(x,s)sβ(γ+1)1F(s,u(s),v(s))ds,
    v(x)=0Gσ2(x,s)sβ(γ+1)1G(s,u(s),v(s))ds,

    where

    Gσ1(x,s)={βΓ(σ1)[xβ(γ+1)xβ(σ1+γ)(xβsβ)σ11],0<sx<,βΓ(σ1)xβ(γ+1),0<xs<, (2.2)
    Gσ2(x,s)={βΓ(σ2)[xβ(γ+1)xβ(σ2+γ)(xβsβ)σ21],0<sx<,βΓ(σ2)xβ(γ+1),0<xs<. (2.3)

    The following result is our main tool.

    Lemma 2.4. (Guo-Krasnosel'skii fixed point theorem; see [37]) P is a cone in a Banach space E, and D1 and D2 are bounded open sets in E with θD1, ¯D1D2. A:P(¯D2D1)P is a completely continuous operator. Consider the following conditions (ⅰ), (ⅱ):

    (i) Aww for wPD1, Aww for wPD2;

    (ii) Aww for wPD1, Aww for wPD2.

    If one of the preceding conditions (ⅰ), (ⅱ) holds, then A has at least one fixed point in P(¯D2D1).

    Next, we present some hypotheses that will play an important role in the subsequent discussion:

    (H1) F,G:(0,+)×(,+)×(,+)(0,+) are continuous and nondecreasing with respect to the second, third variables on (0,+).

    (H2) For (x,u,v)(0,+)×(,+)×(,+),

    F1(x,u,v)=xβ(1+γ)1F(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v),
    F2(x,u,v)=xβ(1+γ)1G(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v),

    such that

    F1(x,u,v)φ1(x)ω1(u)+ψ1(x)ω2(v),
    F2(x,u,v)φ2(x)~ω1(u)+ψ2(x)~ω2(v),

    with ωi,~ωiC((0,+),(0,+)) nondecreasing and φi,ψiL1(0,+), i=1,2.

    (H3) There are positive functions qi,˜qi,i=1,2, with

    qi=0(1+xβ(1+γ))qi(x)dx<,
    ˜qi=0(1+xβ(1+γ))˜qi(x)dx<,

    such that

    xβ(γ+1)1F(x,u,v)F(x,˜u,˜v)∣≤q1(x)u˜u+˜q1(x)v˜v,
    xβ(γ+1)1G(x,u,v)G(x,˜u,˜v)∣≤q2(x)u˜u+˜q2(t)v˜v,

    for any u,v,˜u,˜v(,+) and x(0,+).

    (H4) F,G:(0,+)×(0,+)×(0,+)(0,+) are continuous, such that

    xβ(1+γ)1F(x,u,v)=a1(x)F1(x,u,v),
    xβ(1+γ)1G(x,u,v)=a2(x)G1(x,u,v),

    where a1,a2L1((0,+),(0,+)), F1,G1C((0,+)×(0,+)×(0,+),(0,+)), 0<ττλa1(x)dx<, 0<ττλa2(x)dx<, with τ>0, λ>1. Moreover, xβ(1+γ)1F(x,u,v), xβ(1+γ)1G(x,u,v):[0,+)×(0,+)×(0,+)[0,+) also are continuous.

    Remark 2.1. These conditions ensure the continuity and integrability of nonlinear terms in an infinite interval, which play a very important role in the proof of completely continuity for the relevant integral operators.

    In this section, we use two Banach spaces defined by

    X={uC((0,+),(,+))limx0u(x)1+xβ(1+γ) and limt+u(x)1+xβ(1+γ) exist},

    with the norm

    uX=supx>0u(x)1+xβ(1+γ),

    and

    Y={vC((0,+),(,+))limx0v(x)1+xβ(1+γ) and limx+v(x)1+xβ(1+γ) exist},

    with the norm

    vY=supx>0v(x)1+xβ(1+γ).

    So, (X×Y,(u,v)X×Y) is a Banach space, with the norm (u,v)X×Y=uX+vY.

    Lemma 3.1. If F,G are continuous, then (u,v)X×Y is a solution of system (1.1)(u,v)X×Y is a solution of the following equations:

    {u(x)=0Gσ1(x,s)sβ(γ+1)1F(s,u(s),v(s))ds,v(x)=0Gσ2(x,s)sβ(γ+1)1G(s,u(s),v(s))ds.

    For (u,v)X×Y, we define an operator A:X×YX×Y as follows:

    A(u,v)(x)=(A1(u,v)(x),A2(u,v)(x)),

    where

    A1(u,v)(x)=0Gσ1(x,s)sβ(γ+1)1F(s,u(s),v(s))ds,
    A2(u,v)(x)=0Gσ2(x,s)sβ(γ+1)1G(s,u(s),v(s))ds,

    with Gσi(x,s),i=1,2, given by (2.2) and (2.3).

    Remark 3.1. Let σ1,σ2,β,γ,λ,τR, such that 1<σ1,σ22,β>0,2<γ<1,λ>1,τ>0. If (H2) and (H4) hold, then for (u,v)X×Y with u(x),v(x)>0,

    0sβ(γ+1)1F(s,u(s),v(s))dsητλ2sβ(γ+1)1F(s,u(s),v(s))ds,
    0sβ(γ+1)1G(s,u(s),v(s))dsητλ2sβ(γ+1)1G(s,u(s),v(s))ds,

    where η=max{η1,η2} with η1=1+ιϱ1(λ21),η2=1+ιϱ2(λ21)>1, ϱ1,ϱ2,ι,ι>0.

    Proof. By (H4), for x[τλ2,τ], we know that there exist two constants ϱ1,ϱ2>0, such that

    xβ(γ+1)1F(s,u,v)ϱ1,xβ(γ+1)1G(s,u,v)ϱ2,u,v(0,+).

    So, for (u,v)X×Y with u(x),v(x)>0,

    τλ2sβ(γ+1)1F(s,u(s),v(s))dsττλ2sβ(γ+1)1F(s,u(s),v(s))dsτ(λ21)λ2ϱ1,
    τλ2sβ(γ+1)1G(s,u(s),v(s))dsττλ2sβ(γ+1)1G(s,u(s),v(s))dsτ(λ21)λ2ϱ2,

    and hence,

    λ2τ(λ21)ϱ1τλ2sβ(γ+1)1F(s,u(s),v(s))ds1,
    λ2τ(λ21)ϱ2τλ2sβ(γ+1)1G(s,u(s),v(s))ds1.

    By (H4), we know that there exist two constants ι,ι>0, such that

    xβ(γ+1)1F(x,u(x),v(x))ι,xβ(γ+1)1G(x,u(x),v(x))ι,for  x[0,τλ2].

    Thus,

    τλ20sβ(γ+1)1F(s,u(s),v(s))dsιτλ2,
    τλ20sβ(γ+1)1G(s,u(s),v(s))dsιτλ2.

