Positive solutions of a singular semipositone boundary value problems for fourth order coupled difference equations

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R E S E A R C H

Open Access

Positive solutions of a singular semipositone

boundary value problems for fourth-order

coupled difference equations

Wenjin Li

1

_{, Yupeng Fei}

1*

_{, Biaoan Shan}

1

_{and Yanni Pang}

2

*_{Correspondence: [email protected]} 1_{School of Management, Jilin}

University, Changchun, Jilin 130012, P.R. China

Full list of author information is available at the end of the article

Abstract

In this article, we study the boundary value problems for the fourth-order nonlinear coupled diﬀerence equations

⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩

4_u(i_{– 2) =}_f

1(i,v(i)) +e1(i), i∈[2,T+ 2],

4_v(i_{– 2) =}_f

2(i,u(i)) +e2(i), i∈[2,T+ 2],

u(0) =u(1) =v(1) =v(0) = 0,

u(T+ 3) =u(T+ 4) =v(T+ 4) =v(T+ 3) = 0.

Throughout our nonlinearity may be singular. We establish the existence of positive solutions for the fourth-order coupled systems. The proof relies on Schauder’s ﬁxed point theorem.

MSC: 39A11

Keywords: positive solution; positone and semipositone boundary value problem; singular diﬀerence equation; ﬁxed point theorem

Introduction

In this article, we consider the following boundary value problems of diﬀerence equations

⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩

_u₍_i_{– ) =}_f₍_i_,_v₍_i_{)) +}_e₍_i_), _i_∈_[,_T_{+ ],}

_v₍_i_{– ) =}_f

(i,u(i)) +e(i), i∈[,T+ ], u() =u() =v() =v() = ,

u(T+ ) =u(T+ ) =v(T+ ) =v(T+ ) = ,

()

here [,T+ ] ={, , . . . ,T+ }andu,v,ej: [,T+ ]→R,fj: [,T+ ]×R→R+,j= , .

We will let [a,b] denote the discrete integer set [a,b] ={a,a+ , . . . ,b},C([a,b]) denote set of continuous function on [a,b] (discrete topology) with norm · =maxk∈[a,b]| · |.

Recently, many literature on the boundary value of diﬀerence equations have appeared. We refer the reader to [–] and the references therein, which include work on Agarwal, Eloe, Erber, O’Regan, Henderson, Merdivenci, Yu, Maet al., concerning the existence of positive solutions and the corresponding eigenvalue problems. Recently, the existence of

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positive solutions of fourth-order discrete boundary value problems have been studied by several authors, for examples, see [–] and the references therein.

The fourth-order boundary value problems of ordinary value problems have important application in various branches of pure and applied science. As in entrepreneurial network evolution studies, the research paradigm of ‘U related to V’ was always applied, while U or V was the measurement of the attitudes of the entrepreneurs who answered the ques-tionnaire. According to cognitive psychology studies [, ], questionnaire shows the change of attitude, which is the fourth-order dependent of original signal. So whether this paradigm will lead to meaningful causal outcome, which is basically depended on diﬀer-ence equations.

The fourth-order boundary value problems of ordinary value problems have important application in various branches of pure and applied science. They arise in the mathemati-cal modeling of viscoelastic and inelastic flows, deformation of beams and plate deflection theory [–]. For examples, the deformations of an elastic beam can be described by the boundary value problems of the fourth-order ordinary differential equations. There have been extensive studies on fourth-order boundary value problems with diverse boundary conditions via many methods, for example [–] and the references therein.

The remaining part of the article is organized as follows. In Section “Preliminaries”, some preliminary results will be given. In the remaining sections, by employing a basic applica-tion of Schauder’s ﬁxed point theorem, we state and prove the existence results for (). Our view point sheds some new light on problems with weak force potentials and prove that in some situations weak singularities may stimulate the existence of positive solutions.

Preliminaries

In this section, we state the preliminary information that we need to prove the main re-sults. From [, ], we have the following lemma.

Lemma  x(i)is a solution of equation

⎧ ⎨ ⎩

_x₍_i_{– ) =}_h₍_i_), _i_∈_[,_T_{+ ],} x() =x() =x(T+ ) =x(T+ ) = 

()

if only and if

x(i) =

T+

j=

G(i,j)h(j), i∈[,T+ ], ()

where

G(i,j) =

⎧ ⎨ ⎩

(T+–i)(j–)  (

i (T+)–

(T++i)(j–)

(T+) ), ≤j≤i+ , i(T+–j)

 (

(T+–i)(T++j) (T+) –

T+–j

(T+)), i+  <s≤T+ .

