Research Article Analytic Solution of a Class of...

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Hindawi Publishing Corporation Abstract and Applied Analysis Volume 2013, Article ID 109582, 5 pages http://dx.doi.org/10.1155/2013/109582 Research Article Analytic Solution of a Class of Fractional Differential Equations Yue Hu 1 and Zuodong Yang 2 1 School of Mathematics and Information Science, Henan Polytechnic University, Jiaozuo, Henan 454000, China 2 Institute of Mathematics, School of Mathematical Sciences, Nanjing Normal University, Nanjing, Jiangsu 210023, China Correspondence should be addressed to Yue Hu; [email protected] Received 8 July 2013; Revised 23 October 2013; Accepted 13 November 2013 Academic Editor: Shawn X. Wang Copyright © 2013 Y. Hu and Z. Yang. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We consider the analytic solution of a class of fractional differential equations with variable coefficients by operatorial methods. We obtain three theorems which extend the Garra’s results to the general case. 1. Introduction Recently, Garra [1] studied the analytic solution of a class of fractional differential equations with variable coefficients by using operatorial methods. e proof of his main results is strongly based on operatorial properties of Caputo fractional derivative and the following theorem by Miller and Ross [2]. eorem M.-R. (eorem 1.1 in [1]). Let () be any function of the form () or (ln )(), where > −1, and () = =0 (1) has a radius of convergence >0 and 0<<. en () = + () (2) holds for all ∈ (0, ), provided (1) <+1 and are arbitrary, or (2) ≥+1 and are arbitrary and =0 for = 0,1,...,−1, where = ⌈⌉ (ceiling of the number). Garra’s main results are as follows. eorem Garra (eorem 1.2 in [1]). Consider the following boundary value problem (BVP): (, ) = (, ) , (0, ) = () , (3) in the half plane >0, with analytic boundary condition () such that the conditions of eorem 1.1 (eorem M.-R.) are satisfied. e operatorial solution of BVP (3) is given by (, ) = =0 (!) 2 () , (4) where is the Laguerre derivative and denotes Caputo fractional derivative. On the basis of the previous result, Garra proved (Example 1 in [1]) that if () = , N, then the analytic solution of the BVP (3) is given by (, ) = =0 (!) 2 = [/] =0 Γ ( + 1) Γ ( + 1 − ) (!) 2 , (5) where Γ() = ∫ 0 −1 . Motivated by the results, in the present note, we extend Garra’s results to the general case. 2. Preliminaries Definition 1 (see [3]). For every positive integer , the operator := , . . . , (containing +1 ordinary derivatives) is called the -order Laguerre derivatives and the -exponential function is defined by () := =0 (!) +1 . (6)

Transcript of Research Article Analytic Solution of a Class of...

Hindawi Publishing CorporationAbstract and Applied AnalysisVolume 2013, Article ID 109582, 5 pageshttp://dx.doi.org/10.1155/2013/109582

Research ArticleAnalytic Solution of a Class of Fractional Differential Equations

Yue Hu1 and Zuodong Yang2

1 School of Mathematics and Information Science, Henan Polytechnic University, Jiaozuo, Henan 454000, China2 Institute of Mathematics, School of Mathematical Sciences, Nanjing Normal University, Nanjing, Jiangsu 210023, China

Correspondence should be addressed to Yue Hu; [email protected]

Received 8 July 2013; Revised 23 October 2013; Accepted 13 November 2013

Academic Editor: Shawn X. Wang

Copyright © 2013 Y. Hu and Z. Yang. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

We consider the analytic solution of a class of fractional differential equations with variable coefficients by operatorial methods.Weobtain three theorems which extend the Garra’s results to the general case.

1. Introduction

Recently, Garra [1] studied the analytic solution of a class offractional differential equations with variable coefficients byusing operatorial methods.

The proof of his main results is strongly based onoperatorial properties of Caputo fractional derivative and thefollowing theorem by Miller and Ross [2].

TheoremM.-R. (Theorem 1.1 in [1]). Let 𝑓(𝑡) be any functionof the form 𝑡𝜆𝜂(𝑡) or 𝑡𝜆(ln 𝑡)𝜂(𝑡), where 𝜆 > −1, and

𝜂 (𝑡) =∞

∑𝑛=0

𝑎𝑛𝑡𝑛 (1)

has a radius of convergence 𝑅 > 0 and 0 < 𝑋 < 𝑅. Then

𝐷𝛼𝑡𝐷𝛽

𝑡𝑓 (𝑡) = 𝐷

𝛼+𝛽

𝑡𝑓 (𝑡) (2)

holds for all 𝑡 ∈ (0, 𝑋), provided(1) 𝛽 < 𝜆 + 1 and 𝛼 are arbitrary, or(2) 𝛽 ≥ 𝜆 + 1 and 𝛼 are arbitrary and 𝑎𝑘 = 0 for 𝑘 =0, 1, . . . , 𝑚 − 1, where𝑚 = ⌈𝛽⌉ (ceiling of the number).