    Therefore, we can obtain

    0sβ(γ+1)1F(s,u(s),v(s))ds=τλ20sβ(γ+1)1F(s,u(s),v(s))ds+τλ2sβ(γ+1)1F(s,u(s),v(s))dsιτλ2+τλ2sβ(γ+1)1F(s,u(s),v(s))ds(1+ιϱ1(λ21))τλ2sβ(γ+1)1F(s,u(s),v(s))ds=η1τλ2sβ(γ+1)1F(s,u(s),v(s))ds.

    Similarly,

    0sβ(γ+1)1G(s,u(s),v(s))ds(1+ιϱ2(λ21))τλ2sβ(γ+1)1G(s,u(s),v(s))ds=η2τλ2sβ(γ+1)1G(s,u(s),v(s))ds.

    Take η=max{η1,η2}, and thus

    0sβ(γ+1)1F(s,u(s),v(s))dsητλ2sβ(γ+1)1F(s,u(s),v(s))ds,
    0sβ(γ+1)1G(s,u(s),v(s))dsητλ2sβ(γ+1)1G(s,u(s),v(s))ds,

    hold.

    Define two cones

    K1={uXu(x)>0,x>0;minx[τλ,τ]u(x)1+xβ(1+γ)p(τ)ηuX},
    K2={vYv(x)>0,x>0;minx[τλ,τ]v(x)1+xβ(1+γ)p(τ)ηvY}.

    Obviously, K1×K2={(u,v)X×Yu(x)>0,v(x)>0,x>0;  minx[τλ,τ]u(x)1+xβ(1+γ)p(τ)ηuX,minx[τλ,τ]v(x)1+xβ(1+γ)p(τ)ηvY} is also a cone. For convenience, we first list the following definitions:

    F0=lim(u,v)(0+,0+)supx>0F1(t,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    f=lim(u,v)(+,+)infx>0F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    f0=lim(u,v)(0+,0+)infx>0F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    F=lim(u,v)(+,+)supx>0F1(t,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    G0=lim(u,v)(0+,0+)supx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    g=lim(u,v)(+,+)infx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    g0=lim(u,v)(0+,0+)infx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v,
    G=lim(u,v)(+,+)supx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)u+v.

    Lemma 4.1. If assumptions (H1) and (H2) hold, then A:K1×K2K1×K2 is completely continuous.

    Proof. First, we show A:K1×K2K1×K2. By (H1) and (H2), for (u,v)K1×K2,

    A1(u,v)X=supt>0|A1(u,v)(x)|1+xβ(1+γ)=supx>00Gσ1(x,s)1+xβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))dsβΓ(σ1)0sβ(γ+1)1F(s,u(s),v(s))ds=βΓ(σ1)0sβ(γ+1)1F(s,(1+sβ(1+γ))u(s)1+sβ(1+γ),(1+sβ(1+γ))v(s)1+sβ(1+γ))ds=βΓ(σ1)0F1(s,u(s)1+sβ(1+γ),v(s)1+sβ(1+γ))βΓ(σ1)[ω1(uX)0φ1(s)ds+ω2(vY)0ψ1(s)ds]<+.

    Similarly,

    A2(u,v)YβΓ(σ1)[~ω1(uX)0φ2(s)ds+~ω2(vY)0ψ2(s)ds]<+.

    By (H1) and Lemma 2.2, for (u,v)K1×K2, we have A1(u,v)(x)>0,A2(u,v)(x)>0,x>0. From Lemma 2.2 and Remark 3.1, for x[τλ,τ],τ>0, and λ>1,

    |A1(u,v)(x)|1+xβ(1+γ)=0Gσ1(x,s)1+xβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))ds=τλ20Gσ1(x,s)1+xβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))ds+0τλ2Gσ1(x,s)1+xβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))ds0τλ2Gσ1(t,s)1+tβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))dsβp(τ)Γ(σ1)0τλ2sβ(γ+1)1F(s,u(s),v(s))dsβp(τ)ηΓ(σ1)0sβ(γ+1)1F(s,u(s),v(s))dsp(τ)ηA1(u,v)X.

    So, A1(u,v)(x)1+xβ(1+γ)p(τ)ηA1(u,v)X. Similarly, A2(u,v)(x)1+xβ(1+γ)p(τ)ηA2(u,v)Y. Therefore,

    minx[τλ,τ]A1(u,v)(x)1+xβ(1+γ)p(τ)ηA1(u,v)X,
    minx[τλ,τ]A2(u,v)(x)1+xβ(1+γ)p(τ)ηA2(u,v)Y.

    That is, A:K1×K2K1×K2 is true.

    Second, it will give a simply prove that A is continuous. Let D={(u,v)|(u,v)K1×K2,(u,v)X×YK,K>0}, a bounded subset in K1×K2. Let (un,vn)D be a sequence that converges to (u,v) in K1×K2. Then (un,vn)X×YK. From Lemma 2.2,

    A1(un,vn)A1(u,v)X=supx>0A1(un,vn)(x)A1(u,v)(x)1+xβ(1+γ)βΓ(σ1)0sβ(γ+1)1F(s,un(s),vn(s))ds0sβ(γ+1)1F(s,u(s),v(s))dsβΓ(σ1)0sβ(γ+1)1(F(s,un(s),vn(s))F(s,u(s),v(s)))ds.

    By (H2),

    sβ(γ+1)1F(s,un(s),vn(s))=sβ(γ+1)1F(s,(1+sβ(1+γ))un(s)1+sβ(1+γ),(1+sβ(1+γ))vn(s)1+sβ(1+γ))=F1(s,un(s)1+sβ(1+γ),vn(s)1+sβ(1+γ))φ1(s)ω1(unX)+ψ1(s)ω2(vnY)L1(0,).

    By the continuity of sβ(γ+1)1F(s,u(s),v(s)) and the Lebesgue dominated convergence theorem,

    0sβ(γ+1)1F(s,un(s),vn(s))ds0sβ(γ+1)1F(s,u(s),v(s))ds,n.

    Therefore, A1(un,vn)A1(u,v)X0,n. Similarly, A2(un,vn)A2(u,v)Y0,n.

    So, A(un,vn)A(u,v)X×Y0,n. That is, A is continuous in D. In the end, we know that A(D) is relatively compact on (0,) and is equi-convergent at by [18]. Therefore, A:K1×K2K1×K2 is completely continuous.

    Theorem 4.1. Assume that (H2) and (H4) hold. If F0=0,G0=0,f=,g=, then the system (1.1) has at least one positive solution.

    Proof. We divide the proof into several steps.

    Step 1. A:K1×K2K1×K2 is completely continuous. This result easily follows from Lemma 4.1.

