Shah and Khan SpringerPlus (2016)5:1116 DOI 10.1186/s40064-016-2656-9

Open Access

RESEARCH

Multiple positive solutions to a coupled systems of nonlinear fractional differential equations Kamal Shah*  and Rahmat Ali Khan *Correspondence: [email protected] Department of Mathematics, University of Malakand, Dir(L), Chakdara, Khyber Pakhtunkhwa, Pakistan

Abstract  In this article, we study existence, uniqueness and nonexistence of positive solution to a highly nonlinear coupled system of fractional order differential equations. Necessary and sufficient conditions for the existence and uniqueness of positive solution are developed by using Perov’s fixed point theorem for the considered problem. Further, we also established sufficient conditions for existence of multiplicity results for positive solutions. Also, we developed some conditions under which the considered coupled system of fractional order differential equations has no positive solution. Appropriate examples are also provided which demonstrate our results. Keywords:  Fractional differential equations, Coupled system, Boundary condition, Generalized metric space, Contractions mapping Mathematics Subject Classification:  34A08, 34B37, 35R11

Background In recent years it is mainly proved that fractional differential equations are the best tools in the mathematical modeling of many phenomena in various field of physics, electrochemistry, viscoelasticity, control theory, image and signal processing etc, (see Hilfer 2000; Kilbas et al. 2006; Lakshmikantham et al. 2009; Podlubny 1999; Rossikhin and Shitikova 1997). Also, fractional differential equations are used in the modeling of various phenomena such as nonlinear oscillation of earthquake, Nutting’s law of charge transport in amorphous semi conductors, fluid dynamics, traffic model and non-Markorian diffusion process with memory. For the aforesaid application, see Mainardi (1995), Metzler and Klafer (2000), Scher and Montroll (1975), Wang et al. (2011). Moreover, considerable attention has been given to study mathematical epidemiology in term of fractional order models, this is due to the fact that fractional order models of epidemic disease are more realistic and provide great information as compared to classical order models. In last few years fractals and Chaos are also studied by using the tools of fractional differential equations. Besides from the above discussion, fractional order differential equations have also many applications in the fields of aerodynamics, fluid dynamics, physical chemistry, economics, polymerrheology, regular variation in thermodynamics, biophysics, blood flow phenomena, etc. Due to the important applications and uses of fractional © 2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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differential equations, researchers of various field of mathematics, engineering, physics and computer science etc, gave much attention to study fractional differential equations. Another important application of fractional calculus has been found in condensed matter physics, where the fractional quantum Hall effect is one of the most attracting phenomena. For the present or planned technologies fractional order models are used by the implementation of an optical lattice setup. For detail, see Nielsen et  al. (2013), Hagerstrom et al. (2012) and the reference there in. In networking systems it has been proved that several real networks in their degree of distribution obey a power-law. The presence of highly connected nodes in a scale-free network causes well known robustness against random failures. But on the other hand suffers from vulnerability to malicious attacks at their highly connected nodes. Fractional order models provide more realistic and accurate approach as compared to classical order models to study the afore said phenonmena, see Shang (2014). In last few years, the study of existence and uniqueness of solutions to boundary value problems for fractional order differential equations got much attention from many researchers and a number of research articles are available in the literature, we refer few of them in Deren (2015), Li et al. (2010), Khan and Shah (2015) and the reference therein. The iterative solutions to boundary and initial values problems of nonlinear fractional order differential equations were also studied by some authors (see Ahmad and Nieto 2008; Cui and Zou 2014; Shah et al. 2016 and the references therein). Moreover, existence and multiplicity of positive solutions to nonlinear boundary values problem of fractional order differential equations have been studied by many authors by using classical fixed point theorems, for example see Ahmad and Nieto (2009b), Bai (2008), Cui et al. (2012), Xu et al. (2009). Bai and Lü (2005), have studied the existence of multiple solutions for the following boundary value problems  α D u(t) + f (t, u(t)) = 0, t ∈ (0, 1), 1 < α ≤ 2, u(0) = u(1) = 0. where Dα is the standard Riemann–Liouville fractional derivative of order α, and f : [0, 1] × [0, ∞) → [0, ∞) is continuous function. By means of classical fixed point theorems sufficient conditions were obtained for multiplicity of solutions. Recently, Goodrich (2010), considered the following class of nonlinear fractional differential equations with the given boundary conditions for multiplicity of positive solutions as



Dα u(t) + f (t, u(t)) = 0, (i)

u (0) = 0,

t ∈ (0, 1), n − 1 < α ≤ n,

0 ≤ i ≤ n − 2, and Dδ u(1) = 0,

2 ≤ δ ≤ n − 2.

where n > 3 and f : [0, 1] × [0, ∞) → [0, ∞) is continuous function. Many problems in applied sciences can be modeled as coupled system of differential equations with different type of boundary conditions. The coupled system of fractional order differential equations have many application in computer networking, see Li et al. (2015a, b), Suo et  al. (2013). Boundary values problems for coupled systems with ordinary derivatives are well studied, however, coupled systems with fractional derivatives have attracted the attention quite recently. Most of the biological, physical, computer network model and chemical models etc, are in the form of coupled system (see Anastassiou et al. 2011; Chasnov 2009; Lia et al. 2015). Due to these important applications and uses of coupled

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systems of fractional order differential equations, considerable attention was given to study coupled system for the existence, uniqueness and multiplicity of positive solutions, for detail we refer Miller and Ross (1993), Shah and Khan (2015), Su (2009), Shah et al. (2015) and the references therein. As Bai and Feng (2004), established sufficient conditions for existence of positive solution to a coupled system of fractional differential equations as given by the



Dα u(t) = f (t, v(t)); β

D v(t) = g(t, u(t));

0 < t < 1, 0 < t < 1,

where 1 < α, β < 1, f , g : [0, 1] × R → R are nonlinear continuous functions. Wang et al. (2010), developed sufficient conditions for existence and uniqueness of solution for the coupled system with three point boundary conditions of the form



Dα u(t) = f (t, v(t)), Dβ v(t) = g(t, u(t)); 0 < t < 1, u(0) = v(0) = 0; u(1) = au(ξ ), v(1) = bv(ξ ),

where 1 < α, β < 2, 0 ≤ a, b ≤ 1, 0 < ξ < 1, and f , g : [0, 1] × [0, ∞) → [0, ∞) are nonlinear continuous functions. Rehman and Khan (2010), established sufficient conditions for multiplicity results for positive solutions to the following coupled system of nonlinear boundary value problem of fractional differential equations as given by