Garra’s main results are as follows.

Theorem Garra (Theorem 1.2 in [1]). Consider the followingboundary value problem (BVP):

𝐷𝐿𝑥

𝑓 (𝑥, 𝑡) = 𝐷𝛼

𝑡𝑓 (𝑥, 𝑡) ,

𝑓 (0, 𝑡) = 𝑔 (𝑡) ,(3)

in the half plane 𝑡 > 0, with analytic boundary condition 𝑔(𝑡)such that the conditions of Theorem 1.1 (Theorem M.-R.) aresatisfied. The operatorial solution of BVP (3) is given by

𝑓 (𝑥, 𝑡) =∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)2𝑔 (𝑡) , (4)

where 𝐷𝐿𝑥

is the Laguerre derivative and 𝐷𝛼𝑡denotes Caputo

fractional derivative. On the basis of the previous result, Garraproved (Example 1 in [1]) that if 𝑔(𝑡) = 𝑡𝑚, 𝑚 ∈ N, then theanalytic solution of the BVP (3) is given by

𝑓 (𝑥, 𝑡) =∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝑡𝑚

(𝑘!)2=[𝑚/𝛼]

∑𝑟=0

Γ (𝑚 + 1)

Γ (𝑚 + 1 − 𝛼𝑟)⋅𝑥𝑟𝑡𝑚−𝛼𝑟

(𝑟!)2,

(5)

where Γ(𝛼) = ∫∞0𝑒−𝑡𝑡𝛼−1𝑑𝑡.

Motivated by the results, in the present note, we extendGarra’s results to the general case.

2. Preliminaries

Definition 1 (see [3]). For every positive integer 𝑛, theoperator𝐷𝑛𝐿

𝑥

:= 𝐷𝑥, . . . , 𝐷𝑥𝐷𝑥𝐷 (containing 𝑛+1 ordinaryderivatives) is called the 𝑛-order Laguerre derivatives and the𝑛𝐿-exponential function is defined by

𝑒𝑛 (𝑥) :=∞

∑𝑘=0

𝑥𝑘

(𝑘!)𝑛+1. (6)

2 Abstract and Applied Analysis

In [4], the following result is proved.

Lemma 2. Let 𝑎 be an arbitrary real or complex number. Thefunction 𝑒𝑛(𝑎𝑥) is an eigenfunction of the operator 𝐷𝑛𝐿

𝑥

; thatis,

𝐷𝑛𝐿𝑥

𝑒𝑛 = 𝑎𝑒𝑛 (𝑎𝑥) . (7)

For 𝑛 = 0, we have 𝐷0𝐿 := 𝐷. Thus, (7) leads to theclassical property of the exponential function𝐷𝑒𝑎𝑥 = 𝑎𝑒𝑎𝑥.

Similarly, the spectral properties can be obtained [3] byusing the general Laguerre derivatives

𝐷1𝐿 + 𝑚𝐷 = 𝐷 (𝑥𝐷 + 𝑚) (8)

(here 𝑚 is a real or complex constant) or more generally theoperator

𝐷(𝑥𝐷 + 𝑚)𝑛 =𝑛

∑𝑘=0

(𝑛𝑘)𝐷𝑘𝐿𝑚

𝑛−𝑘 (9)

(𝑛 ∈ N) and the corresponding eigenfunctions

∑𝑘=0

𝑥𝑘

𝑘! (𝑘 + 𝑚)!(10)

or∞

∑𝑘=0

𝑥𝑘

𝑘!((𝑘 + 𝑚)!)𝑛. (11)

Throughout this paper, we use the Caputo fractionalderivatives as in [1].

Definition 3 (see [5]). The Caputo derivative of fractionalorder 𝛼 of function 𝑓(𝑡) is defined as

𝐷𝛼𝑓 (𝑡) :=

{{{{{{{{{

1

Γ (𝑚 − 𝛼)∫𝑡

0

𝑓(𝑚) (𝜏)

(𝑡 − 𝜏)𝛼+1−𝑚𝑑𝜏, 𝑚 − 1 < 𝛼 < 𝑚

𝑑𝑚𝑓

𝑑𝑡𝑚, 𝛼 = 𝑚,

(12)

where𝑚 := ⌈𝛼⌉.

