Open access peer-reviewed chapter

Existence Theory of Differential Equations of Arbitrary Order

By Kamal Shah and Yongjin Li

Submitted: November 1st 2017Reviewed: February 16th 2018Published: May 23rd 2018

DOI: 10.5772/intechopen.75523

Downloaded: 725


The aims of this chapter are devoted to investigate a system of fractional-order differential equations (FDEs) with multipoint boundary conditions. Necessary and sufficient conditions are investigated for at most one solution to the proposed problem. Also, results for the existence of at least one or two positive solutions are developed by using a fixed-point theorem of concave-type operator for the considered problem. Further, we extend the conditions for more than two solutions and established some adequate conditions for multiplicity results to the proposed problem. Also, a result devoted to Hyers-Ulam stability is discussed. Suitable examples are provided to verify the established results.


  • fractional differential equations
  • coupled system
  • boundary condition
  • concave operator
  • Mathematics subject classification: 26A33
  • 34A08
  • 35B40

1. Introduction

Arbitrary-order differential equations are the excellent tools in the description of many phenomena and process in different fields of science, technology, and engineering (see [1, 2]). Therefore, considerable attention has been paid to the subject of differential equations of arbitrary order (see [3, 4, 5] and the references therein). The area devoted to the existence of positive solutions to fractional differential equations and their system especially coupled systems was greatly studied by many authors (for details see [6, 7, 8, 9]). In all these articles, the concerned results were obtained by using classical fixed point theorems like Banach contraction principle, Leray-Schauder fixed point theorem, and fixed point theorems of cone type. The aforesaid area has been very well explored for both ordinary- and arbitrary-order differential equations. Existence and uniqueness results for nonlinear and linear, classical, as well as arbitrary-order differential equations have been investigated in many papers (see few of them as [10, 11, 12, 13]).

Another warm area of research in the theory of fractional-order differential equations (FDEs) is devoted to the multiplicity of solutions. Plenty of research articles are available on this topic in literature. In [14], the author studied the given boundary value problem (BVP) for existence of multiple solutions:


where Dis the Riemann-Liouville derivative of non-integer order and I=01. In same line, Kaufmann and Mboumi [15] studied the given boundary value problem of fractional differential equations for multiplicity of positive solutions:


where Dis the Riemann-Liouville derivative and ϕCIR,HCI×RR.

In the last few decades, the theory devoted to the multiplicity of solutions is very well extended to coupled systems of nonlinear FDEs, and we refer to few papers in [16, 17, 18]. Wang et al. [19] established some conditions under which the given system of three point BVP


has a solution, where θ1,θ212and μ,νI,ξ01,Hi:01×RRfor i=1,2are nonlinear functions.

In the last few decades, another important aspect devoted to stability analysis of FDEs with initial/boundary conditions has been given much attention. This is because stability is very important from the numerical and optimization point of view. Various forms of stabilities were studied for the aforesaid FDEs including exponential, Mittag-Leffler, and Lyapunov stability. Recently, Hyers-Ulam stability has given more attention. This concept was initially introduced by Ulam and then by Hyers (for details see [20, 21, 22]). Now, many articles have been written on this concept (see [23, 24, 25, 26, 27]). So far, the aforementioned stability has not yet well studied for multipoint BVPs of FDEs. Motivated by the aforesaid discussion, we propose the following coupled system of four-point BVP provided as


where j=0,1,2,m2,m3,I=01,η,ξ01, H1,H2:01×0R+×0R+0R+are continuous functions, and Dθ1,Dθ2stand for Riemann-Liouville fractional derivative of order θ1,θ2in sequel. We obtain necessary and sufficient conditions for the existence of solution to system (1) by using another type of fixed point result based on a concave-type operator with increasing or decreasing property. The idea then extends to form some conditions which ensure multiplicity of solutions to the considered problem. Also, we discuss some results about the Hyers-Ulam stability for the considered problem. Further by providing examples, we illustrate the established results.


2. Preliminaries

In the current section, we review few fundamental lemmas and results found in [2, 4, 6, 28, 29].

Arbitrary-order integral of functionψ:0Ris recalled as

whereθ1>0is a real number and also the integral is pointwise defined onR+

Arbitrary-order derivative in Riemann-Liouville sense for a functionψ0Ris given by

Dθ1ψt=ddtm0ttsmθ11Γmθ1ψsds,θ1>0,where m=θ1+1.

[16] Letθ1>0,then for arbitraryCjR,j=1,2,,m,m=θ1+1,and the solution of

is provided by

[17, 28] Consider a Banach spaceEwith a closed setCE. Then,Cis said to be partially ordered ifpqsuch thatqpC.Further,Cis said to be a cone if it holds the given conditions:

  1. pCand for a real constantκ0the relationκpCholds.