 α β   D u(t) + a(t)f (v(t)) = 0; D v(t) + b(t)g(u(t)) = 0; 0 < t < 1, u(0) = u′ (0) = u′′ (0) = · · · = u(n−2) (0) = u(n−1) (0) = 0; u(1) = 0,   v(0) = v′ (0) = v′′ (0) = · · · = v(n−2) (0) = v(n−1) (0) = 0; v(1) = 0,

where n − 1 < α, β ≤ n,  > 0, f , g : [0, ∞) → [0, ∞) are continuous. Jalili and Samet (2014), studied existence and uniqueness as well as multiplicity of positive solutions to the following coupled system of boundary value problems of fractional differential equations  α D u(t) + f (t, u(t), v(t)) = 0; t ∈ (0, 1); n − 1 < α ≤ n,      Dβ v(t) + g(t, u(t), v(t)) = 0; t ∈ (0, 1); n − 1 < β ≤ n,

 u(i) (0) = v(i) (0) = 0, i = 0, 1, 2 . . . n − 2,     γ D u(1) = Dδ v(1) = 0,

where 2 < γ , δ ≤ n − 2, n > 3, f , g : I × [0, ∞) × [0, ∞) → [0, ∞) are continuous. The aim of this paper is to study the existence, uniqueness as well as non-existence conditions for positive solution to the following system of non-linear fractional order differential equations with four point boundary conditions  α D u(t) + φ(t, u(t), v(t)) = 0; t ∈ (0, 1); n − 1 < α ≤ n,      Dβ v(t) + ψ(t, u(t), v(t)) = 0; t ∈ (0, 1); n − 1 < β ≤ n, (1)  u(0) = u′ (0) = u′′ (0) = · · · = u(n−2) (0) = 0; Dδ u(1) = Dδ u(η),     v(0) = v′ (0) = v′′ (0) = · · · = v(n−2) (0) = 0; Dγ v(1) = µDγ v(ξ ).

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where n > 3, α − δ ≥ 1, β − γ ≥ 1 and 0 < δ, γ ≤ n − 2, , µ ∈ (0, ∞), 0 < η, , ξ < 1, φ, ψ : [0, 1] × [0, ∞) × [0, ∞) → [0, ∞) are continuous functions and Dα , Dβ stand for Riemann–Liouville fractional derivative of order α, β respectively. Sufficient conditions are developed for uniqueness of solution of system (1), by using Perov’s fixed theorem. Moreover by means of some classical fixed point theorems of cone type, we develop necessary and sufficient conditions under which the considered system has at least one , two or more positive solutions. Also, we develop conditions for nonexistence of positive solution for system (1). We also provide some examples to illustrate our main results.

Preliminaries To proceed further, we recall some basic definitions and well known results of functional analysis, fixed point theory and fractional calculus (see i.e. Agarwal et al. 2004; Deimling 1985; Jalili and Samet 2014; Krasnoselskii 1964; Miller and Ross 1993; Podlubny 1993; Szilárd 2003; Zeidler 1986). Definition 1  The fractional integral of order α > 0 of a function y : (0, ∞) → R is defined by  t 1 I α y(t) = (t − s)α−1 y(s)ds, Ŵ(α) 0 provided the integral is pointwise defined on (0, ∞). Definition 2  The Riemann–Liouville fractional derivative of order α > 0 of a function y ∈ C n [0, 1] is defined by  n  t 1 d α D0+ y(t) = (t − s)n−α−1 y(s)ds, Ŵ(n − α) dt 0 where n = ⌈α⌉ + 1, ⌈α⌉ represents integer part of α. The following results need in the sequel, Lemma 3 Ahmad and Nieto (2009a, b) Ci ∈ R, i = 0, 1, 2, . . . , n, n = [α] + 1, we have

Let

α>0

then

for

arbitrary

I α Dα y(t) = y(t) + C1 t α−1 + C2 t α−2 + . . . + Cn t α−n .

Lemma 4  Agarwal et al. (2004) Let X be a Banach space with P ⊆ X closed and convex. Let U be a relatively open subset of P with 0 ∈ U and A : U → U be a continuous and compact(completely continuous) mapping. Then either 1. The mapping A has a fixed point in U or 2. There exist u ∈ ∂U and  ∈ (0, 1) with u = Au. Lemma 5  Podlubny (1993) Let P be a cone of real Banach space X and let 1 and 2 be two bounded open sets in X such that 0 ∈ 1 ⊂ 1 ⊂ 2. Let operator A : P ∩ (�2 \ �1 ) → P be completely continuous operator. If one of the following two conditions holds:

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(1) (2)

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�Au� ≤ �u� for all u ∈ P ∩ ∂�1 ; �Au� ≥ �u�, for all u ∈ P ∩ ∂�2 ; �Au� ≥ �u� for all u ∈ P ∩ ∂�1 ; �Au� ≤ �u�, for all u ∈ P ∩ ∂�2.

Then A has at least one fixed point in P ∩ (�2 \ �1 ). Definition 6  Jalili and Samet (2014) For a nonempty set X, the mapping d : X × X → Rn is called a generalized metric on X if for all x, y, and u, v ∈ X with v � = x, v �= u, v �= y satisfies

M1 .  d (u, v) = 0Rn if and only if u = v, for all u, v ∈ X; d (u, v) = d(v, u), for all u, v ∈ X; (symmetric property) M2 .  d (x, y) ≤ d(x, v) + d(v, u) + d(u, y), for all x, y, u, v, ∈ X (tetrahedral inequality). M3 .  Moreover the pairs (X, d) is called generalized metric space. n×n }, the system of all n × n Definition 7  Jalili and Samet (2014) Let M = {Mn,n ∈ R+ matrices with positive element. For any matrix A the spectral radius is defined by ρ(A) = sup{|i |, i = 1, 2, . . . , n}, where i , (i = 1, 2, . . . , n) are the eigenvalues of the matrix A.

Lemma 8  Jalili and Samet (2014) Let (M, d) be a complete generalized metric space and let A : M → M be an operator such that there exists a matrix A ∈ M with

d(Au, Av) ≤ Ad(u, v), for all u, v ∈ M. if ρ(A) < 1, then A has a fixed point w ∈ M. Further, for any u0 the iterative sequence un+1 = Aun converges to w.