    Step 2. We show that there exist R1>0 and D1={(u,v)X×Y,(u,v)X×Y<R1} such that A(u,v)X×Y(u,v)X×Y, (u,v)(K1×K2)D1.

    Because F0=0,G0=0, we choose R1>0, such that

    F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ1(u+v),
    G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ2(u+v),

    for 0<u+vR1,x>0, where ϵ1,ϵ2>0 satisfy

    ϵ112Γ(σ1)β0a1(s)ds,ϵ212Γ(σ2)β0a2(s)ds.

    So, for (u,v)K1×K2 and (u,v)X×Y=R1, by Lemma 2.2,

    A1(u,v)(x)1+xβ(1+γ)=0Gσ1(x,s)1+xβ(1+γ)sβ(γ+1)1F(s,u(s),v(s))dsβΓ(σ1)0sβ(γ+1)1F(s,u(s),v(s))ds,
    A2(u,v)(x)1+xβ(1+γ)=0Gσ2(x,s)1+xβ(1+γ)sβ(γ+1)1G(s,u(s),v(s))dsβΓ(σ2)0sβ(γ+1)1G(s,u(s),v(s))ds.

    By (H4),

    A1(u,v)(x)1+xβ(1+γ)βΓ(σ1)0a1(s)F1(s,u(s),v(s))ds=βΓ(σ1)0a1(s)F1(s,(1+sβ(1+γ))u(s)1+sβ(1+γ),(1+sβ(1+γ))v(s)1+sβ(1+γ))dsβΓ(σ1)0a1(s)ϵ1u(s)+v(s)1+sβ(1+γ)dsβΓ(σ1)ϵ1(u,v)X×Y0a1(s)ds12(u,v)X×Y.

    Similarly,

    A2(u,v)(x)1+xβ(1+γ)βΓ(σ2)ϵ2(u,v)X×Y0a2(s)ds12(u,v)X×Y.

    Therefore,

    A(u,v)X×Y(u,v)X×Y, for (u,v)K1×K2, and (u,v)X×Y=R1.

    Let D1={(u,v)X×Y,(u,v)X×Y<R1}. Then,

    A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D1.

    Step 3. We show that there exist R2>0 and D2={(u,v)X×Y,(u,v)X×Y<R2} such that

    A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D2.

    Because f=,g=, there exists R>0, such that

    F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)m1(u+v),
    G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)m2(u+v),

    for u+vR,x>0, where m1,m2>0 satisfy

    m112η1ηΓ(σ1)βp2(τ)τλτa1(s)ds,m212η2ηΓ(σ2)βp2(τ)τλτa2(s)ds,η=max{η1,η2}.

    Let R2max{R1,ηRp(τ)}, and D2={(u,v)X×Y,(u,v)X×Y<R2}. Then, D1D2.

    Thus, for  (u,v)K1×K2, (u,v)X×Y=R2, we have

    u(x)1+xβ(1+γ)minx[τλ,τ]u(x)1+xβ(1+γ)p(τ)η1uX,
    v(x)1+xβ(1+γ)minx[τλ,τ]v(x)1+xβ(1+γ)p(τ)η2vY.

    So,

    u(x)+v(x)1+xβ(1+γ)p(τ)η1uX+p(τ)η2vYp(τ)η(uX+vY)=p(τ)η(u,v)X×Y=p(τ)ηR2R.

    By (H4), for x[τλ,τ], we can obtain

    A1(u,v)(x)1+xβ(1+γ)βp(τ)η1Γ(σ1)0sβ(γ+1)1F(s,u(s),v(s))ds=βp(τ)η1Γ(σ1)0a1(s)F1(s,u(s),v(s))ds=βp(τ)η1Γ(σ1)0a1(s)F1(s,(1+sβ(1+γ))u(s)1+sβ(1+γ),(1+sβ(1+γ))v(s)1+sβ(1+γ))dsβp(τ)η1Γ(σ1)m10a1(s)u(s)+v(s)1+sβ(1+γ)dsβp(τ)η1Γ(σ1)m10a1(s)dsp(τ)η1uX+βp(τ)η1Γ(σ1)m10a1(s)dsp(τ)η2vYβp(τ)η1Γ(σ1)m1τλτa1(s)dsp(τ)η1uX+βp(τ)η1Γ(σ1)m1τλτa1(s)dsp(τ)η2vY=βp2(τ)η1Γ(σ1)m1τλτa1(s)ds(1η1uX+1η2vY)βp2(τ)η1Γ(σ1)m1τλτa1(s)ds1η(u,v)X×Y12(u,v)X×Y.

    Similarly, A2(u,v)(x)1+xβ(1+γ)12(u,v)X×Y. Therefore,

    A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D2.

    Finally, by Lemma 2.4, A has a fixed point in (K1×K1)(¯D2D1). So, (1.1) has at least one positive solution.

    Theorem 4.2. Assume that (H2) and (H4) hold. If f0=,g0=,F=0,G=0, then (1.1) has at least one positive solution.

    Proof. We divide the proof into several steps.

    Step 1. A:K1×K2K1×K2 is completely continuous. This result easily follows from Lemma 4.1.

    Step 2. We show that there exist r1>0 and D1={(u,v)X×Y,(u,v)X×Y<r1} such that

    A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D1.

    Because f0=,g0=, there exists r1>0 such that

    F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)M1(u+v),
    G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)M2(u+v),

    for 0<u+vr1,x>0, where M1,M2>0, satisfy

    M112η1ηΓ(σ1)βp2(τ)τλτa1(s)ds,M212η2ηΓ(σ2)βp2(τ)τλτa2(s)ds,η=max{η1,η2}.

    Let D1={(u,v)X×Y,(u,v)X×Y<r1}. So, for (u,v)K1×K2 with (u,v)X×Y=r1, and x[τλ,τ], then by (H4),

    A1(u,v)(x)1+xβ(1+γ)βp(τ)η1Γ(σ1)0sβ(γ+1)1F(s,u(s),v(s))ds=βp(τ)η1Γ(σ1)0a1(s)F1(s,u(s),v(s))ds=βp(τ)η1Γ(σ1)0a1(s)F1(s,(1+sβ(1+γ))u(s)1+sβ(1+γ),(1+sβ(1+γ))v(s)1+sβ(1+γ))dsβp(τ)η1Γ(σ1)M10a1(s)u(s)+v(s)1+sβ(1+γ)dsβp(τ)η1Γ(σ1)M10a1(s)dsp(τ)η1uX+βp(τ)η1Γ(σ1)M10a1(s)dsp(τ)η2vYβp(τ)η1Γ(σ1)M1τλτa1(s)dsp(τ)η1uX+βp(τ)η1Γ(σ1)M1τλτa1(s)dsp(τ)η2vY=βp2(τ)η1Γ(σ1)M1τλτa1(s)ds(1η1uX+1η2vY)βp2(τ)η1Γ(σ1)M1τλτa1(s)ds1η(u,v)X×Y12(u,v)X×Y.