()

Lemma  The Green’s function G(i,j)deﬁned by () have properties Ci(T+  –i)(j– )(T+  –j)≤G(i,j)≤(j– )(T+  –j),

G(i,j)≤i(T+  –i),

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For our constructions, we shall consider the Banach spaceE=C([,T+ ]) equipped with the standard normx=max≤i≤T+|x(i)|,x∈E. We deﬁne a coneP⊂Eby

P=x∈E|x(i)≥Ci(T+  –i)x,i∈[,T+ ].

Deﬁne an operatorA(u,v) = (A(u),A(v)) :P×P→E×Eby

Au(i) = T+

j=

G(i,j)(f(i,v(i)) +e(i)), i∈[,T+ ],v∈P,

Av(i) =

T+

j=

G(i,j)(f(i,u(i)) +e(i)), i∈[,T+ ],u∈P.

()

Notice from () and Lemma  that, foru∈P, we have

Au(i) =

T+

j=

G(i,j) f i,v(i)+e(i)≤

T+

j=

(j– )(T+  –j) f i,v(i)+e(i),

Av(i) =

T+

j=

G(i,j) f i,u(i)+e(i)≤

T+

j=

(j– )(T+  –j) f i,u(i)+e(i),

thenAv ≤T_j_=+(j– )₍_T_{+  –}_j₎₍_f₍_i_,_v₍_i_{)) +}_e₍_i_{)) and}_Au_≤T+

j= (j– )(T+  – j)(f(i,u(i)) +e(i)).

On the other hand, we have

Au(i)≥Ci(T+  –i)

T+

j=

(j– )(T+  –j) f i,v(i)+e(i)

≥Ci(T+  –i)Au,

Av(i)≥Ci(T+  –i)

T+

j=

(j– )(T+  –j) f i,u(i)+e(i)

≥Ci(T+  –i)Av.

Thus,A(P×P)⊂P×P. In addition, standard arguments show thatAis completely con-tinuous.

Now, we deﬁne the functionγk:E→Eby

γk(i) = T+

j=

G(i,j)

i₍_T_{+  –}_i₎ek(j), i∈[,T+ ],k= , ,

which is the unique solution of

⎧ ⎨ ⎩

_x₍_i_{– ) =}_e

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Throughout this article, we use the following notations

γk*=min

k,i γk(i), γ * k=max

k,i γk(i).

The case

γ

1*≥0,

γ

2*≥0

Theorem  Assume that there exists bk ,bˆk and <αk< such that

≤bˆk(i)

xαk ≤fk(i,x)≤ bk(i)

xαk , for all x> ,i∈[,T+ ],k= , . (H) Ifγ*≥,γ*≥, then there exists a positive solution of ().

Proof A solution of () is just a ﬁxed point of the completely continuous mapA(x,y) = (Ax,Ay) :P×P→P×P, from () we have

(Au)(t) =

T+

j=

G(i,j)f i,v(i)+γ(i),

(Av)(t) =

T+

j=

G(i,j)f i,u(i)+γ(i).

By a direct application of Schauder’s fixed point theorem, the proof is finished if we prove thatAmaps the closed convex set defined as

K=(u,v)∈P×P:ri(T+  –i)≤u(i)≤Ri(T+  –i),

ri(T+  –i)≤v(i)≤Ri(T+  –i),i∈[,T+ ]

,

into itself, whereR>r> ,R>r>  are positive constants to be ﬁxed properly. For convenience, we introduce the following notations

βk(i) = T+

j=

G(i,j)

i₍_T_{+  –}_i₎bk(j), βˆk(i) = T+

j=

G(i,j)

i₍_T_{+  –}_i₎bˆk(j), k= , .