Main results This section is concerned to the existence, uniqueness and multiplicity results of positive solutions for boundary value problem (1). We begin with the following lemma. Lemma 9  Let y ∈ C[0, 1] then the boundary value problem

Dα u(t) + y(t) = 0; 0 < t < 1; n − 1 < α ≤ n,

(2)

u(0) = u′ (0) = · · · = u(n−2) (0) = 0; Dδ u(1) = Dδ u(η),

where 0 < η < 1,  ∈ (0, ∞), 0 < δ ≤ n − 2, has a unique positive solution given by

u(t) =

1

Gα (t, s)y(s)ds,

0

where Gα (t, s) is a Green’s function given by Gα (t, s)  α−1 (1 − s)α−δ−1 − (t − s)α−1 − d t α−1 (η − s)α−δ−1 ;  1  d1 t 1  d1 t α−1 (1 − s)α−δ−1 − (t − s)α−1 ; = d t α−1 (1 − s)α−δ−1 − d1 t α−1 (η − s)α−δ−1 ; Ŵ(α)    1 α−1 d1 t (1 − s)α−δ−1 ;

0 ≤ s ≤ t ≤ η ≤ 1, 0 ≤ η ≤ s ≤ t ≤ 1, 0 ≤ t ≤ η ≤ s ≤ 1, 0 ≤ η ≤ t ≤ s ≤ 1,

(3)

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where d1 =

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1 . 1−ηα−δ−1

Proof  By applying I α and using Lemma 3, the general solution of linear boundary value problem (2) is given by

u(t) = −I α y(t) + c1 t α−1 + c2 t α−2 + · · · + cn t α−n .

(4)

With the help of boundary and initial conditions of Eq. (2) and Dδ [t α−1 ] = we get c2 = c3 = · · · = cn = 0 and using d1 = 1−η1α−δ−1 , we have

d1 c1 = Ŵ(α)



0

1

α−δ−1

(1 − s)

y(s)ds − 



0

η

α−δ−1

(η − s)

y(s)ds

Ŵ(α) α−δ−1  , Ŵ(α−δ) t



and thus (4) becomes

   η 1 d1 t α−1 α−δ−1 α−δ−1 u(t) = −I y(t) + (η − s) y(s)ds (1 − s) y(s)ds −  Ŵ(α) 0 0  t 1 (t − s)α−1 y(s)ds =− Ŵ(α) 0    η 1 d1 t α−1 α−δ−1 α−δ−1 + (1 − s) y(s)ds −  (η − s) y(s)ds Ŵ(α) 0 0  1 = Gα (t, s)y(s)ds, α

0

where Gα (t, s) is the Green’s function of linear boundary value problem (2). Similarly, we 1 can obtain v(t) = 0 Gβ (t, s)z(s)ds, where Gβ (t, s) is the Green’s function for the second equation of the system (1) and given by  d t β−1 (1 − s)β−γ −1 − (t − s)β−1 − d2 µt β−1 (ξ − s)β−γ −1 ;   2 1  d2 t β−1 (1 − s)β−γ −1 − (t − s)β−1 ; Gβ (t, s) = d t β−1 (1 − s)β−γ −1 − d2 t β−1 µ(ξ − s)β−γ −1 ; Ŵ(β)    2 β−1 d2 t (1 − s)β−γ −1 ;

where d2 =

0 ≤ s ≤ t ≤ ξ ≤ 1, 0 ≤ ξ ≤ s ≤ t ≤ 1, 0 ≤ t ≤ ξ ≤ s ≤ 1, 0 ≤ ξ ≤ t ≤ s ≤ 1,

1 . 1−µξ β−γ −1

(5)



We recall the following lemma found in Jalili and Samet (2014) and Yang (2012). The proof is omitted because the proof is like the proof given in Goodrich (2010). Lemma 10  Let G(t, s) = (Gα (t, s), Gβ (t, s)) be the Green’s function of (1). This Green’s function G(t, s) has some properties given by

(P1 )  G(t, s) is continuous function on the unit square for all (t, s) ∈ [0, 1] × [0, 1]; (P2 )  G (t, s) ≥ 0 for all t, s ∈ [0, 1] and G(t, s) > 0 for all t, s ∈ (0, 1); (P3 )  max G(t, s) = G(1, s), ∀ s ∈ [0, 1]; 0≤t≤1

(P4 ) min G(t, s) ≥ γ (s)G(1, s) t∈[ω,1−ω]

for

γ = min{γα = ωα−1 , γβ = ωβ−1 }.

each

ω ∈ (0, 21 ), s ∈ (0, 1),

where

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Further we define some fundamental results which will be used throughout in this paper. Let us define X = {u(t)|u ∈ C[0, 1]} endowed with the norm ||u|| = maxt∈[0,1] |u(t)|. Further the norm for the product space we define as ||(u, v)|| = ||u|| + ||v||. Obviously (X × X, || · ||) is a Banach space. Let J = [ω, 1 − ω], then, we define the cone P ⊂ X × X by

P = {(u, v) ∈ X × X : min[u(t) + v(t)] ≥ γ ||(u, v)||}. t∈J

Pr = {(u, v) ∈ P : �(u, v)� ≤ r}, ∂Pr = {(u, v) ∈ P : �(u, v)� = r}. Now inview of Lemma 9, we can write system (1) as an equivalent coupled system of integral equations given as

 �1     u(t) = Gα (t, s)φ(s, u(s), v(s))ds,     0

(6)

 �1     v(t) = Gβ (t, s)ψ(s, u(s), v(s))ds.    0

Let A : X × X → X × X be the operator defined as

A(u, v)(t) =



1

Gα (t, s)φ(s, u(s), v(s))ds, 0  = A1 (u, v)(t), A2 (u, v)(t) .

1

Gβ (t, s)ψ(s, u(s), v(s))ds

0



.

(7)

Then the fixed point of operator A coincides with the solution of coupled system (1). Theorem  11  Assume that f , g : [0, 1] × [0, ∞) × [0, ∞) → [0, ∞) are continuous. Then A(P) ⊂ P and A : P → P is completely continuous, where A is defined in (7). Proof  To derive A(P) ⊂ P, let (u, v) ∈ P,then by Lemma 10, we have A(u, v) ∈ P. Further from property (P4 ) and for all t ∈ J , we get

A1 (u(t), v(t)) =

1

Gα (t, s)φ(s, u(s), v(s))ds ≥ γα

0

1

Gα (1, s)φ(s, u(s)), v(s)ds.