    Similarly, A2(u,v)(x)1+xβ(1+γ)12(u,v)X×Y. Thus,

    A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D1.

    Step 3. We show that there exist r2>0 and D2={(u,v)X×Y,(u,v)X×Y<r2} such that

    A(u,v)X×Y(u,v)X×Y for (u,v)(K1×K2)D2.

    Because F=0,G=0, there exists r>0, such that

    F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ1(u+v),
    G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ2(u+v),

    for u+v>r,x>0, where ϵ1,ϵ2>0 satisfy

    ϵ112Γ(σ1)β0a1(s)ds,ϵ212Γ(σ2)β0a2(s)ds.

    Let D2={(u,v)X×Y,(u,v)X×Y<r2}, where r2>max{r1,r}. Then D1D1. We define two functions U1,U2 as follows:

    U1:(,+)(,+),U1(a)=sup0<u+vasupx>0F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v),
    U2:(,+)(,+),U2(a)=sup0<u+vasupx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v).

    For (u,v)K1×K2 and (u,v)X×Y=r2,

    U1(r2)=sup0<u+vr2supx>0F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ1sup0<u+vr2(u+v)=ϵ1r2=ϵ1(u,v)X×Y,
    U2(r2)=sup0<u+vr2supx>0G1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ϵ2sup0<u+vr2(u+v)=ϵ2r2=ϵ2(u,v)X×Y.

    By Lemma 2.2 and (H4),

    A1(u,v)(x)1+xβ(1+γ)βΓ(σ1)0sβ(γ+1)1F(s,u(s),v(s))ds=βΓ(σ1)0a1(s)F1(s,u(s),v(s))ds=βΓ(σ1)0a1(s)F1(s,(1+sβ(1+γ))u(s)1+sβ(1+γ),(1+sβ(1+γ))v(s)1+sβ(1+γ))dsβΓ(σ1)0a1(s)sup0<u+vr2supx>0F1(x,(1+xβ(1+γ))u,(1+xβ(1+γ))v)ds=βΓ(σ1)0a1(s)U1(r2)dsβΓ(σ1)0a1(s)dsϵ1(u,v)X×Y12(u,v)X×Y.

    Similarly, A2(u,v)(x)1+xβ(1+γ)12(u,v)X×Y. Therefore, A(u,v)X×Y(u,v)X×Y, for (u,v)(K1×K2)D2. Finally, by Lemma 2.4, A has a fixed point in (K1×K1)(¯D2D1). So, the system (1.1) has at least one positive solution.

    In the section, we obtain the multiplicity of positive solution of (1.1) by using the monotone iterative technique.

    Theorem 5.1. If (H1) and (H2) hold, then (1.1) has two positive solutions (u,v) and (w,z) satisfying 0(u,v)X×YΥ and 0(w,z)X×YΥ, where Υ is a positive preset constant. Moreover, limn(un,vn)=(u,v) and limn(wn,zn)=(w,z), where (un,vn) and (wn,zn) are given by

    (un(x),vn(x))=(A1(un1,vn1)(x),A2(un1,vn1)(x)),n=1,2,, (5.1)

    with

    (u0(x),v0(x))=(Υ1[1+xβ(γ+1)],Υ2[1+xβ(γ+1)]),Υ1,Υ2>0,Υ1+Υ2Υ,

    and

    (wn(x),zn(x))=(A1(wn1,zn1)(x),A2(wn1,zn1)(x)),n=1,2,, (5.2)

    with (w0(x),z0(x))=(0,0). In addition,

    (w0(x),z0(x))(w1(x),z1(x))(wn(x),zn(x))(w,z)(u,v)(un(x),vn(x))(u1(x),v1(x))(u0(x),v0(x)). (5.3)

    Proof. First, from Lemma 4.1, A(K_{1}\times K_{2})\subset K_{1}\times K_{2} for (u, v)\in K_{1}\times K_{2} . Let

    \Upsilon_{1} = \frac{\beta}{\Gamma(\sigma_{1})}[\omega_{1}(\Upsilon)\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\Upsilon)\int_0^\infty \psi_{1}(s)ds] < \infty,
    \Upsilon_{2} = \frac{\beta}{\Gamma(\sigma_{2})}[\widetilde{\omega_{1}}(\Upsilon)\int_0^\infty \varphi_{2}(s)ds+\widetilde{\omega_{2}}(\Upsilon)\int_0^\infty \psi_{2}(s)ds] < \infty,

    and \Upsilon\geq\Upsilon_{1}+\Upsilon_{2} with D_{\Upsilon} = \{(u, v)\in K_{1}\times K_{2}:\|(u, v)\|_{X\times Y}\leq\Upsilon\} . For any (u, v)\in D_{\Upsilon} , from (H_{2}) and Lemma 2.2,

    \begin{eqnarray*} \|A_{1}(u, v)\|_{X}& = &\sup\limits_{x > 0}\frac{|A_{1}(u, v)(x)|}{1+x^{-\beta(1+\gamma)}}\\ & = &\sup\limits_{x > 0}\mid\int_0^\infty \frac{G_{\sigma_{1}}(x, s)}{1+t^{-\beta(1+\gamma)}}s^{\beta(\gamma+1)-1}F(s, u(s), v(s))ds\mid \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}\int_0^\infty \mid s^{\beta(\gamma+1)-1}F(s, u(s), v(s))ds\mid \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}[\omega_{1}(\frac{\mid u(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\frac{\mid v(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \psi_{1}(s)ds]\\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}[\omega_{1}(\|u\|_{X})\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\|v\|_{Y})\int_0^\infty \psi_{1}(s)ds]\\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}[\omega_{1}(\Upsilon)\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\Upsilon)\int_0^\infty \psi_{1}(s)ds] = \Upsilon_{1}. \end{eqnarray*}

    Similarly, \|A_{2}(u, v)\|_{Y}\leq\Upsilon_{2} for (u, v)\in D_{\Upsilon} . Thus,

    \|A(u, v)\|_{X\times Y} = \|A_{1}(u, v)\|_{X}+\|A_{2}(u, v)\|_{Y}\leq\Upsilon_{1}+\Upsilon_{2}\leq\Upsilon.

    That is, A(D_{\Upsilon})\subset D_{\Upsilon} . We construct two sequences as follows:

    (u_{n}, v_{n}) = A(u_{n-1}, v_{n-1}), (w_{n}, z_{n}) = A(w_{n-1}, z_{n-1}), \ \ n = 1, 2, 3, \ldots.