Lemma 4 (see [5]). Let 𝑓(𝑡) = 𝑡𝜆 for some 𝜆 ≥ 0. Then

𝐷𝛼𝑓 (𝑡)

=

{{{{{{{

0, 𝑖𝑓 𝜆 ∈ 0, 1, . . . , 𝑚 − 1,Γ (𝜆 + 1)

Γ (𝜆 + 1 − 𝛼)𝑡𝜆−𝛼, 𝑖𝑓 𝜆 ∈ N, 𝜆 ≥ 𝑚 𝑜𝑟

𝜆 ∉ N, 𝜆 > 𝑚 − 1,

(13)

where𝑚 =: ⌈𝛼⌉.

Lemma 5. Let 𝜆 > 0. Let 𝛼 > 0, 𝛽 > 0, and 𝛼 + 𝛽 ≤ 𝜆. Then

𝐷𝛼+𝛽𝑡𝜆 = 𝐷𝛼𝐷𝛽𝑡𝜆 (𝑡 > 0) . (14)

Proof. This follows immediately from Lemma 4.

Remark 6. In general, for 𝛼, 𝛽 > 0, 𝐷𝛼𝐷𝛽 = 𝐷𝛼+𝛽 is not true.For example,

𝐷0.6𝐷0.6𝑡 =Γ (2)

Γ (0.8)𝑡−0.2, (15)

but

𝐷0.6+0.6𝑡 = 𝐷1.2𝑡 = 0. (16)

Lemma 7. Suppose 𝛼 > 0, and let 𝑔(𝑡) = ∑∞𝑘=0𝑎𝑘𝑡𝑘 have a

radius of convergence 𝑅 > 0 and 0 < 𝑋 < 𝑅. If [𝛼𝑘] = ⌈𝛼(𝑘 +1)⌉ − 1 for some 𝑘 ∈ N, then

𝐷(𝑘+1)𝛼𝑔 (𝑡) = 𝐷𝛼𝐷𝑘𝛼𝑔 (𝑡) . (17)

Proof. Let ⌈𝛼(𝑘 + 1)⌉ = 𝑚. Then𝑚 − 1 < 𝛼(𝑘 + 1) ≤ 0.By lemmas 4 and 5, we have

𝐷(𝑘+1)𝛼𝑔 (𝑡) = 𝐷(𝑘+1)𝛼(

∑𝑙=0

𝑎𝑙𝑡𝑙)

= 𝐷(𝑘+1)𝛼(∞

∑𝑙=𝑚

𝑎𝑙𝑡𝑙) .

(18)

Since [𝛼𝑘] = ⌈𝛼(𝑘 + 1)⌉ − 1 = 𝑚 − 1, together withDefinition 3, we have

𝐷𝛼𝐷𝛼𝑘(𝑚−1

∑𝑙=0

𝑎𝑙𝑡𝑙) = 0, (19)

and therefore (18) implies that

𝐷(𝑘+1)𝛼𝑔 (𝑡) = 𝐷𝛼𝐷𝛼𝑘(

∑𝑙=0

𝑎𝑙𝑡𝑙) = 𝐷𝛼𝐷𝛼𝑘𝑔 (𝑡) . (20)

This completes the proof.

3. Main Results

We first study the following BVP in the plane 𝑡 > 0:

𝐷𝑛𝐿𝑥

𝑓 (𝑥, 𝑡) = 𝐷𝛼

𝑡𝑓 (𝑥, 𝑡) ,

𝑓 (0, 𝑡) = 𝑔 (𝑡) .(21)

Theorem 8. Let 𝑔(𝑡) = 𝑡𝜆 with 𝜆 > 0 and assume that 𝜆/𝛼 is apositive integer. Then the operatorial form solution of BVP (21)is

𝑓 (𝑥, 𝑡) =𝜆/𝛼

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝜆

(𝑘!)𝑛+1

=𝜆/𝛼

∑𝑘=0

Γ (𝜆 + 1)

Γ (𝜆 + 1 − 𝛼𝑘)⋅𝑥𝑘

(𝑘!)𝑛+1𝑡𝜆−𝛼𝑘.