(8)

0

Also from (P3 ), we obtain

A1 (u(t), v(t)) =

1

Gα (t, s)φ(s, u(s), v(s))ds ≤

0

Thus from (8) and (9), we have

A1 (u(t), v(t)) ≥ γ �A1 (u, v)�, for all t ∈ J . Similarly, one can write that

1 0

Gα (1, s)φ(s, u(s)), v(s)ds.

(9)

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A2 (u(t), v(t)) ≥ γ �A2 (u, v)�, for all t ∈ J . Thus A1 (u(t), v(t)) + A2 (u(t), v(t)) ≥ γ �A(u, v)�, for all t ∈ J , min[A1 (u(t), v(t)) + A2 (u(t), v(t))] ≥ γ �A(u, v)�. t∈J

Hence we have A(u, v) ∈ P ⇒ A(P) ⊂ P. Next by similar proof of Theorem  1 of Shah and Khan (2015) and using Arzela´ -Ascoli’s theorem, one can easily prove that A : P → P  is completely continuous.  Theorem 12  Assume that φ and ψ are continuous on [0, 1] × [0, ∞) × (0, ∞) → [0, ∞) , and there exist fi (t), gi (t), (i = 1, 2) : (0, 1) → [0, ∞) that satisfy

|φ(t, u, v) − φ(t, u, ¯ v¯ )| ≤ f1 (t)|u − u| ¯ + g1 (t)|v − v¯ |, for t ∈ (0, 1) and u, v, u, ¯ v¯ ≥ 0; (A1 ) 

|ψ(t, u, v) − ψ(t, u, ¯ v¯ )| ≤ f2 (t)|u − u| ¯ + g2 (t)|v − v¯ |, for t ∈ (0, 1) and u, u, u, ¯ v¯ ≥ 0; (A2 )  2×2 (A3 )  } is a matrix given by ρ (A) < 1, where A ∈ {M2,2 ∈ R+

�

1

Gα (1, s)f1 (s)ds   0 � 1  Gβ (1, s)f2 (s)ds 0

1

�



Gα (1, s)g1 (s)ds  . � 1  Gβ (1, s)g2 (s)ds 0

0

Then the system (1) has a unique positive solution (u, v) ∈ P. Proof  Let us define a generalized metric d : X 2 × X 2 → R2 by   ||u − u|| ¯ d((u, v), (u, ¯ v¯ )) = , for all (u, v), (u, ¯ v¯ ) ∈ X × X. ||v − v¯ || Obviously (X × X, d) is a generalized complete metric space. Then for any (u, v), (u, ¯ v¯ ) ∈ X × X and using property (P3 ) we get

|A1 (u, v)(t) − A1 (u, ¯ v¯ )(t)| ≤ max

t∈[0,1]

1

|Gα (t, s)|[|φ(s, u(s), v(s)) − φ(s, u(s), ¯ v¯ (s))|]ds

0

≤

1

≤

1

Gα (1, s)[f1 (s)�u − u� ¯ + g1 (s)�v − v¯ �]ds

0

f1 (s)Gα (1, s)ds�u − u� ¯ +

0

Similarly we can show that

|A2 (u, v) − A2 (u, ¯ v¯ )| ≤



0

+

1

f2 (s)Gβ (1, s)ds�u − u� ¯ 

0

1

g2 (s)Gβ (1, s)ds�v − v¯ �.

1 0

g1 (s)Gα (1, s)ds�v − v¯ �.

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Thus we have

|A(u, v) − A(u, ¯ v¯ )| ≤ Ad((u, v), (u, ¯ v¯ )),

∀(u, v), (u, ¯ v¯ ) ∈ X × X,

where

�

1

Gα (1, s)f1 (s)ds   A =  �0 1  Gβ (1, s)f2 (s)ds 0

1

�



Gα (1, s)g1 (s)ds  . � 1  Gβ (1, s)g2 (s)ds 0

0

As ρ(A) < 1, by means of Lemma 8, system (1) has a unique positive solution. 



Theorem  13  Let φ and ψ are continuous on [0, 1] × [0, ∞) × (0, ∞) → [0, ∞) and there exist ci , di , ei (i = 1, 2) : (0, 1) → [0, ∞) satisfying:

φ (t, u(t), v(t)) ≤ c1 (t) + d1 (t)u(t) + e1 (t)v(t), t ∈ (0, 1), u, v ≥ 0; (A4 ) 

ψ (t, u(t), v(t)) ≤ c2 (t) + d2 (t)u(t) + e2 (t)v(t), t ∈ (0, 1), u, v ≥ 0; (A5 ) 

1 1 (A6 )  �1 = Gα (1, s)c1 (s)ds < ∞, �1 = Gα (1, s)[d1 (s) + e1 (s)]ds < 12 ; 0

0

1 1 (A7 )  �2 = Gβ (1, s)c2 (s)ds < ∞, �2 = Gβ (s, s)[d2 (s) + e2 (s)]ds < 12 . 0

0

 Then the system (1) has at least one positive solution in 

  �1 (u, v) ∈ P : �(u, v)� ≤ r , where max 1 , − �1 2

�2 1 2

− �2



< r.

   �1 , Proof Defined � = (u, v) ∈ P : �(u, v)� < r with max 1 2 − �1

�2 1 2

− �2



< r.

Also inview of Theorem  11, the operator A :  → P is completely continuous. Let (u, v) ∈ �, such that (u, v) < r. Then, we have

  1    �A1 (u, v)� = max  Gα (t, s)φ(s, u(s), v(s))ds t∈[0,1] 0  1   1 ≤ Gα (1, s)d1 (s)|u(s)|ds + Gα (1, s)c1 (s)ds + 0

≤



0

1

Gα (1, s)c1 (s)ds + r

r ≤ �1 + r�1 < , 2

0

Gα (1, s)e1 (s)|v(s)|ds

0

0



1

1

Gα (1, s)[d1 (s) + e1 (s)]ds





similarly, A2 (u, v) < 2r , thus A(u, v) < r. Therefore, thank to Lemma 4, we have A(u, v) ∈ �. Therefore A :  → . Let there exist ζ ∈ (0, 1) and (u, v) ∈ ∂� such that (u, v) = ζ A(u, v). Then inview of assumptions (A4 ), (A5 ) and using Property (P4 ) of Lemma 10, we obtain for all t ∈ [0, 1]