Given (u,v)∈K, by the nonnegative sign ofGandfk,k= , , we have

(Au)(i) =i(T+  –i)

T+

j=

G(i,j)

i₍_T_{+  –}_i₎f j,v(j)

+γ(i)

≥i(T+  –i)

T+

j=

G(i,j)

i₍_T_{+  –}_i₎

ˆ

b(j)

vα₍_j₎+i

₍_T_{+  –}_i₎_γ *

≥i(T+  –i)

T+

j=

G(i,j)

i₍_T_{+  –}_i₎

ˆ

b(j) Rα



≥i(T+  –i)βˆ*·



Rα 

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Note for every (u,v)∈K

Similarly, by the same strategy, we have

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ˆ

β*·R–α≥, β*·R

α₊_γ* ≤R

and these inequalities hold forRbig enough becauseαi< . The proof is complete.

The case

γ

*

1≤0,

γ

2*≤0

The aim of this section is to show that the presence of a weak singular nonlinearity makes it possible to ﬁnd positive solutions ifγ_*≤,γ_*≤.

then there exists a positive solution of ().

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h(r) possesses a minimum at

γ*≥g(r), which are just condition (). The second inequalities hold directly from the

choice ofRandR, so it remains to prove thatR=

β_*

r_α >r,R=

β_*

rα_ >r. This is easily

veriﬁed through elementary computations:

R= β

where <r< +∞is a unique positive solution of the equation

r–αα

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If we ﬁxR= β* rα 

, then the ﬁrst inequality of () holds ifrsatisﬁes

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or equivalently,

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*.

Takingr=r, then the ﬁrst inequality in () holds ifγ*≥f(r), which is just

condi-tion (). The second inequality holds directly by the choice ofR, and it would remain to prove thatr<R andr<R. These inequalities hold forR big enough andr small enough. The proof is complete.

Similarly, we have the following theorem.

Theorem  Assume (H) are satisﬁed. Ifγ_*≤,γ*≥and

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα )α

,

where <r< +∞is a unique positive solution of the equation

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*,

The case

γ

1*< 0 <

γ

1*,

γ

2*< 0 <

γ

2*

Theorem  Assume (H) is satisﬁed. Ifγ*<  <γ*,γ*<  <γ*and

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα )α

, ()

γ*≥r–βˆ*·

rαα 

(β* +γ*r

α )α

, ()

where <r< +∞is a unique positive solution of the equation

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*,

and <r< +∞is a unique positive solution of the equation

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*, then there exists a positive solution of ().

Proof We follow the same strategy and notation as in the proof of ahead theorem. In this case, to prove thatA:K→K, it is suﬃcient to ﬁndr<R,r<Rsuch that

ˆ β* Rα 

+γ*≥r,

β_*

rα 

+γ_*≤R, ()

ˆ β* Rα 

+γ*≥r,

β_*

rα 

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If we ﬁxR= β*

, then the ﬁrst inequality of () holds ifrsatisﬁes

γ*≥g(r) :=r–βˆ*·

γ*≥g(r), which are just condition () and (). The second inequalities hold directly

by the choice ofRandR, and it would remain to prove thatr<Randr<R. This is

easily veriﬁed through elementary computations.

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=(ααβ

On the other hand,

r–αα

Now if we can prove

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The case

γ

*

1≤0,

γ

2*< 0 <

γ

2*(

γ

2*≤0,

γ

1*< 0 <

γ

1*)

Theorem  Assume (H) are satisﬁed. Ifγ_*≤,γ*<  <γ*and

γ*≥

 – 

αα

ˆ β*

(β_*)α



–αα

, ()

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα )α

, ()

where <r< +∞is a unique positive solution of the equation

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*, ()

Proof In this case,to prove thatA:K→K, it is suﬃcient to ﬁndr<R,r<Rsuch that

ˆ β* Rα 

+γ*≥r,

β_*

rα 

≤R, ()

ˆ β* Rα 

+γ*≥r,

β_*

rα 

+γ_*≤R. ()

If we ﬁxR= β

*  rα_,R=

β*_

rα_ +γ *

, then the ﬁrst inequality of () holds ifrsatisﬁes

γ*≥r–

ˆ β* Rα 

=r– βˆ* (β_*)α ·r

αα

 , ()

or equivalently

γ*≥f(r) :=r–

ˆ β*

(β_*)α ·r

αα

 . ()

Then the functionf(r) possesses a minimum at

r=

αα·

ˆ β*

(β* )α



–αα

, ()

i.e.,f(r) =minr(,+∞)f(r).

