# The Periodic Solutions of the Compound Singular Fractional Differential System with Delay

Authors: XuTing Wei; XuanZhu Lu
Journal: International Journal of Differential Equations
 (2010)

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## Abstract

The paper gives sufficient conditions on the existence of periodic solution for a class of compound singular fractional differential systems with delay, involving Nishimoto fractional derivative. Furthermore, for the particular functions, the necessary conditions on the existence of periodic solution are also derived. Especially, for two-dimensional compound singular fractional differential equation with delay, the criteria of existence of periodic solution are obtained. Finally, two examples are presented to verify the validity of criteria.


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## Body

## 1. Introduction

In real life, there are many phenomena with time delay. The mathematical model derived from engineering, physics, mechanics, control theory, chemical reactions, biology, and medicine was made with a significant amount of delay, such as the limited signal transmission speed human reaction time to the outside world. Therefore, the delay is widespread in nature and society, in the introduction of time-delay differential equations there can be a more accurate description and explanation of various phenomena and processes.Fractional calculus is the promotion of classical calculus. The study found that fractional calculus was very suitable to describe long memory and hereditary properties of various materials and processes [1, 2]. In the recent years, fractional calculus becomes a research hotspot, its field of concern has become wide, such as the numerical method of the equation in [3], the existence and uniqueness of equations in [4], fractional Brownian motion, fractional reaction-diffusion equation and random walk [5, 6], fractional wavelet transform [7], and fractional control [8].Most of the above-mentioned studies, utilize the Riemann-liouville fractional derivative definition, which due to its nature of its definition is simple and relatively good. But Nishimoto definition of fractional calculus [9, 10], has not received a lot of attention, this may be part of the naturalization due to the complexity of its definition, but compared to Riemann-liouville fractional calculus, it has a better nature, relevant results more concise useful.The existence of periodic solutions of differential equations is one of the important research directions of biomathematics [11–15], which has a wide range of applications, such as the existence of periodic orbits of celestial movement and its stability.In [12], the author discussed the following system:(1.1)Eẋ(t)=Ax(t)+Bx(t-τ1)+Cx(t-τ2),
and obtained sufficient and necessary conditions for the existence of periodic solutions for the system. Taking into account the periodic solutions of the fractional time-delay system will be a very important practical significance; we are tried to generalize the corresponding results to the case of fractional order.For the above reasons we consider the following compound singular fractional differential system with delay:(1.2)HDαx(t)=Ax(t)+Bx(t-τ1)+Cx(t+τ2),
where Dα denotes Nishimoto fractional derivative of order α,  α>0. H,A,B, and C are constant system matrices of appropriate dimensions, and τ1 and τ2 are constants with τ1>0,τ2>0,|H|=0.


## 2. Definitions and Notations

In this section we introduce the definitions of fractional derivative/integral and related basic properties used in the paper; more information can be obtained from [9, 10].Definition 2.1 (see [9]).
If the functionf(z) is analytic (regular) inside and on C, here C:={C-,C+},  C- is a contour along the cut joining the points z and -∞+iℑ(z), which starts from the point at -∞, encircles the point z once counter-clockwise, and returns to the point at -∞, C+ is a contour along the cut joining the points z and +∞+iℑ(z), which starts from the point at +∞, encircles the point z once counter-clockwise, and returns to the point at +∞,
(2.1)fυ(z)=(f(z))υ:=Γ(υ+1)2πi∫cf(ς)(ς-z)υ+1dς(υ∈RZ-;Z-:={-1,-2,-3,…}),f-n(z):=limυ→-n{fυ(z)}(n∈N:={1,2,3,…}),
where ς,  -π≤arg(ς-z)≤π, for C-, and 0≤arg(ς-z)≤2π, for C+.
Thenfυ(z)(υ>0) is said to be the fractional derivative of f(z) of order υ and fυ(z)(υ<0) is said to be the fractional integral of f(z) of order -υ, provided that |fυ(z)|<∞(υ∈R).
Let us recall the following useful properties associated with the definition introduced above [9].Property 1.
For a constantλ,
(2.2)(eλz)υ=λυeλz(λ≠0;υ∈R;z∈C).Property 2.
For a constantλ,
(2.3)(e-λz)υ=e-iπυλυe-λz(λ≠0;υ∈R;z∈C).Property 3.
If the functionf(z) is singlevalued and analytic in some domain Ω⊆C, then
(2.4)(fμ(z))υ=fμ+υ(z)=(fυ(z))μ.Property 4.
For a constantλ,
(2.5)(zλ)υ=e-iπυΓ(υ-λ)Γ(-λ)zλ-υ(υ∈R;z∈C;|Γ(υ-λ)Γ(-λ)|<∞).
In the following section of this paper, we let Dα denote the α order Nishimoto derivative.