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|u(t)| ≤ ζ



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1

Gα (t, s)|φ(s, u(s), v(s))|ds

0

≤ζ



<ζ

r 2

1

Gα (1, s)c1 (s)ds +   0 ≤ ζ �1 + r�1



1

Gα (1, s)d1 (s)u(s)ds +

0



1

Gα (1, s)e1 (s)v(s)ds

0



r which implies that �u� < ζ , 2 similarly, one can show that v < ζ 2r . From which, we have �(u, v)� < ζ r, with ζ ∈ (0, 1) which is a contradiction that (u, v) ∈ ∂� as r = �(u, v)�. Thus by mean of Lemma 4, A has at least one fixed point (u, v) ∈ �.   Next we use the following assumptions and notations:

φ , ψ : [0, 1] × [0, ∞) × [0, ∞) → [0, ∞) are continuous (C1 )  = ψ(t, 0, 0) = 0 uniformly with respect to t on [0, 1]. (C2 )  Gα (1, s), Gβ (1, s) defined in Lemma 10 satisfy 0<

1

Gα (1, s)ds < ∞, 0 <

0

1

and

φ(t, 0, 0)

Gβ (1, s)ds < ∞;

0

(C3 ) Let these limit hold φσ = φσ =

ψ(t, u, v) φ(t, u, v) , ψσ = , lim sup (u,v)→(σ ,σ ) t∈[0,1] u + v (u,v)→(σ ,σ ) t∈[0,1] u + v lim

sup

φ(t, u, v) ψ(t, u, v) , ψσ = lim inf , where σ ∈ {0, ∞}. (u,v)→(σ ,σ ) t∈[0,1] u + v (u,v)→(σ ,σ ) t∈[0,1] u + v lim

σα = max

t∈[0,1]

inf

1

Gα (t, s)ds, σβ = max

t∈[0,1]

0

1

Gβ (t, s)ds.

0

Theorem 14  If the assumptions (C1 ) − (C2 ) hold and one of the following conditions is also satisfied: 







1−ω 1−ω   (D1 )  φ0 γ 2 Gα (1, s)ds > 1, φ ∞ σα < 1 and ψ0 γ 2 Gβ (1, s)ds > 1, ψ ∞ σβ < 1. ω

ω

Moreover, φ0 = ψ0 = ∞ and φ ∞ = ψ ∞ = 0;

(D2 )  there exist two constants 1 , 2 with 0 < 1 ≤ 2 such that φ(t, ·, ·) and ψ(t, ·, ·) are nondecreasing on [0, 2 ] for all t ∈ [0, 1],

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Then boundary value problem (1) has at least one positive solution.

−1  1−ω � 1  γα Gα (1, s)ds , φ(t, γα 1 , γβ 1 ) ≥ 2 ω

ψ(t, γα 1 , γβ 1 ) ≥



1  γβ 2

1−ω � ω

−1

Gβ (1, s)ds

2 2 and φ(t, 2 , 2 ) ≤ , ψ(t, 2 , 2 ) ≤ , forall t ∈ [0, 1]. 2σα 2σβ Proof As A defined in (7) is completely continuous. 

Case I. Let the conditions of (D1 ) hold. Taking φ0 exists a constant ǫ1 > 0 such that

γα2

1−ω 

Gα (1, s)ds

ω



> 1, then there

φ(t, u, v) ≥ (φ0 − r1 )(u(t) + v(t)), ψ(t, u, v) ≥ (ψ0 − r2 )(u(t) + v(t)), for all t ∈ [0, 1], u, v ∈ [0, ǫ1 ], where r1 > 0, and satisfies the conditions

γ2 (φ0 − r1 ) α 2

1−ω 

Gα (1, s)ds ≥ 1, (ψ0 − r1 )

ω

γβ2 2

1−ω 

Gβ (1, s)ds ≥ 1.

ω

So for t ∈ [0, 1], (u, v) ∈ ∂Pǫ1 , we have

A1 (u, v)(t) =

1

Gα (t, s)φ(s, u(s), v(s))ds ≥ γα

0

1

Gα (1, s)φ(s, u(s), v(s))ds

0

≥ (φ0 − r1 )

γα2 2

1

�(u, v)� . 2

Gα (1, s)ds�(u, v)� ≥

0

Analogously

A2 (u, v)(t) =

1

Gβ (t, s)ψ(s, u(s), v(s))ds ≥ γβ

0

≥ (ψ0 − r2 )

1

Gβ (1, s)φ(s, u(s), v(s))ds

0

γβ2 2

1

Gβ (1, s)ds�(u, v)� ≥

�(u, v)� . 2

0

Therefore, we have

�A(u, v)� ≥ �A1 (u, v)� + �A2 (u, v)� ≥ �(u, v)�.

(10)

Also for φ ∞ σα < 1 and ψ ∞ σβ < 1, there exists a constant say ǫ¯2 > 0 such that φ(t, u, v) ≤ (φ ∞ + r2 )(u + v), ψ(t, u, v) ≤ (ψ ∞ + r2 )(u + v), for t ∈ [0, 1], u, v ∈ (ǫ¯2 , ∞), where r2 > 0 satisfies the conditions σα (φ ∞ + r2 ) ≤ 1, σβ (ψ ∞ + r2 ) ≤ 1. Let K = maxt∈[0,1],u,v∈[0,ǫ¯2 ] φ(t, u, v), L = maxt∈[0,1],u,v∈[0,ǫ¯2 ] ψ(t, u, v), then φ(t, u, v) ≤ K +(φ ∞ + r2 )(u, v), ψ(t, u, v) ≤ L + (ψ ∞ + r2 )(u, v). Now setting

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max{ǫ1 , ǫ¯2 , K σα (1−σα (φ ∞ +r2 ))−1 } ≤

ǫ2 ∞ −1 2 , max{ǫ1 , ǫ¯2 , Lσβ (1−σβ (ψ +r2 )) }

≤

ǫ2 2.

So for any t ∈ [0, 1], (u, v) ∈ ∂Pǫ2 , we obtain

A1 (u, v)(t) =

1

Gα (t, s)φ(s, u(s), v(s))ds ≤ γα

≤

1

Gα (1, s)(K + (φ ∞ + r2 )[u(s) + v(s)]ds

0

1

Gα (1, s)φ(s, u(s), v(s))ds

0

0

≤K



1

Gα (1, s)ds + (φ

0

∞

+ r2 )

1

Gα (1, s)ds�(u, v)�

0

ǫ2 ǫ2 ǫ2 − σα (φ ∞ + r2 ) + (φ ∞ + r2 )σα �(u, v)� < . < 2 2 2 Similarly A2 (u, v)(t) <

ǫ2 2,

as (u, v) ∈ ∂Pǫ2, thus, we have

(11)

A(u, v) < (u, v).