  2. pandpCyield that0C,where0is zero element of Banach spaceE

[17, 28] A closed and convex setCofEis said to be a normal cone if it obeys the given properties:

  1. For0pqE,there existsβ>0,such thatpEβqE;

  2. pq,for allp,qEyields that there exist constantsa,b>0such thatapqbq.

Asis an equivalence relation, therefore defines a setCf=pE:pfforfC. Obviously, one can derive thatCfCforf0.

The operatorS:CCis said to beλconcave for everyθ,λ01,pC,if and only ifSλpθλSp.

The operatorS:CCis said to be to be increasing ifp,qC,pqgives thatSpSq.

[17, 28] Assume thatS:CCis increasingλconcave operator for a normal coneCproduced by Banach spaceE,such that there existsp0withSfCf. Then,Shas a unique fixed pointpCf

[30] LetEbe a Banach space withCB, which is closed and convex. LetEbe a relatively open subset ofCwith0EandS:E¯Cbe a continuous and compact operator. Then.

  1. The operatorShas a fixed point inE¯,

  2. There existw∂ℰandλ01withw=λSw.

[30] For a Banach spaceEtogether with a coneC, there exist two relatively open subsetsA1andA2ofEsuch that0A1A¯1A2. Moreover, for a completely continuous operatorS:CA¯2\A1C,one of the given conditions holds:

  1. Sppfor allpCA1;Spp, for allpCA2;

  2. Sppfor allpCA1;Spp, for allpCA2

Then,Shas at least one fixed point inCA¯2\A1.


3. Main results

LetφC01R,η01andλ1=1ηθ11<1, and then the unique solution to BVP of linear FDE

is given by
whereGtsis the Green’s function defined by

Proof.In view of Lemma 2.3, we may write Eq. (2) as


In view of conditions pjtt=0=0,j=0,1,m2,m3,, Eq. (5) suffers from singularity; therefore, we have C2=C3==Cn=0.Hence, Eq. (5) becomes


Applying boundary condition ptt=1=ptt=ηand d=1η1θin Eq. (6), one has


where Gtsis Green’s function given in Eq. (4).

In view of Theorem 3.1 and using λ1=1ηθ11,λ2=1ξθ21, the corresponding coupled system of integral equations to the proposed system (1) is given as


where G1ts,G2tsare Green’s functions, which can be similarly computed like in Theorem 3.1. Further, they are continuous on I×Iand satisfy the following properties:

  1. maxtIG1tsλ1+11sθ11λ1=G11s,forallsI,maxtIG1tsλ2+11sθ21λ2=G21s,forallsI;

  2. mintθ1θG1tsγ1s2G1sfor every θs01;

    mintθ1θG2tsγ2s2G1sfor every θs01;

    Further, taking that γ=infγ1=θθ11γ2=θθ21.

Let us define a Banach space by E=ptpCIendowed with a norm pE=maxtIpt. Further, in the norm for the product space, we define it as pqE×E=pE+qE. Clearly, E×EE×Eis a Banach space. Onward, we define the cone CE×Eby


Consider an operator S:E×EE×Edefined by


It is to be noted that the fixed points of the operator Scorrespond with the solution of the system (1) under consideration.

Under the continuity ofH1,H2:I×R+0×R+0R+0, the operatorSsatisfies thatSCCandS:CCis completely continuous.

Proof.To derive SCC, let pqC, and then we have


Also, we get


Thus, from Eqs. (10) and (11), we have

S1ptqtγS1pqE,for everytI.

Similarly, we can obtain

S2ptqtγS2pqE,for everytI.

Hence, we have SpqCSCC.

Let us consider


Then, we consider t1<t2I,such that


By the same fashion, we obtain for S2as


The right hand sides of Eqs. (12) and (13) are approaching to zero at t1t2.Thus, the operator Sis equi-continuous. Therefore, thanks to the Arzelá-Ascoli theorem, we receive that S=S1S2:CCis completely continuous.

Due to continuity ofH1andH2onI×R+0×R+0R+, there existφj,ψj,σjj=12:01R+0fort01,p,q0such that

along with the following conditions:
  1. Δ1=01G11sφ1sds<,Λ1=01G11sψ1s+σ1sds<1;

  2. Δ2=01G21sφ2sds<,Λ2=01G21sψ2s+σ2sds<1

are satisfied. Then, the system(1) has at least one solutionpqwhich lies in

Proof.Let E=pqC:pqE×E<rwith min2Δ112Λ12Δ212Λ2<r.

Define the operator S:E¯Cas in Eq. (9).