    Obviously, (u_{0}(x), v_{0}(x)), (w_{0}(x), z_{0}(x))\in D_{\Upsilon} . Because A(D_{\Upsilon})\subset D_{\Upsilon} , (u_{n}, v_{n}), (w_{n}, z_{n})\in D_{\Upsilon}, n = 1, 2, \ldots . We need to show that there exist (u^{\ast}, v^{\ast}) and (w^{\ast}, z^{\ast}) satisfying \lim\limits_{n\rightarrow \infty}(u_{n}, v_{n}) = (u^{\ast}, v^{\ast}) and \lim\limits_{n\rightarrow \infty}(w_{n}, z_{n}) = (w^{\ast}, z^{\ast}) which are two monotone sequences for approximating positive solutions of the system (1.1).

    For x\in(0, +\infty), (u_{n}, v_{n})\in D_{\Upsilon} , from Lemma 2.2 and (5.1),

    \begin{eqnarray*} u_{1}(x)& = &A_{1}(u_{0}, v_{0})(x) = \int_0^\infty G_{\sigma_{1}}(x, s)s^{\beta(\gamma+1)-1}F(s, u_{0}(s), v_{0}(s))ds \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}\int_0^\infty (1+t^{-\beta(1+\gamma)})s^{\beta(\gamma+1)-1}F(s, u_{0}(s), v_{0}(s))ds \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}(1+x^{-\beta(1+\gamma)})[\omega_{1}(\frac{\mid u_{0}(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\frac{\mid v_{0}(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \psi_{1}(s)ds] \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}(1+x^{-\beta(1+\gamma)})[\omega_{1}(\|u_{0}\|_{X})\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\|v_{0}\|_{Y})\int_0^\infty \psi_{1}(s)ds] \\ &\leq&\frac{\beta}{\Gamma(\sigma_{1})}(1+x^{-\beta(1+\gamma)})[\omega_{1}(\Upsilon)\int_0^\infty \varphi_{1}(s)ds+\omega_{2}(\Upsilon)\int_0^\infty \psi_{1}(s)ds] \\ & = &(1+x^{-\beta(1+\gamma)})\Upsilon_{1} = u_{0}(x) \end{eqnarray*}

    and

    \begin{eqnarray*} v_{1}(x) = A_{2}(u_{0}, v_{0})(x)& = &\int_0^\infty G_{\sigma_{2}}(x, s)s^{\beta(\gamma+1)-1}G(s, u_{0}(s), v_{0}(s))ds \\ &\leq&\frac{\beta}{\Gamma(\sigma_{2})}\int_0^\infty (1+x^{-\beta(1+\gamma)})s^{\beta(\gamma+1)-1}G(s, u_{0}(s), v_{0}(s))ds \\ &\leq&\frac{\beta}{\Gamma(\sigma_{2})}(1+t^{-\beta(1+\gamma)})[\widetilde{\omega_{1}}(\frac{\mid u_{0}(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \varphi_{2}(s)ds+\widetilde{\omega_{2}}(\frac{\mid v_{0}(s)\mid}{1+s^{-\beta(1+\gamma)}})\int_0^\infty \psi_{2}(s)ds] \\ &\leq&\frac{\beta}{\Gamma(\sigma_{2})}(1+t^{-\beta(1+\gamma)})[\widetilde{\omega_{1}}(\|u_{0}\|_{X})\int_0^\infty \varphi_{2}(s)ds+\widetilde{\omega_{2}}(\|v_{0}\|_{Y})\int_0^\infty \psi_{2}(s)ds] \\ &\leq&\frac{\beta}{\Gamma(\sigma_{2})}(1+x^{-\beta(1+\gamma)})[\widetilde{\omega_{1}}(\Upsilon)\int_0^\infty \varphi_{2}(s)ds+\widetilde{\omega_{2}}(\Upsilon)\int_0^\infty \psi_{2}(s)ds] \\ & = &(1+x^{-\beta(1+\gamma)})\Upsilon_{2} = v_{0}(x), \end{eqnarray*}

    that is,

    (u_{1}(x), v_{1}(x)) = (A_{1}(u_{0}, v_{0})(x), A_{2}(u_{0}, v_{0})(x))\leq((1+x^{-\beta(1+\gamma)})\Upsilon_{1}, (1+x^{-\beta(1+\gamma)})\Upsilon_{2}) = (u_{0}(x), v_{0}(x)).

    So, by the condition (H_{1}) ,

    (u_{2}(x), v_{2}(x)) = (A_{1}(u_{1}, v_{1})(x), A_{2}(u_{1}, v_{1})(x))\leq(A_{1}(u_{0}, v_{0})(x), A_{2}(u_{0}, v_{0})(x)) = (u_{1}(x), v_{1}(x)).

    For x\in(0, +\infty) , the sequences \{(u_{n}, v_{n})\}_{n = 0}^{\infty} satisfy (u_{n+1}(x), v_{n+1}(x))\leq(u_{n}(x), v_{n}(x)) . By the iterative sequences (u_{n+1}, v_{n+1}) = A(u_{n}, v_{n}) and the complete continuity of the operator A , (u_{n}, v_{n})\rightarrow (u^{\ast}, v^{\ast}) , and A(u^{\ast}, v^{\ast}) = (u^{\ast}, v^{\ast}) .

    Similarly, for the sequences \{(w_{n}, z_{n})\}_{n = 0}^{\infty} , we have

    \begin{eqnarray*} (w_{1}(x), z_{1}(x))& = &(A_{1}(w_{0}, z_{0})(x), A_{2}(w_{0}, z_{0})(x))\\ & = &(\int_0^\infty G_{\sigma_{1}}(x, s)s^{\beta(\gamma+1)-1}F(s, w_{0}(s), z_{0}(s))ds, \int_0^\infty G_{\sigma_{2}}(x, s)s^{\beta(\gamma+1)-1}G(s, w_{0}(s), z_{0}(s))ds)\\ &\geq&(0, 0) = (w_{0}(x), z_{0}(x)). \end{eqnarray*}

    Then, by the condition (H_{1}) ,

    (w_{2}(x), z_{2}(x)) = (A_{1}(w_{1}, z_{1})(x), A_{2}(w_{1}, z_{1})(x))\geq(A_{1}(w_{0}, z_{0})(x), A_{2}(w_{0}, z_{0})(x)) = (w_{1}(x), z_{1}(x)).

    Analogously, for x\in(0, +\infty) , we have (w_{n+1}(x), z_{n+1}(x))\geq(w_{n}(x), z_{n}(x)) . By the iterative sequences (w_{n+1}, z_{n+1}) = A(w_{n}, z_{n}) and the complete continuity of the operator A , (w_{n}, z_{n})\rightarrow (w^{\ast}, z^{\ast}) , and A(w^{\ast}, z^{\ast}) = (w^{\ast}, z^{\ast}) .