On the analogy of (), we obtain

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα  )α

, ()

or equivalently,

γ*≥h(r) :=r–βˆ*·

rαα 

(β_*+γ_*rα  )α

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According to

On the other hand,

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Combing () and (),

R= ααβ*βˆ*



+α_r– +α_

+α

 . ()

In what follows, we will verify thatR>r. In fact,

(αα) +α_+α_

–αα –·

_ˆ

β*

β_* +α

–αα

·

_ˆ

β*

β_* α(+α)

–αα

< ,

sinceβˆi*≤βi*,i= , . Thus

ααβ*(–α)βˆ*

+α_

–αα · _α__α__β*(–α)  βˆ*

+α_

–αα _<_α__α__β*

βˆ*. ()

On the other hand,

r–αα  β

*(+α)

 ≤ααβ*βˆ*, r–αα

 β *(+α)

 ≤ααβ*βˆ*.

Thus one can see easily note that

r≤ ααβ*(–α)βˆ*



–αα_, _()

r≤ ααβ*(–α)βˆ*



–αα_. _()

From () and (),

r+α  r

+α

 ≤ ααβ*(–α)βˆ*

+α

–αα _α__α__β*(–α)  βˆ*

+α

–αα_. _()

Combing () and (),

r+α  r

+α

 <ααβ*βˆ*.

Therefore,

rr +α +α

 < ααβ*βˆ*



+α_.

Recall (), we obtainr<Rimmediately. The proof is complete. Similarly, we have the following theorem.

Theorem  Assume (H) are satisﬁed. Ifγ_*≤,γ*<  <γ*and

γ*≥

 – 

αα

·

αα

ˆ β*

(β_*)α



–α_α_

,

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα )α

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where <r< +∞is a unique positive solution of the equation r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*, then there exists a positive solution of ().

The case

γ

1*≥0,

γ

2*< 0 <

γ

₂*(

γ

2*≥0,

γ

1*< 0 <

γ

₁*)

Theorem  Assume (H) are satisﬁed. Ifγ*≥,γ*<  <γ*and

γ*≥r–βˆ*·

rαα 

(β_*+γ_*rα )α

,

where <r< +∞is a unique positive solution of the equation r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*,

then there exists a positive T -periodic solution of ().

Proof In this case, to prove thatA:K→K, it is suﬃcient to ﬁndr<R,r<Rsuch that

ˆ β* Rα 

≥r, β

*  rα 

+γ_*≤R. ()

ˆ β* Rα 

+γ*≥r,

β_*

rα 

+γ_*≤R. ()

If we ﬁxR= β* rα_ +γ

* ,R=

β_*

rα_ +γ *

, then the ﬁrst inequality of () satisﬁes

ˆ β*·

β_*

rα 

+γ_* –α

+γ*≥r,

or equivalently

γ*≥l(r) :=r–

ˆ β*

(β* +γ*r

α  )α

·rαα  .

Then the functionl(r) possesses a minimum atr,i.e.,l(r) =minr(,+∞)l(r).

Notel(r) = , then we have  –ααβ*βˆ*r

αα–

 β*+γ*·r

α 

––α

= . Therefore,

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*.

Note thatβˆi*,βi*> ,i= , . And takingr=r,R=

β*_

rα_ +γ *

,r=_R_, it is suﬃcient to

ﬁndr<R,r<Rsuch that Rα_–≤β_*, Rα

 β*+γ*≤R

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Similarly, we have the following theorem.

Theorem  Assume (H) are satisﬁed. Ifγ*≥,γ*<  <γ*and

γ*≥r–βˆ*·

rαα 

(β* +γ*r

α )α

,

where <r< +∞is a unique positive solution of the equation

r–αα

 β*+γ*·r

α 

+α

=ααβ*βˆ*, then there exists a positive solution of ().

Competing interests

The authors declare that they have no competing interests. Authors’ contributions

WL, YF, BS, and YP have critically revised the manuscript and have made substantial contributions to conception. WL designed the research and drafted the manuscript. YF, BS and YP revised it critically for important intellectual content. All authors read and approved the ﬁnal manuscript.

Author details

1_{School of Management, Jilin University, Changchun, Jilin 130012, P.R. China.}2_{School of Mathematics, Jilin University,}

Changchun, Jilin 130012, P.R. China. Acknowledgement

This study was supported by the National Science Founds of China (NO. 71172019). Received: 7 May 2012 Accepted: 29 May 2012 Published: 2 July 2012 References

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doi:10.1186/1687-1847-2012-97