(22)

Abstract and Applied Analysis 3

Proof. Let 𝜆/𝛼 = 𝑚. By Lemmas 4 and 5, we have

𝐷𝑛𝐿𝑥

𝑓 (𝑥, 𝑡) =𝑚

∑𝑘=1

𝑘𝑛+1𝐷𝛼𝑘𝑡𝑡𝜆

(𝑘!)𝑛+1=𝑚

∑𝑘=1

𝑥𝑘−1𝐷𝛼𝑘𝑡𝑡𝜆

[(𝑘 − 1)!]𝑛+1

=𝑚−1

∑𝑘=0

𝐷𝛼(𝑘+1)𝑡

𝑡𝜆

(𝑘!)𝑛+1= 𝐷𝛼𝑡(𝑚−1

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝜆

(𝑘!)𝑛+1)

= 𝐷𝛼𝑡(𝑚

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝜆

(𝑘!)𝑛+1) = 𝐷𝛼

𝑡𝑓 (𝑥, 𝑡) .

(23)

In the fifth previous equality, we use the fact that

𝐷𝛼𝑡(𝑥𝑚𝐷𝛼𝑚𝑡𝜆

(𝑘!)𝑛+1) = 𝐷𝛼

𝑡(𝑥𝑚𝐷𝜆𝑡𝜆

(𝑘!)𝑛+1) = 0. (24)

Remark 9. We point out that the result of Example 1 in [1] isincorrect. A counterexample is as follows. Let 𝑔(𝑡) = 𝑡 and𝛼 = 0.6. By Lemma 5, we have

𝐷𝐿𝑥

([1/0.6]

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)2𝑡)

= 𝐷𝑥𝑥𝐷𝑥 (𝑡 +𝑥𝐷0.6𝑡

(1!)2𝑡) = 𝐷0.6

𝑡(𝑡) .

(25)

On the other hand, we have

𝐷0.6𝑡([1/0.6]

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)2𝑡)

= 𝐷0.6𝑡(𝑡 +

𝑥𝐷0.6𝑡

(1!)2𝑡) = 𝐷0.6

𝑡(𝑡) + 𝑥𝐷

0.6

𝑡(𝐷0.6𝑡𝑡) .

(26)

Note that by Remark 6, we have

𝐷0.6+0.6𝑡

𝑡 = 0,

𝐷0.6𝑡𝐷0.6𝑡𝑡 =

1

Γ (0.8)𝑡−0.2 ̸= 0.

(27)

Hence,

𝐷𝐿𝑥

([1/0.6]

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)2𝑡) ̸=𝐷0.6

𝑡([1/0.6]

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)2𝑡) . (28)

Theorem 10. Let 𝑔(𝑡) satisfy the assumptions of Lemma 7. If[𝛼𝑘] = ⌈𝛼(𝑘 + 1)⌉ − 1, (𝑘 = 1, 2, . . .), then the operatorial formsolution of BVP (21) is given by

𝑓 (𝑥, 𝑡) = 𝑒𝑛 (𝑥𝐷𝛼

𝑡) 𝑔 (𝑡) =

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

(𝑘!)𝑛+1𝑔 (𝑡) . (29)

Proof. By Lemmas 2 and 7, we have

𝐷𝑛𝐿𝑥

𝑓 (𝑥, 𝑡) = 𝐷𝑛𝐿𝑥

(𝑒𝑛 (𝑥𝐷𝛼

𝑡) 𝑔 (𝑡))

= 𝐷𝛼𝑡𝑒𝑛 (𝑥𝐷

𝛼

𝑡) 𝑔 (𝑡) = 𝐷

𝛼

𝑡𝑓 (𝑥, 𝑡) .

(30)

This completes the proof.

The following generalization of the Theorem 10 can beproved similarly.

Theorem 11. Let 𝑚 be a real or complex constant and 𝑛 ∈ N.Consider the following BVP:

𝐷𝑥(𝑥𝐷𝑥 + 𝑚)𝑛𝑓 (𝑥, 𝑡) = 𝐷

𝛼

𝑡𝑓 (𝑥, 𝑡) ,

𝑓 (0, 𝑡) = 𝑔 (𝑡) ,(31)

in the half plane 𝑡 > 0, with analytic boundary condition 𝑔(𝑡)such that the conditions of Lemma 7 are satisfied. If [𝛼𝑘] =⌈𝛼(𝑘 + 1)⌉ − 1, (𝑘 = 1, 2, . . .), then the operatorial solution ofBVP (31) is given by

𝑓 (𝑥, 𝑡) =∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

𝑘!((𝑘 + 𝑚)!)𝑛𝑔 (𝑡) . (32)