    Finally, we prove that (u^{\ast}, v^{\ast}) and (w^{\ast}, z^{\ast}) are the minimal and maximal positive solutions of (1.1). Assume that (\varsigma(x), \mu(x)) is any positive solution of (1.1). Then, A(\varsigma(x), \mu(x)) = (\varsigma(x), \mu(x)) , and

    (w_{0}(x), z_{0}(x)) = (0, 0)\leq(\varsigma(x), \mu(x))\leq((1+x^{-\beta(1+\gamma)})\Upsilon_{1}, (1+x^{-\beta(1+\gamma)})\Upsilon_{2}) = (u_{0}(x), v_{0}(x)).

    Therefore,

    (w_{1}(x), z_{1}(x)) = (A_{1}(w_{0}, z_{0})(x), A_{2}(w_{0}, z_{0})(x))\leq(\varsigma(x), \mu(x))\leq(A_{1}(u_{0}, v_{0})(x), A_{2} (u_{0}, v_{0})(x)) = (u_{1}(x), v_{1}(x)).

    That is, (w_{1}(x), z_{1}(x))\leq(\varsigma(x), \mu(x))\leq(u_{n}(x), v_{n}(x)) . So, (5.3) holds. By (H_{1}) , (0, 0) is not a solution of (1.1). From (5.1), (w^{\ast}, z^{\ast}) and (u^{\ast}, v^{\ast}) are two extreme positive solutions of (1.1), which can be constructed via limitS of two monotone iterative sequences in (5.1) and (5.2).

    Example 6.1. We consider the following system:

    \begin{align} \begin{cases} D_{1}^{-\frac{3}{2}, \frac{5}{3}}u(x)+x^{\frac{3}{2}}(\frac{u}{1+x^{\frac{1}{2}}})^{2}e^{-x}+x^{\frac{3}{2}} (\frac{v}{1+x^{\frac{1}{2}}})^{2}e^{-x} = 0, t\in (0, +\infty), \\ D_{1}^{-\frac{3}{2}, \frac{3}{2}}v(x)+x^{\frac{5}{2}}e^{-2x^{2}}(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u} {1+x^{\frac{1}{2}}})^{2}) +x^{\frac{5}{2}}e^{-2x^{2}}(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2}), x\in (0, +\infty), \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{\frac{1}{6}, \frac{1}{3}}u(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{\frac{1}{6}, \frac{1}{3}}u(x) = 0, \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{0, \frac{1}{2}}v(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{0, \frac{1}{2}}v(x) = 0, \end{cases} \end{align} (6.1)

    where \sigma_{1} = \frac{5}{3}, \sigma_{2} = \frac{3}{2}, \gamma = -\frac{3}{2}, \beta = 1 ,

    F(x, u, v) = x^{\frac{3}{2}}e^{-x}[(\frac{u}{1+x^{\frac{1}{2}}})^{2}+(\frac{v}{1+x^{\frac{1}{2}}})^{2}],
    G(x, u, v) = x^{\frac{5}{2}}e^{-2x^{2}}[(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2}) +(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2})].

    First, for F_{1}(x, u, v) = x^{\beta(1+\gamma)-1}F(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = e^{-x}(u^{2}, v^{2}) , we choose \omega_{1}(u) = u^{2}\in C((0, +\infty), (0, +\infty)) , \omega_{2}(v) = v^{2}\in C((0, +\infty), (0, +\infty)) , and \varphi_{1}(x) = \psi_{1}(x) = e^{-x}\in L^{1}(0, +\infty) . Then,

    \mid F_{1}(x, u, v)\mid\leq\varphi_{1}(x)\omega_{1}(\mid u\mid)+\psi_{1}(t)\omega_{2}(\mid v\mid), \ \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    Similarly, for F_{2}(x, u, v) = x^{\beta(1+\gamma)-1}G(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = xe^{-2x^{2}}[u^{2}\ln(u^{2}+1)+ v^{2}\ln(v^{2}+1)] , we choose \widetilde{\omega_{1}}(u) = u^{2}\ln(u^{2}+1)\in C((0, +\infty), (0, +\infty)) , \widetilde{\omega_{2}}(v) = v^{2}\ln(v^{2}+1)\in C((0, +\infty), (0, +\infty)) , and \varphi_{2}(x) = \psi_{2}(x) = xe^{-2x^{2}}\in L^{1}(0, +\infty) . Then,

    \mid F_{2}(x, u, v)\mid\leq\varphi_{2}(x)\widetilde{\omega_{1}}(\mid u\mid)+\psi_{2}(x)\widetilde{\omega_{2}}(\mid v\mid), \ \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    So, the condition (H_{2}) holds. Obviously, F, G:(0, +\infty)\times(0, +\infty)\times(0, +\infty)\rightarrow (0, +\infty) are continuous.

    x^{-\frac{3}{2}}F(x, u, v) = e^{-x}[(\frac{u}{1+x^{\frac{1}{2}}})^{2}+(\frac{v}{1+x^{\frac{1}{2}}})^{2}] = a_{1}(x)F_{1}(x, u, v),
    x^{-\frac{3}{2}}G(x, u, v) = xe^{-2x^{2}}[(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2}) +(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2})] = a_{2}(x)G_{1}(x, u, v),

    where a_{1}(x) = e^{-x}, a_{2}(x) = xe^{-2x^{2}} , F_{1}(x, u, v) = (\frac{u}{1+x^{\frac{1}{2}}})^{2}+(\frac{v}{1+x^{\frac{1}{2}}})^{2} , G_{1}(t, u, v) = (\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2}) +(\frac{u}{1+x^{\frac{1}{2}}})^{2}\ln(1+(\frac{u}{1+x^{\frac{1}{2}}})^{2}) . So, x^{-\frac{3}{2}}f(x, u, v), x^{-\frac{3}{2}}G(x, u, v):[0, +\infty)\times(0, +\infty)\times(0, +\infty)\rightarrow [0, +\infty) are continuous. Hence, the condition (H_{4}) holds. Finally,

    F_{0} = \lim\limits_{(u, v)\rightarrow (0^{+}, 0^{+})} \frac{u^{2}+v^{2}}{u+v} = 0, G_{0}^{\ast} = \lim\limits_{(u, v)\rightarrow (0^{+}, 0^{+})} \frac{u^{2}\ln(u^{2}+1)+v^{2}\ln(v^{2}+1)}{u+v} = 0,
    f_{\infty} = \lim\limits_{(u, v)\rightarrow (+\infty, +\infty)} \frac{u^{2}+v^{2}}{u+v} = \infty, g_{\infty}^{\ast} = \lim\limits_{(u, v)\rightarrow (+\infty, +\infty)} \frac{u^{2}\ln(u^{2}+1)+v^{2}\ln(v^{2}+1)}{u+v} = \infty.