Case II. If assumptions in (D2 ) hold, then inview of definition of P for (u, v) ∈ ∂P1, we have �(u, v)� = 1 , for t ∈ J . Then from (D2 ), we have

A1 (u, v)(t) =

�1

1−ω �

Gα (t, s)φ(s, u(s), v(s))ds ≥ γα

Gα (1, s)φ(s, u(s), v(s))ds

ω

0



≥ γα

1−ω � ω





1 Gα (1, s)ds γα 2

Similarly one can also obtain A2 (u, v)(t) ≥

−1

1−ω � ω

Gα (1, s)ds

1 2 , for (u, v)

=

1 . 2

∈ ∂P1, we get

(12)

�A(u, v)� = �A1 (u, v)� + �A2 (u, v)� ≥ �(u, v)�.

Also for (u, v) ∈ ∂P2, we get that �(u, v)� = 2 for t ∈ [0, 1]. Then from (D2 ), one can get

A1 (u, v)(t) =

1

Gα (t, s)φ(s, u(s), v(s))ds ≤

0

≤

2 2σα

1

Gα (1, s)φ(s, u(s), v(s))ds

0

1

Gα (1, s)ds =

2 . 2

0

Similarly, one can also obtain that A2 (u, v)(t) ≤

2 2 , (u, v)

�A(u, v)� = �A1 (u, v)� + �A2 (u, v)� ≤ �(u, v)�.

∈ ∂P2. Hence, we have

(13)

Now inview of application of Lemma 5 to (10) and (11) or (12) and (13) implies that A has a fixed point (u1 , v1 ) ∈ P ǫ, or (u1 , v1 ) ∈ P¯ ǫi ,i (i = 1, 2) such that u1 (t) ≥ γα �u1 � > 0

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and v1 (t) ≥ γβ �v1 � > 0, t ∈ [0, 1]. From which it follows that boundary value problem  (1) has at least one positive solution.  Theorem  15  Assume that (C1 ) − (C3 ) hold. Further the following conditions are also satisfied: 

(D3 )  If φ 0 σα < 1; φ ∞ γα2

1−ω  ω





Gα (1, s)ds > 1 and ψ 0 σβ < 1; ψ ∞ γβ2

1−ω 

Gα (1, s)ds

ω



> 1,

Then the boundary value problem (1) has at least one positive solutions. Moreover, if φ 0 = ψ 0 = 0 and φ ∞ = ψ ∞ = ∞, then the the boundary value problem (1) has at least one positive solution.  Proof  Proof is similar as like the proof of Theorem 14. Theorem  16  Assume that (C1 ) − (C3 ) hold. Also the following conditions are also satisfied:



(D4 )  If φ0 γα2

1−ω 

Gα (1, s)ds

ω

(1, s)ds) > 1, ψ∞



γβ2

is also hold;



1−ω 



> 1, φ∞ γα2

Gα (1, s)ds

ω

(D5 )  there exists a > 0 such that maxt∈[0,1],(u,v)∈∂Pa φ(t, u, v) < 

a 2σα



1−ω 

Gα (1, s)ds

ω





> 1 and ψ0 γβ2

1−ω 

Gβ

ω

> 1. Moreover, φ0 = ψ0 = φ∞ = ψ∞ = ∞

and maxt∈[0,1],(u,v)∈∂Pa ψ(t, u, v) <

a 2σβ  .

Then

¯ v¯ ) the boundary value problem (1) has at least two positive solutions (u, v), (u, such that

(14)

0 < �(u, v)� < a < �(u, ¯ v¯ )�. Proof Let (D4 ) hold. Select ǫ,  such that 0 < ǫ < ρ < .     1−ω 1−ω   2 2 φ0 γα Gα (1, s)ds > 1 and ψ0 γβ Gβ (1, s)ds > 1. ω

Now

if

ω

Then like the proof of Theorem 14, we have

(15)

�A(u, v)�  ≥ |(u, v)�, for(u, v) ∈ ∂Pǫ .   1−ω 1−ω   Gα (1, s)ds > 1 and ψ∞ γβ2 Gβ (1, s)ds > 1. Now, if φ∞ γα2 ω

ω

Then like the proof of Theorem 14, we have

�A(u, v)� ≥ �(u, v)�,

(16)

for (u, v) ∈ ∂P .

Also from (D5 ), (u, v) ∈ ∂Pρ , we get

A1 (u, v)(t) = ≤



1

Gα (t, s)φ(s, u(s), v(s))ds

0



0

1

Gα (1, s)φ(s, u(s), v(s))ds <

ρ 2σα



0

1

Gα (1, s)ds =

ρ . 2

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Similarly, we have A1 (u, v)(t) <

�A(u, v)� < |(u, v)�,

ρ 2

as (u, v) ∈ ∂Pρ. Hence, we have

for (u, v) ∈ ∂Pρ .

(17)

Now by the use of Lemma 5 to (15) to (17) gives that A has a fixed point (u, v) ∈ ∂P ǫ,ρ ¯ v¯ ) ∈ ∂P ρ, . Hence it implies that the boundary value problem (1) and a fixed point in (u, ¯ v¯ ) such that �(u, v)� �= ρ and �(u, ¯ v¯ )� �= ρ. has at least two positive solutions (u, v), (u,  Thus relation (14) holds. The proof is completed.  Theorem  17  Assume that (C1 ) − (C3 ) hold and also the following conditions are satisfied:

σα φ0 < 1 and φ∞ σα < 1; σβ ψ0 < 1, and ψ∞ σβ < 1; (D6 )  (D7 )  there exist ρ > 0 such that −1  1−ω � ρ 2 γα Gα (1, s)ds , max φ(t, u, v) > t∈J ,(u,v)∈∂Pρ 2 ω



ρ max ψ(t, u, v) > γβ2 t∈J ,(u,v)∈∂Pρ 2

−1

1−ω � ω

Gβ (1, s)ds

such that Then the boundary value problem (1) has at least two positive solutions.

0 < �(u, v)� < ρ < �(u, ¯ v¯ )�. Proof  Proof is like the proof of Theorem 16. 