Let pqEthat is pqE×E<r.Then, we have


Thus, from Eq. (14), we have


Similarly, one can derive that


Thus, from Eqs. (15) and (16), we get


Therefore, SpqE¯.Hence, by Theorem 3.2 the operator S:E¯Eis completely continuous.

Consider the eigenvalue problem:


Under the assumption that pqis a solution of Eq. (18) for ρ01,we have

ptρmaxtI01G1tsH1(spsqs)dsρ01G11sφ1sds+01G1(1s)(ψ1sps+σ1sqs)dsρΔ1+rΛ1which implies thatpE<r2.

Similarly, we can obtain that qE<r2,so pqE×E<r,which implies that pqdoes not belong to Efor all ρ01.Therefore, due to Theorem 2.10, Shas a fixed point in E¯

Assume that the given hypothesis holds:

(H1) The nonlinear functions H1and H2are continuous on I×R+0×R+0R+0

(H2) For all tI,we have




(H3) For all tIsuch that


(H4) For p,q0, there exist real numbers 0<λ,μ<1,such that for each tI,τ01, we have


Under the assumptionsH1H4,the BVP(1) has a unique solution inCfwhereft=tθ11tθ21.

Proof.Let maxλμ=κand pqC. For each tI,using H4, we have


Analogously, we also get


In view of partial order on E×Einduced by the cone C, we get SτpτqτκSptqt,τ01,pqC.Which yields that Sis τconcave and nondecreasing operator with respect to the partial order by using hypothesis H4. Hence, taking fCfor each tIdefined by


Suppose that




Also, from Green’s functions, we can obtain that


Due to nondecreasing property of H1,H2in view of H3,we get μ>0,ν>0. Therefore, applying (19) together with H4,one has


Similarly, we can get


Then, we obtain


Like the aforesaid process, applying Eq. (19) together with H4,for each tI, one has


With same fashion, we can obtain


Thus, we have


From Eqs. (20) and (21), we produce


which implies that SfCf. So, thanks to Lemma 2.9, we see that the operator Sis concave; hence, it has at most one fixed point pqCfwhich is the corresponding solution of BVPs (1).

Now, we define the following:

(C1) Hjj=12:I×R+0×R+0R+0is uniformly bounded and continuous on Iwith respect to t.

(C2) Green’s functions G11s,G21ssatisfy


(C3) Let these limits hold:


Assume that the conditionsC1C3together with given assumptions are satisfied:

(H5) H1,0γ12θ1θG11sds>1,H1,γ12θ1θG11sds>1and


Moreover,H1,0=H2,0=H1,=H2,=also hold:

(H6) There exists constantα>0such that




Then, the system(1) of BVPs has at least two positive solutionspq,p¯q¯which obeying


Proof.Assume that H5holds, and consider ϵ,α,λsuch that 0<ϵ<α<λ. Further we define a set by


Now, if


Then, obviously, we can obtain that


Now, if H1,γ12θ1θG11sds>1andH2,γ22θ1θG21sds>1.

Then, like the proof of Eq. (23), we have


Also, from H5and pqCΩα,we get


From which we have


Similarly, we have S1pqE×E<α2as pqCΩα. Hence, we have


Now, applying Lemma 2.11 to Eqs. (23) and (25) yields that Shas a fixed point pqCΩ¯α\Cεand a fixed point in p¯q¯CΩ¯λ\Ωα.Hence, we conclude that the system of BVPs (1) has at least two positive solutions pq,p¯q¯such that pqE×Eαand p¯q¯E×Eα. Thus, relation (22) holds.

Consider thatC1C3together with the following hypothesis are satisfied:

(H7) δ1H1,0<1, δ1H1,<1;δ2H1,0<1,andδ2H2,<1;

(H8) There existρ>0such that


such that


Then, the proposed coupled system of BVPs(1) has at least two positive solutions.

Proof.Proof is like the proof of Theorem 3.4.

Analogously, we deduce from Theorem 3.5 and 3.6 the following results for multiplicity of solutions to the system (1) of BVPs.

Under the conditionsC1C3, there exist2kpositive numbersaj,âj,j=1,2kwitha1<γ1â1<â1<a2<γ1â2<â2ak<γ1âk<âkanda1<γ2â1<â1<a2<γ2â2<â2ak<γ2âk<âksuch that.