    Therefore, from Theorem 4.1, (6.1) has at least one positive solution (u(x), v(x)) . Further,

    \begin{cases} u(x) = \frac 3{2\Gamma(\frac 23)}[x^{\frac 12}\int_0^\infty s^{-\frac 32}F(s, u(s), v(s))ds-x^{-\frac 83}\int_x^\infty (x-s)^{\frac 23}s^{-\frac 32}F(s, u(s), v(s))ds], \\ v(x) = \frac 2{\sqrt{\pi}}[x^{\frac 12}\int_0^\infty s^{-\frac 32}G(s, u(s), v(s))ds-\int_x^\infty (x-s)^{\frac 12}s^{-\frac 32}G(s, u(s), v(s))ds]. \end{cases}

    Example 6.2. We consider the following system:

    \begin{align} \begin{cases} D_{1}^{-\frac{3}{2}, \frac{3}{2}}u(x)+x^{\frac{5}{2}}e^{-2x^{2}+1}[\arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\frac{1}{\pi}]+ x^{\frac{5}{2}}e^{-2x^{2}+1} [\arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\pi] = 0, x\in (0, +\infty), \\ D_{1}^{-\frac{3}{2}, \frac{7}{6}}v(x)+x^{\frac{3}{2}}e^{-x}[\arctan(\ln((\frac{u}{1+x^{\frac{1}{2}}})^{2}+1))+\frac{3}{2}\pi]+ x^{\frac{3}{2}}e^{-x} [\arctan(\ln((\frac{v}{1+x^{\frac{1}{2}}})^{2}+1))+1], x\in (0, +\infty), \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{0 \frac{1}{2}}u(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{0, \frac{1}{2}}u(x) = 0, \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{-\frac{1}{3}, \frac{5}{6}}v(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{-\frac{1}{3}, \frac{5}{6}}v(x) = 0, \end{cases} \end{align} (6.2)

    where \sigma_{1} = \frac{3}{2}, \sigma_{2} = \frac{7}{6}, \gamma = -\frac{3}{2}, \beta = 1 ,

    F(x, u, v) = x^{\frac{5}{2}}e^{-2x^{2}+1}[\arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\frac{1}{\pi}]+x^{\frac{5}{2}}e^{-2x^{2}+1} [\arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\frac{1}{\pi}],
    G(x, u, v) = x^{\frac{3}{2}}e^{-x}[\arctan(\ln((\frac{u}{1+x^{\frac{1}{2}}})^{2}+1))+\frac{3}{2}\pi]+x^{\frac{3}{2}}e^{-x} [\arctan(\ln((\frac{v}{1+x^{\frac{1}{2}}})^{2}+1))+1].

    First, for

    F_{1}(x, u, v) = x^{\beta(1+\gamma)-1}F(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = xe^{-2x^{2}+1}[\arctan u^{2}+\frac{1}{\pi}+\arctan v^{2}+\pi],

    we choose \omega_{1}(u) = \arctan u^{2}+\frac{1}{\pi}\in C((0, +\infty), (0, +\infty)), \omega_{2}(v) = \arctan v^{2}+\pi\in C((0, +\infty), (0, +\infty)) , and \varphi_{1}(x) = \psi_{1}(x) = xe^{-2x^{2}+1}\in L^{1}(0, +\infty) . Then,

    \mid F_{1}(x, u, v)\mid\leq\varphi_{1}(x)\omega_{1}(\mid u\mid)+\psi_{1}(x)\omega_{2}(\mid v\mid), \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    Similarly, for

    F_{2}(x, u, v) = x^{\beta(1+\gamma)-1}g(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = e^{-x}[\arctan(\ln(u^{2}+1))+ \frac{3}{2}\pi+\arctan(\ln(v^{2}+1))+1],

    we choose \widetilde{\omega_{1}}(u) = \arctan(\ln(u^{2}+1))+\frac{3}{2}\pi\in C((0, +\infty), (0, +\infty)) , \widetilde{\omega_{2}}(v) = \arctan(\ln(v^{2}+1))+1\in C((0, +\infty), (0, +\infty)) , and \varphi_{2}(x) = \psi_{2}(x) = e^{-x}\in L^{1}(0, +\infty) . Then,

    \mid F_{2}(x, u, v)\mid\leq\varphi_{2}(x)\widetilde{\omega_{1}}(\mid u\mid)+\psi_{2}(x)\widetilde{\omega_{2}}(\mid v\mid), \ \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    That is, (H_{2}) holds. Second, F, G:(0, +\infty)\times(0, +\infty)\times(0, +\infty)\rightarrow (0, +\infty) are continuous. And

    x^{-\frac{3}{2}}F(x, u, v) = xe^{-2x^{2}+1}[\arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\frac{1}{\pi}+\arctan(\frac{v} {1+x^{\frac{1}{2}}})^{2}+\pi] = a_{1}(x)F_{1}(x, u, v),
    x^{-\frac{3}{2}}G(x, u, v) = e^{-x}[\arctan(\ln((\frac{u}{1+x^{\frac{1}{2}}})^{2}+1))+\frac{3}{2}\pi+\arctan(\ln((\frac{v} {1+x^{\frac{1}{2}}})^{2}+1))+1] = a_{2}(x)G_{1}(x, u, v),

    where a_{1}(x) = xe^{-2x^{2}+1}, a_{2}(x) = e^{-x} , F_{1}(x, u, v) = \arctan(\frac{u}{1+x^{\frac{1}{2}}})^{2}+\frac{1}{\pi}+\arctan(\frac{v}{1+x^{\frac{1}{2}}})^{2}+\pi , G_{1}(x, u, v) = \arctan(\ln((\frac{u}{1+x^{\frac{1}{2}}})^{2}+1))+\frac{3}{2}\pi+\arctan(\ln((\frac{v}{1+x^{\frac{1}{2}}})^{2}+1))+1 . So, x^{-\frac{3}{2}}F(x, u, v), x^{-\frac{3}{2}}G(x, u, v):[0, +\infty)\times(0, +\infty)\times(0, +\infty)\rightarrow [0, +\infty) are continuous. That is, (H_{4}) holds. In addition,

    f_{0} = \lim\limits_{(u, v)\rightarrow (0^{+}, 0^{+})} \frac{\arctan u^{2}+\frac{1}{\pi}+\arctan v^{2}+\pi}{u+v} = \infty,
    g_{0}^{\ast} = \lim\limits_{(u, v)\rightarrow (0^{+}, 0^{+})} \frac{\arctan(\ln(u^{2}+1))+\frac{3}{2}\pi+\arctan(\ln(v^{2}+1))+1}{u+v} = \infty,
    F_{\infty} = \lim\limits_{(u, v)\rightarrow (+\infty, +\infty)} \frac{\arctan u^{2}+\frac{1}{\pi}+\arctan v^{2}+\pi}{u+v} = 0,
    G_{\infty}^{\ast} = \lim\limits_{(u, v)\rightarrow (+\infty, +\infty)} \frac{\arctan(\ln(u^{2}+1))+\frac{3}{2}\pi+\arctan(\ln(v^{2}+1))+1}{u+v} = 0.