Similarly for multiplicity the following theorems can be easily deduced: Theorem  18  Let (C1 ) − (C3 ) hold. If there exist 2m positive numbers wk , w ˆ k , k = 1, 2 . . . m with w1 < γα w ˆ1 < w ˆ 1 < w2 < γα w ˆ2 < w ˆ 2 . . . wm < γα w ˆm < w ˆm ˆ1 < w ˆ 1 < w2 < γβ w ˆ2 < w ˆ 2 . . . wm < γβ w ˆm < w ˆ m such that and w1 < γβ w 

1

(D8 )  φ (t, u(t), v(t)) ≥ wk γα Gα (1, s)ds

and

0

−1

, for (t, u, v) ∈ [0, 1] × [γα wk , wk ] × [γβ wk , wk ],

φ(t, u(t), v(t)) ≤ σα−1 w ˆ k , for (t, u, v) ∈ [0, 1] × [γα w ˆ k, w ˆ k ] × [γβ wk , wk ], k = 1, 2 . . . m,



ψ(t, u(t), v(t)) ≥ wk γβ (D9 ) 

�1 0

−1

Gβ (1, s)ds)

, for(t, u, v) ∈ [0, 1] × [γβ wk , wk ]

× [γα wk , wk ], andψ(t, u(t), v(t)) ≤ σβ−1 w ˆ k , for (t, u, v) ∈ [0, 1] × [γα wk , wk ] × [γβ w ˆ k, w ˆ k ], k = 1, 2 . . . m.

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Then the boundary value problem (1) has at least m-positive solutions (uk , vk ), satisfying

wk ≤ �(uk , vk )� ≤ w ˆ k , k = 1, 2 . . . m. Theorem  19  Suppose that (C1 ) − (C3 ) holds. If there exist 2m positive numbers wk , w ˆ k , k = 1, 2 . . . m, with w1 < w ˆ 1 < w2 < w ˆ 2 . . . < wm < w ˆ m such that

(D10 ) φ and ψ are non-decreasing on [0, w ˆ m ] for all t ∈ [0, 1]; 

(D11 )  φ (t, u(t), v(t)) ≥ wk γα

1−ω 

−1

Gα (1, s)ds)

ω



k = 1, 2 . . . m, ψ(t, u(t), v(t)) ≥ wk γβ ψ(t, u(t), v(t)) ≤

w ˆk , k = 1, 2 . . . m. σβ

, φ(t, u(t), v(t)) ≤

1−ω � ω

−1

Gβ (1, s)ds)

w ˆk , σα

,

Then the boundary value problem (1) has at least m-positive solutions (uk , vk ), satisfying wk ≤ �(uk , vk )� ≤ w ˆ k , v = 1, 2 . . . m.

Examples We conclude the paper with the following examples. Example 20  Consider the coupled system as follow

 5 t + 1 �5�   [Ŵ D 2 u(t) + |u(t)| + cos |v(t)|] = 0, t ∈ [0, 1], u, v ≥ 0,   4 2    2    D 52 v(t) + t + 1 [sin |u(t)| + v(t)] = 0, t ∈ [0, 1], u, v ≥ 0, 4  (i) (i)  u (0) = v (0) = 0, i = 0, 1    � � � �   1 1 1 1 1 1 1 1    D 2 u(1) = D 2 u , D 2 v(1) = D 2 v . 2 2 2 2

Since φ(t, u(t), v(t)) =

(18)

t+1 5 t2 + 1 [Ŵ( )|u(t)| + cos |v(t)|], ψ(t, u(t), v(t)) = [sin |u(t)| + |v(t)|]. 4 2 4

Also as 0 < γ , δ < n − 2, n = [2.5] + 1 = 3 and γ = δ = 12 . Then

t+1 5 t+1 |u2 − u1 | + |v2 − v1 |, |φ(t, u2 , v2 ) − φ(t, u1 , v1 )| ≤ Ŵ( ) 2 4 4 t2 + 1 t2 + 1 |u2 − u1 | + |v2 − v1 |. |ψ(t, u2 , v2 ) − ψ(t, u1 , v1 )| ≤ 4 4

Shah and Khan SpringerPlus (2016)5:1116

where f1 (t) =



t+1 5 4 Ŵ( 2 ), g1 (t)

�1

Page 16 of 20

=

 Gα (1, s)f1 (s)ds)    0 A=  �1   Gβ (1, s)f2 (s)ds) 0

t+1 4 , f2 (t)

�1

= g2 (t) =

t2 + 1 , from which we have 4



 Gα (1, s)g1 (s(ds) 5    36 0   = 22  �1  √ 135 π Gβ (1, s)g2 (s)ds) 0

 5 √ 27 π  . 22  √ 135 π

By standard calculation, we can obtain that ρ(A) = 0.2479 < 1, hence by the use of Theorem 13 boundary value problem (18) has a unique positive solution. Example 21  Consider the system of non-linear fractional differential equations.

� �  7 5  D 3 u(t) + a(t) u(t) + v(t) = 0, D 2 v(t) + b(t) 3 u(t) + v(t) = 0,    u(0) = u′ (0) = 0, v(0) = v′ (0) = 0, � � � �   1 1 1 1 1 1 1 1   D 3 u(1) = D 3 u , D 2 v(1) = D 2 v , 3 2 2 2

t ∈ (0, 1),

(19)

√ √ where φ(t, u, v) = a(t) u(t) + v(t) and ψ(t, u, v) = b(t) 3 u(t) + v(t), δ = 13 , γ = 21 . and a(t), b(t) : [0, 1] → [0, ∞) are continuous.