Proof. Using spectral properties of Laguerre derivative,together with Lemma 7, we have

𝐷𝑥(𝑥𝐷𝑥 + 𝑚)𝑛𝑓 (𝑥, 𝑡) = 𝐷𝑥(𝑥𝐷𝑥 + 𝑚)

𝑛

× (∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡

𝑘!((𝑘 + 𝑚)!)𝑛𝑔 (𝑡))

=∞

∑𝑘=1

𝑘(𝑘 + 𝑚)𝑛𝑥𝑘−1𝐷𝛼𝑘𝑡

𝑘!((𝑘 + 𝑚)!)𝑛𝑔 (𝑡)

=∞

∑𝑘=0

𝑥𝑘𝐷𝛼(𝑘+1)

𝑘!((𝑘 + 𝑚)!)𝑛+1𝑔 (𝑡)

= 𝐷𝛼(∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘

𝑘!((𝑘 + 𝑚)!)𝑛𝑔 (𝑡))

= 𝐷𝛼𝑓 (𝑥, 𝑡) .

(33)

This completes the proof.

Remark 12. Conditions [𝛼𝑘] = ⌈𝛼(𝑘 + 1)⌉ − 1 (𝑘 = 1, 2, . . .)are very harsh. However, if we remove them,Theorems 10 and11 no longer remain valid. It should be noted that the mainresult in [1] (Theorem 1.2 in [1]) is incorrect in general case. Acounterexample is as follows. Let 𝑔(𝑡) = 𝑒𝑡 and 𝛼 = 0.6. Onehas

𝐷𝐿𝑥

(∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝑒𝑡

(𝑘!)2) = 𝐷𝑥𝑥𝐷𝑥 (

𝑥

(1!)2𝐷0.6𝑡𝑒𝑡 +

𝑥2

(2!)2𝐷1.2𝑡𝑒𝑡

+𝑥3

(3!)2𝐷1.8𝑡𝑒𝑡 + ⋅ ⋅ ⋅ )

= 𝐷0.6𝑡𝑒𝑡 +

𝑥

(1!)2𝐷1.2𝑡𝑒𝑡

+𝑥2

(2!)2𝐷1.8𝑡𝑒𝑡 + ⋅ ⋅ ⋅ .

(34)

4 Abstract and Applied Analysis

On the other hand, we have

𝐷𝛼𝑡(∞

∑𝑘=0

𝑥𝑘𝐷𝛼𝑘𝑡𝑒𝑡

(𝑘!)2)

= 𝐷0.6𝑡(𝑒𝑡 +

𝑥

(1!)2𝐷0.6𝑡𝑒𝑡 +

𝑥2

(2!)2𝐷1.2𝑡𝑒𝑡 + ⋅ ⋅ ⋅ ) .

(35)

Set 𝑥 = 1 in (34) and (35). By Definition 3 and Lemma 4,we deduce that all terms in (34) are positive and cannotcontain negative exponent of variable 𝑡. For example,

𝐷0.6𝑡𝑒𝑡 = 𝐷0.6

𝑡(1 + 𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ ) = 𝐷0.6

𝑡(𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ )

=1

Γ (1.4)𝑡0.4 +

1

Γ (2.4)𝑡1.4 + ⋅ ⋅ ⋅ ,

𝐷1.8𝑡𝑒𝑡 = 𝐷1.8

𝑡(1 + 𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ ) = 𝐷1.8

𝑡(𝑡2

2!+𝑡3

3!+ ⋅ ⋅ ⋅ )

=1

Γ (1.2)𝑡0.2 +

1

Γ (2.2)𝑡1.2 + ⋅ ⋅ ⋅ ,

and so forth.(36)

Similarly, all terms in (35) are also positive, except thatsome terms contain negative exponent of variable 𝑡. Forexample,

𝐷0.6𝑡𝐷0.6𝑡𝑒𝑡 = 𝐷0.6

𝑡𝐷0.6𝑡(1 + 𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ )

= 𝐷0.6𝑡𝐷0.6𝑡(𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ )

=1

Γ (0.8)𝑡−0.2 + ⋅ ⋅ ⋅ ,

𝐷0.6𝑡𝐷6.6𝑡𝑒𝑡 = 𝐷0.6

𝑡𝐷6.6𝑡(1 + 𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ )

= 𝐷0.6𝑡𝐷6.6𝑡(𝑡 +

𝑡2

2!+ ⋅ ⋅ ⋅ )

=1

Γ (0.8)𝑡−0.2 + ⋅ ⋅ ⋅ ,

and so forth.