    Therefore, from Theorem 4.2, (6.2) has at least one positive solution (u(x), v(x)) . Further,

    \begin{cases} u(x) = \frac 2{\sqrt{\pi}}[x^{\frac 12}\int_0^\infty s^{-\frac 32}F(s, u(s), v(s))ds-\int_x^\infty (x-s)^{\frac 12}s^{-\frac 32}F(s, u(s), v(s))ds], \\ v(x) = \frac 6{\Gamma(\frac 16)}[x^{\frac 12}\int_0^\infty s^{-\frac 32}G(s, u(s), v(s))ds-x^{\frac 13}\int_x^\infty (x-s)^{\frac 16}s^{-\frac 32}G(s, u(s), v(s))ds]. \end{cases}

    Example 6.3. We consider the following system:

    \begin{align} \begin{cases} D_{1}^{-\frac{3}{2}, \frac{5}{3}}u(x)+x^{\frac{3}{2}}\frac{e^{-x}}{3}\mid\frac{u}{1+x^{\frac{1}{2}}}\mid+ x^{\frac{5}{2}}\ln(\mid\frac{v}{1+x^{\frac{1}{2}}}\mid+1)\frac{e^{-2x^{2}+1}}{10} = 0, x\in (0, +\infty), \\ D_{1}^{-\frac{3}{2}, \frac{3}{2}}v(x)+x^{\frac{5}{2}}e^{-2x^{2}+1}\arctan(\mid\frac{u}{1+x^{\frac{1}{2}}}\mid+ \frac{1}{\sqrt{\pi}})+x^{\frac{5}{2}}\frac{e^{-2x^{2}+1}}{5} \mid\frac{v}{1+x^{\frac{1}{2}}}\mid = 0, x\in (0, +\infty), \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{\frac{1}{6}, \frac{1}{3}}u(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{\frac{1}{6}, \frac{1}{3}}u(x) = 0, \\ \lim\nolimits_{x\rightarrow 0} x^{\frac{1}{2}}I^{0, \frac{1}{2}}v(x) = 0, \lim\nolimits_{x\rightarrow \infty} x^{-\frac{1}{2}}I^{0, \frac{1}{2}}v(x) = 0, \end{cases} \end{align} (6.3)

    where \sigma_{1} = \frac{5}{3}, \sigma_{2} = \frac{3}{2}, \gamma = -\frac{3}{2}, \beta = 1 ,

    F(x, u, v) = x^{\frac{3}{2}}\frac{e^{-x}}{3}\mid\frac{u}{1+x^{\frac{1}{2}}}\mid+x^{\frac{5}{2}}\ln(\mid\frac{v} {1+x^{\frac{1}{2}}}\mid+1)\frac{e^{-2x^{2}+1}}{10},
    G(x, u, v) = x^{\frac{5}{2}}e^{-2x^{2}+1}\arctan(\mid\frac{u}{1+x^{\frac{1}{2}}}\mid+\frac{1}{\sqrt{\pi}})+ x^{\frac{5}{2}}\frac{e^{-2x^{2}+1}}{5} \mid\frac{v}{1+x^{\frac{1}{2}}}\mid.

    Obviously, F, G:(0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty)\rightarrow (0, +\infty) are continuous and nondecreasing with respect to the second and the third variables on (0, +\infty) . That is, (H_{1}) holds. Next,

    F_{1}(x, u, v) = x^{\beta(1+\gamma)-1}F(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = \frac{e^{-x}}{3}\mid u\mid+x\frac{e^{-2x^{2}+1}}{10}\ln(\mid v\mid+1).

    We choose \omega_{1}(u) = \mid u\mid\in C((0, +\infty), (0, +\infty)) , \omega_{2}(v) = \ln(\mid v\mid+1)\in C((0, +\infty), (0, +\infty)) , and \varphi_{1}(x) = \frac{e^{-x}}{3}, \psi_{1}(x) = \frac{xe^{-2x^{2}+1}}{10}\in L^{1}(0, +\infty) . Then,

    \mid F_{1}(x, u, v)\mid\leq\varphi_{1}(x)\omega_{1}(\mid u\mid)+\psi_{1}(x)\omega_{2}(\mid v\mid), \ \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    Similarly, for

    F_{2}(x, u, v) = x^{\beta(1+\gamma)-1}G(x, (1+x^{-\beta(1+\gamma)})u, (1+x^{-\beta(1+\gamma)})v) = xe^{-2x^{2}+1}\arctan(\mid u\mid+\frac{1}{\sqrt{\pi}})+x\frac{e^{-2x^{2}+1}}{5}\mid v\mid,

    we choose \widetilde{\omega_{1}}(u) = \arctan(\mid u\mid+\frac{1}{\sqrt{\pi}})\in C((0, +\infty), (0, +\infty)) , \widetilde{\omega_{2}}(v) = \mid v\mid\in C((0, +\infty), (0, +\infty)) , and \varphi_{2}(x) = xe^{-2x^{2}+1}, \psi_{2}(x) = x\frac{e^{-2x^{2}+1}}{5}\in L^{1}(0, +\infty) . Then,

    \mid F_{2}(x, u, v)\mid\leq\varphi_{2}(x)\widetilde{\omega_{1}}(\mid u\mid)+\psi_{2}(x)\widetilde{\omega_{2}}(\mid v\mid), \ \ (0, +\infty)\times(-\infty, +\infty)\times(-\infty, +\infty).

    That is, (H_{2}) holds. Therefore, from Theorem 5.1, (6.3) has two positive solutions (u^{\ast}, v^{\ast}) and (w^{\ast}, z^{\ast}) with (0, 0)\leq (u^{\ast}(x), v^{\ast}(x)), (w^{\ast}(x), z^{\ast}(x))\leq ((1+x^{\frac 12})\Upsilon_{1}, (1+x^{\frac 12})\Upsilon_{2}) , where \Upsilon_{1}+\Upsilon_{2}\leq \Upsilon , and \Upsilon satisfies

    \frac{95.58}{191.86}\Upsilon-0.69\arctan (\Upsilon+0.56)\geq \frac 1{36}.

    This paper studies the Erdélyi-Kober fractional coupled system (1.1), where the variable is in an infinite interval. We give some proper conditions and set a special Banach space. We obtain the existence of at least one positive solution for (1.1) by using the Guo-Krasnosel'skii fixed point theorem, and we get the existence of at least two positive solutions for (1.1) by using the monotone iterative technique. Our methods and results are different from ones in [18]. Moreover, we give three examples to show the plausibility of our main results. For future work, we intend to use other fixed point theorems to solve some Erdélyi-Kober fractional differential equations.

    The authors declare they have not used artificial intelligence (AI) tools in the creation of this article.

    This paper is supported by the Fundamental Research Program of Shanxi Province (202303021221068).

    The authors declare that they have no competing interests.



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