## 3. The Main Results

In this section, we discuss some problems to the system of the system (1.2).Theorem 3.1.
The sufficient condition for the existence of the nonconstant periodic solutions of system (1.2) is that the following equation exists pure imaginary roots
(3.1)det(λαH-A-Be-τ1λ-Ceτ2λ)=0.Proof.
Assume thatηi is pure imaginary root of (3.1), let x(t)=Keηit(K∈Rn), substituting x(t) in (1.2), then
(3.2)((ηi)αH-A-Be-τ1(ηi)-Ceτ2(ηi))K=0.
As ηi is pure imaginary roots of (3.1), note that
(3.3)det((ηi)αH-A-Be-τ1(ηi)-Ceτ2(ηi))=0.
So (3.1) exists nonzero solution K, then, x(t)=Keηit is the nonconstant periodic solution of (1.2).
If the system have nonconstant periodic solutions, then we may wonder whether the solution satisfy (3.1), in fact, as you will see it holds when the function satisfy some conditions. We know that if the function f(t) is a continuous smooth periodic function with period 2l,then it can be expressed as its fourier series form
(3.4)f(t)=∑k=-∞+∞Ckeikπt/l,
where Ck=(1/2l)∫-llf(ξ)e-ikπt/ldξ.
And as we also know that its fourier series expansion has the following properties:(3.5)f′(t)=∑k=-∞+∞Ckikπleikπt/l,
we can even get the following relation if f(t) satisfy some more strictly condition:
(3.6)f(k)(t)=∑k=-∞+∞Ck(ikπl)keikπt/l.
To obtain the similar property of our fractional derivative, what conditions the function should satisfy? We give the following function space.Definition 3.2.
If the periodic functionf(t) is continuous and smooth on R, its α(α>1) order Nishimoto derivative exists, then we let Ω(t) denote the corresponding function space whose elements have the following property:
(3.7)Dαf(t)=Dα(∑k=-∞+∞Ckeikπt/l)=∑k=-∞+∞CkDα(eikπt/l)=∑k=-∞+∞Ck(ikπtl)αeikπt/l,
it is easy to know from the definition that eiλt∈Ω(t)(λ∈R), and so Ω(t) is nonempty.Theorem 3.3.
Ifx(t) is the non-constant periodic solution of (1.2), and further x(t)∈Ω(t), one can obtain the necessity of Theorem 3.1.Proof.
Suppose the period ofx(t) is 2l, x(t) is continuous and differentiable because of α>1, then we can denote it in the form of its fourier series:
(3.8)x(t)=∑k=-∞+∞Ckeikπt/l.
Since x(t)∈Ω(t), we have
(3.9)Dαx(t)=∑k=-∞+∞Ck(ikπtl)αeikπt/l,
where Ck=(1/2l)∫-llx(ξ)e-ikπt/ldξ.
We put (3.8) and (3.9) into (1.2), and obtain
(3.10)∑k=-∞∞[(ikπl)αH-A-Be-ikπτ1/l-Ceikπτ2/l]Ckeikπt/l=0,
then multiply (1/2l)e-imπt/l(m=0,±1,±2,…) on both sides of (3.10) and integrate it from -l to l, hence
(3.11)∑k=-∞∞[(ikπl)αH-A-Be-ikπτ1/l-Ceikπτ2/l]Ck12l∫-llei(k-m)πt/ldt=0.
It is easy to deduce that
(3.12)12l∫-llei(k-m)πt/ldt={1,k=m,0,k≠m,
recalling (3.10), it reduces to
(3.13)[(ikπl)αH-A-Be-ikπτ1/l-Ceikπτ2/l]Cm=0(m=0,±1,±2,…).
Thus, if there are no pure imaginary roots in (3.1), then for every k we have Ck=0, according to (3.8), we conclude that x(t)=constant vector which conflicts the suppose that x(t) is the non-constant periodic solution of (1.2).