(H9) H1tptqtγ101G11sdsaj,fortpqI×γ1ajaj×γ2ajaj,and


(H10) H2tptqtγ201G21sdsaj,fortpqI×γ1ajaj×γ2ajaj,and


Then, system(1) of BVPs has at leastksolutionspjqj,satisfying


Further, if assumptionsC1C3hold such that there exist2kpositive numbersbj,b̂j,j=1,2k, with


together with following hypothesis hold:

(H11) H1tpqandH2tpqare nondecreasing on0b̂kfor alltI;


Then, system(1) of BVPs has at leastksolutionspjqj,satisfying



4. Hyers-Ulam stability

([31, Definition 2]) Consider a Banach spaceE×Esuch thatS1,S2:E×EE×Ebe the two operators. Then, the operator system provided by

is called Hyers-Ulam stability if we can findCii=1,2,3,4>0,such that for eachρii=12>0and for each solutionpqE×Eof the inequalities given by
there exist a solutionp¯q¯E×Eof system(26) which satisfy

Ifλi,fori=1,2,,nbe the (real or complex) eigenvalues of a matrixMCn×n, then the spectral radiusρMis defined by


Further, the matrix will converge to zero ifρM<1..

([31, Theorem 4]) Consider a Banach spaceE×EwithS1,S2:E×EE×Ebe the two operators such that

and if the matrix
converges to zero ([31, Theorem 1]), then the fixed points corresponding to operatorial system(26) are Hyers-Ulam stable.

For the stability results, the following should be hold:

(H13) Under the continuity of Hi,i=1,2, there exist ai,bic01,i=1,2and pq,p¯q¯such that


In this section, we study Hyers-Ulam stability for the solutions of our proposed system. Thanks to Definition 4.1 and Theorem 4.3, the respective results are received.

Suppose that the assumptionsH13along with condition that matrix

is converging to zero. Then, the solutions of(1) are Hyers-Ulam stable.

Proof.In view of Theorem 4.3, we have


From which we get


Hence, we get


where M=01G11sa1sds01G11sb1sds01G21sa2sds01G21sb2sds. Hence, we received the required results.


5. Illustrative examples

Consider the given system of BVPs


Clearly,H1tpq0,H2tpq0,atpq=00,andH1tpq0,H2tpq0,atpq=11. Simple computation yields thatH1,H2are nondecreasing for everyt01.Also, forτ,t01,andp,q0, one hasmax1413=13,


Thus, all the conditions ofTheorem 3.4 are fulfilled, so the system(32) of BVPs has unique positive solution inBfwhereft=t52t92.

Consider the following system of BVPs:


It is obvious thatH1tpq0,H2tpq0,atpq=00,andH1tpq0,H2tpq0,atpq=11. Also, an easy computation yields thatH1,H2are nondecreasing for eacht01.Moreover, forτ,t01,andp,q0, we see thatmax32=3,


Thus, all the assumption ofTheorem 3.4 is fulfilled, so the coupled system(33) has a unique positive solution inBfwhereft=t34t43.

Consider the following system of BVPs:


From system(33), we see that

whereφ1t=t40,φ2t=t250,ψ1t=t20,ψ2t=t260,σ1t=t220,σ2t=t60. Also, η=ξ=12,λ1=λ2=0.17677.Thus, by computation, we have

Upon computation, we get


Similarly, we can also compute.

Λ1=01G11sψ1s+σ1sds=0.03092853<1,Λ2=01G21sψ2s+σ2sds=0.00289<1.Further, we see thatmax0.0076260.00185=0.007626. So, all the conditions ofTheorem 3.3 are satisfied. So, the BVP(34) has at least one solution and the solution lies in


Taking the following system of BVPs


It is simple to check thatH1,0=H2,0=H1,=H2,=.Also, for anytpqI×I×I, we see that


Thus, all the assumptions of Theorem 3.5 are satisfied with takingα=1, so the coupled system(35) has two solutions satisfying0<pqE×E<1<pqE×E.

Consider the following coupled systems of boundary value problems:


Here,a1t=Γ52t16,b1t=Γ52t232,a2t=Γ529t216π,b2t=Γ529t32π. Moreover


Here,ρM=4.61×102<1.Therefore, matrixMconverges to zero, and hence the solutions of(36) are Hyers-Ulam stable by usingTheorem 4.4.


6. Conclusion

We have developed a comprehensive theory on existence of solutions and its Hyers-Ulam stability for system of multipoint BVP of FDEs. The concerned theory has been enriched by providing suitable examples.



We are very thankful to the reviewers for his/her careful reading and suggestion which improved this chapter very well.


Conflict of interest

We declare the there is no conflict of interest regarding this chapter.


Research funder

This work has been supported by the National Natural Science Foundation of China (11571378).

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Kamal Shah and Yongjin Li (May 23rd 2018). Existence Theory of Differential Equations of Arbitrary Order, Differential Equations - Theory and Current Research, Terry E. Moschandreou, IntechOpen, DOI: 10.5772/intechopen.75523. Available from:

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