(37)

Thus, we conclude that

𝐷𝐿𝑥

(∞

∑𝑘=0

𝑥𝑘𝐷0.6𝑘𝑡𝑒𝑡

(𝑘!)2) ̸=𝐷0.6

𝑡(∞

∑𝑘=0

𝑥𝑘𝐷0.6𝑘𝑡𝑒𝑡

(𝑘!)2) . (38)

4. Conclusion and Discussion

In this paper, we point out that Garra’s results are incorrectand give some necessary counterexamples. In addition, we

established three theorems (Theorems 8, 10, and 11) whichcorrect and extend the corresponding results of [1].

Different from integer-order derivative, there are manykinds of definitions for fractional derivatives, includingRiemann-Liouville, Caputo, Grunwald-Letnikov, Weyl,Jumarie, Hadamard, Davison and Essex, Riesz, Erdelyi-Kober, and Coimbra (see [1, 6–8]). These definitions aregenerally not equivalent to each other. Every derivative hasits own serviceable range. In other words, all these fractionalderivatives definitions have their own advantages anddisadvantages. For example, the Caputo derivative is veryuseful when dealing with real-world problem, since it allowstraditional initial and boundary conditions to be included inthe formulation of the problem and the Laplace transformof Caputo fractional derivative is a natural generalizationof the corresponding well-known Laplace transform ofinteger-order derivative. So, the Caputo fractional-ordersystem is often used in modelling and analysis. However, thefunctions that are not differentiable do not have fractionalderivative, which reduces the field of application of Caputoderivative (see [1, 8, 9]).

When solving fractional-order systems, the law of expo-nents (semigroup property) is the most important. Unlikeinteger-order derivative, for 𝛼 > 0 and 𝛽 > 0, 𝛼 derivative ofthe𝛽 derivative of a function is, in general, not equal to the𝛼+𝛽 derivative of such function. About the semigroup propertyof the fractional derivatives, under suitable assumptions offractional order, there have existed some studies, but only afew studies provide valuable judgment methods (see [1, 9]).

Fortunately, if we define Ω as the class of all functions𝑓 which are infinitely differentiable everywhere and aresuch that 𝑓 and all its derivatives are of order 𝑡−𝑁 for all𝑁, 𝑁 = 1, 2, . . ., then, for all functions of class Ω, Weylfractional derivatives possess the semigroup property [1].This has brought us great convenience for studying Weylfractional differential equations.Wewill considered this topicin a forthcoming paper.

Acknowledgments

The authors are grateful to the anonymous reviewers forseveral comments and suggestions which contributed tothe improvement of this paper. This work is supportedby the National Natural Science Foundation of China (no.11171092) and the Natural Science Foundation of JiangsuHigher Education Institutions of China (no. 08KJB110005).

References

[1] R. Garra, “Analytic solution of a class of fractional differentialequations with variable coefficients by operatorial methods,”Communications in Nonlinear Science and Numerical Simula-tion, vol. 17, no. 4, pp. 1549–1554, 2012.

[2] K. S. Miller and B. Ross, An Introduction to the FractionalCalculus and Fractional Differential Equations, John Wiley &Sons, New York, NY, USA, 1993.

[3] G. Bretti and P. E. Ricci, “Laguerre-type special functions andpopulation dynamics,” Applied Mathematics and Computation,vol. 187, no. 1, pp. 89–100, 2007.

Abstract and Applied Analysis 5

[4] G. Dattoli and P. E. Ricci, “Laguerre-type exponentials, andthe relevant 𝐿-circular and 𝐿-hyperbolic functions,” GeorgianMathematical Journal, vol. 10, no. 3, pp. 481–494, 2003.

[5] K. Diethelm, The Analysis of Fractional Differential Equations,Springer, Berlin, Germany, 2010.

[6] I. Podlubny, Fractional Differential Equations, vol. 198, Aca-demic Press, New York, NY, USA, 1999.

[7] P. L. Butzer and U. Westphal, An Introduction to FractionalCalculus, World Scientific, Singapore, 2000.

[8] A. Atangana and A. Secer, “A note on fractional order deriva-tives and table of fractional derivatives of some special func-tions,” Abstract and Applied Analysis, vol. 2013, Article ID279681, 8 pages, 2013.

[9] C. Li andW. Deng, “Remarks on fractional derivatives,”AppliedMathematics andComputation, vol. 187, no. 2, pp. 777–784, 2007.

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