## 4. Two-Dimensional Case

For the case of the two-dimensional compound singular fractional differential system with delay, there is(4.1)HDαx(t)=Ax(t)+Bx(t-τ1)+Cx(t+τ2),
where α>0,H=(1000),A=(a11a12a21a22),B=(b11b12b21b22),C=(c11c12c21c22),x(t)=(x1(t)x2(t)), andx1(t),x2(t) is scalar function.Using Theorem3.1, we obtained the following theorem.Theorem 4.1.
If one of the following equations exists non-zero real root, then system(4.1) exists non-constant periodic solution, further more, if x(t)∈Ω(t), then the conclusion is sufficient and necessary
(4.2)|A|+|B|cos(2τ1y)+|C|cos(2τ2y)+Ecos(τ1y)+Fcos(τ2y)+Gcos((τ2-τ1)y)-b22|y|αcos(-τ1y+π2α)-c22|y|αcos(τ2y+π2α)-a22|y|αcos(π2α)=0-|B|sin(2τ1y)+|C|sin(2τ2y)-Esin(τ1y)+Fsin(τ2y)+Gsin((τ2-τ1)y)-b22|y|αsin(-τ1y+π2α)-c22|y|αsin(τ2y+π2α)-a22|y|αsin(π2α)=0,
or
(4.3)|A|+|B|cos(2τ1y)+|C|cos(2τ2y)+Ecos(τ1y)+Fcos(τ2y)+Gcos((τ2-τ1)y)-b22|y|αcos(-τ1y-π2α)-c22|y|αcos(τ2y-π2α)-a22|y|αcos(-π2α)=0-|B|sin(2τ1y)+|C|sin(2τ2y)-Esin(τ1y)+Fsin(τ2y)+Gsin((τ2-τ1)y)-b22|y|αsin(-τ1y-π2α)-c22|y|αsin(τ2y-π2α)-a22|y|αsin(-π2α)=0,
where E=|a11a12b21b22|+|b11b12a21a22|,  F=|a11a12c21c22|+|c11c12a21a22|,  G=|b11b12c21c22|+|c11c12b21b22|.Proof.
First of all we know that(4.4)(yi)α={yαeπαi/2,y≥0,|y|αe-παi/2,y<0,
according to Theorem 3.1, we have
(4.5)h(yi)=|yiI-A-Be-τ1yi-Ceτ2yi|=|(yi)α-a11-b11e-τ1yi-c11eτ2yi-a12-b12e-τ1yi-c12eτ2yi-a21-b21e-τ1yi-c21eτ2yi-a22-b22e-τ1yi-c22eτ2yi|=|A|+|B|e-τ1yi+|C|e2τ2yi+|E|e-τ1yi+Feτ2yi+Ge(τ2-τ1)yi-b22(yi)αe-τ1yi-c22(yi)αeτ2yi-a22(yi)α.
then
(4.6)ℜ[h(yi)]=|A|+|B|cos(2τ1y)+|C|cos(2τ2y)+Ecos(τ1y)+Fcos(τ2y)+Gcos((τ2-τ1)y)-b22|y|αcos(-τ1y±π2α)-c22|y|αcos(τ2y±π2α)-a22|y|αcos(±π2α)=0,ℑ[h(yi)]=-|B|sin(2τ1y)+|C|sin(2τ2y)-Esin(τ1y)+Fsin(τ2y)+Gsin((τ2-τ1)y)-b22|y|αsin(-τ1y±π2α)-c22|y|αsin(τ2y±π2α)-a22|y|αsin(±π2α)=0,
where ℜ[z] and ℑ[z] denote the real and imaginary parts of z, respectively. Then using Theorem 3.1, if there exists y∈R,y≠0,h(yi)=0, we obtain that
(4.7)ℜ[h(yi)]=0,ℑ[h(yi)]=0,
hence system (4.1) exists non-constant periodic solution. This proved the theorem.


## 5. Examples

In this section we give some concrete examples to illustrate our conclusions.Example 5.1.
We consider the following two-dimensional compound singular fractional differential system with delay:(5.1)Dαx1(t)=-x2(t)+x1(t-1)+x2(t-1)+x2(t+1),0=x1(t)+x1(t-1)+x1(t+1),
where H=(1000),A=(0-110),B=(1110),C=(0110),τ1=1,τ2=1,α>1. so we have |A|=1,|B|=-1,|C|=-1,E=0,F=0,G=-2.
Using the discriminant of Theorem4.1, we have
(5.2)2cos(2y)=-1,0=0,
the solution isy=kπ+π/3(k=0,±1,±2,…).
According to Theorem4.1 , system (5.1) has non-constant periodic solution. We suppose thaty=π/3, as
(5.3)((yi)αI-A-Be-yi-Ceyi)K=0
exists non-zero solution, it means that
(5.4)([(π3)αcos(πα2)-32]+[(π3)αsin(πα2)+32]i000)(k1k2)=(00).
So we have k1=0,  k2 for any real number. Supposed that k2=1, then we obtained a non-constant periodic solution of system (5.1):
(5.5)x(t)=e(π/3)it(01).
We can verify thatx(t) is a non-constant periodic solution of system (5.1).Example 5.2.
Consider the following two-dimension compounded with singular fractional differential equation delay system:(5.6)D1/2x1(t)=-x2(t)+x1(t-1)+x2(t+1),0=x1(t)+x2(t)+x1(t-1)+x2(t-1)+x1(t+1)+x2(t+1).
where H=(1000),A=(0-111),B=(0111),C=(0111),τ1=1,τ2=1,α=1/2.
So we have|A|=1,|B|=-1,|C|=-1,E=0,F=0,G=-2.
Using the discriminant of Theorem4.1, we obtained
(5.7)(2cosy+1)|y|1/2cosπ4+2cos2y+1=0,(2cosy+1)|y|1/2sinπ4=2sin2y.
Through the simplication of this equation, we have
(5.8)2sin(2y+(π/4))=-1/2,
and the solution is y=kπ+(1/2)arcsin(-1/2√2)-π/8,k=(0,±1,±2,…).
According to Theorem4.1, there exists non-constant periodic solution in the system.



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*Source: 509286-2009-12-30.xml*