φ(t, u, v) = ∞, similarly ψ 0 = ∞. Also by simple calculation we can u+v get that φ ∞ = 0 = ψ ∞ . Thus by Theorem 14, boundary value problem (19) has at least one positive solution. Now φ 0 = lim

(u,v)→0

Example 22  Consider the following boundary value problem

 9 14  D 2 u(t) + (1 − t 2 )[u(t) + v(t)]2 = 0, D 3 v(t) + [u(t) + v(t)]3 = 0,     (i) u (0) = v(i) = 0, i = 0, 1, 2, 3, � � � �   5 5 1 5 1 1 5 1   2 2 2 2 D u(1) = D u , D v(1) = D v .  4 4 3 3

t ∈ (0, 1),

(20)

from (20), we have γ = δ = 52 , as n = 5, 0 < γ , δ < 3. By simple calculation we obtain that φ 0 = ψ 0 = 0 and φ ∞ = ψ ∞ = ∞. Thus by Theorem 15, the boundary value problem (20) has a positive solution. Example 23  Consider the following boundary value problem  2 2 9   D 2 u(t) + (1 + t )[u (t) + v(t)] = 0, t ∈ (0, 1),   2  (4t + 4)σα    3 2    D 143 v(t) + (t + 1)[u(t) + v (t)] = 0, t ∈ (0, 1), 3 (4t + 4)σβ   (i) (i)   u (0) = v = 0, i = 0, 1, 2, 3,   � � � �    5 5 1 5 1 1 5 1   D 2 u(1) = D 2 u , D 2 v(1) = D 2 v . 4 4 3 3

(21)

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where γ = δ = 52 , as n = 5, 0 < γ , δ < 3. Also, by simple calculation we obtain that φ0 = ψ0 = ∞ and φ∞ = ψ∞ = ∞. Further for all (t, u, v) ∈ [0, 1] × [0, 1] × [0, 1], we have

σβ−1 (t 3 + 1)2 (t 2 + 1)2 σα−1 , ψ(t, u, v) ≤ φ(t, u, v) ≤ = = . 4(t 2 + 1)σα 2 4(t 3 + 1)σβ 2 Thus all the assumptions of Theorem  16 are satisfied and also a = 1. Hence by Theorem 16, the boundary value problem (21) has at least two positive solutions (u1 , v1 ) and (u2 , v2 ) which satisfy

0 < (u1 , v1 ) < 1 < (u2 , v2 ).

Non‑existence of positive solution In this section, we discuss the non-existence of positive solution to the coupled system (1) of fractional order differential equations. Theorem  24  Assume that (C1 ) − (C3 ) hold and φ(t, u, v) < (u,v) and 2σα (u,v) ψ(t, u, v) < 2σβ for all t ∈ [0, 1], u > 0, v > 0, then the boundary value problem (1) has no positive solution. Proof  On contrary let (u,  v) be the positive solution of boundary value problem (1) . Then (u, v) ∈ P for 0 < t < 1 and

�(u, v)� = �u� + �v� = max |u(t)| + max |v(t)| t∈[0,1]

≤ max

t∈[0,1]

1

t∈[0,1]

Gα (t, s)|φ(s, u(s), v(s))|ds + max

t∈[0,1]

0

<

1 0

1

Gβ (t, s)|ψ(s, u(s), v(s)|ds

0

�(u, v)� ds + Gα (1, s) 2σα

1

Gβ (1, s)

0

�(u, v)� ds 2σβ

⇒ �(u, v)� < �(u, v)�, which is contradiction. So boundary value problem (1) has no positive solution. Hence proof is completed.    −1 1−ω  γα2 Gα (1, s)ds , Theorem 25  Let(C1 ) − (C3 )holds and if φ(t, u(t), v(t)) > �(u,v)� 2 ω

ψ(t, u(t), v(t)) >

�(u,v)� 2



γβ2

1−ω  ω

Gβ (1, s)ds

−1

for all t ∈ [0, 1], u > 0 and v > 0. Then

boundary value problem (1) has no positive solution. Proof  Proof is just like the proof of Theorem 24, so we omit it.



Example 26  In this section we provide an example which illustrate the results of Theorems 24 and 25 respectively. Consider the system of non linear fractional differential equations:

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 5 (2u2 (t) + u(t))(20 + cos v(t))   = 0, t ∈ [0, 1],  D 2 u(t) +   u(t) + v(t) + 1    2    D 52 v(t) + (2v (t) + v(t))(20 + cos v(t)) = 0, t ∈ [0, 1], u(t) + v(t) + 1   (i) (i)  u (0) = v (0) = 0, i = 0, 1,    � � � �   1 1 1 1 1 1 1 1    D 2 u(1) = D 2 v(1) = . D2u D2v 10 2 10 2

(22)

Since (C1 ) − (C3 ) holds and also γ = δ = 12 , as n = 3, 0 < γ , δ < 1.

φ 0 = ψ 0 = 20, φ ∞ = ψ ∞ = 43, 20�(u, v)� < φ(t, u(t), v(t)) < 43�(u, v)� 20(u, v) < ψ(t, u(t), v(t)) < 43(u, v) and φ(t, u(t), v(t)) < 43(u, v) <

(u, v) , σα

where σα ≈ 0.876126 and σβ ≈ .475675 �(u, v)� ≈ 1.1413�(u, v)� implies that φ(t, u(t), v(t)) Case I: Now φ(t, u(t), v(t)) < σα < 43�(u, v)� ≈ 1.1413�(u, v)� and ψ(t, u(t), v(t)) < 43�(u, v)� ≈ 2.1022�(u, v)�. Thus by Theorem 24, boundary value problem (22) has no positive solution. 

2 3

−1

Case II: Also φ(t, u(t), v(t)) > 20�(u, v)� > �(u, v)�γα2 � Gα (1, s)ds 

ψ(t, u(t), v(t)) > 2�(u, v)� > �(u, v)�γβ2

2

�3 1 3

−1

Gβ (s, s)ds

1 3

≈ 15.8688�(u, v)� and

≈ 12.678�(u, v)�.

Then by Theorem 25, boundary value problem (22) has no solution.

Conclusion In this article, we have developed sufficient conditions for the multiplicity results of positive solutions to a highly nonlinear coupled system of fractional order differential equations. Our paper is the generalization of the work carried out in Goodrich (2010), Jalili and Samet (2014), Rehman and Khan (2010). In Jalili and Samet (2014), the authors studied the concerned coupled system with homogenous boundary conditions involving fractional order derivative, but we extended this work with taking non-homogenous boundary condition involved fractional order derivative of Riemann- Liouville type. By using classical fixed point theorems, we have successfully developed conditions under which the considered coupled system has multiple solutions. Moreover, uniqueness and non existence results have also been established. Numerous examples have been provided which justify the results developed by us. Authors’ contributions Both authors contribute the manuscript equally. Both authors read and approved the final manuscript. Acknowlegements We are really thankful to the anonymous referees for their valuable suggestions and comments which improved the quality of this Manuscript. We are also thankful to the editor for giving us some directions to produce motivation in this article for Refs. Nielsen et al. (2013), Shang (2014). The authors are also grateful to Springerplus for granting full fee waiver.

Shah and Khan SpringerPlus (2016)5:1116

Competing interests The authors declare that they have no competing interests. Received: 13 February 2016 Accepted: 18 June 2016

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