# Solvability for fractional order boundary value problems at resonance

Zhigang Hu* and Wenbin Liu

Author Affiliations

Department of Mathematics, China University of Mining and Technology, Xuzhou 221008, People's Republic of China

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Boundary Value Problems 2011, 2011:20  doi:10.1186/1687-2770-2011-20

 Received: 10 May 2011 Accepted: 5 September 2011 Published: 5 September 2011

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### Abstract

In this paper, by using the coincidence degree theory, we consider the following boundary value problem for fractional differential equation

where denotes the Caputo fractional differential operator of order α, 2 < α ≤ 3. A new result on the existence of solutions for above fractional boundary value problem is obtained.

Mathematics Subject Classification (2000): 34A08, 34B15.

##### Keywords:
Fractional differential equations; boundary value problems; resonance; coincidence degree theory

### 1 Introduction

Fractional calculus is a generalization of ordinary differentiation and integration on an arbitrary order that can be noninteger. This subject, as old as the problem of ordinary differential calculus, can go back to the times when Leibniz and Newton invented differential calculus. As is known to all, the problem for fractional derivative was originally raised by Leibniz in a letter, dated September 30, 1695.

In recent years, the fractional differential equations have received more and more attention. The fractional derivative has been occurring in many physical applications such as a non-Markovian diffusion process with memory [1], charge transport in amorphous semiconductors [2], propagations of mechanical waves in viscoelastic media [3], etc. Phenomena in electromagnetics, acoustics, viscoelasticity, electrochemistry, and material science are also described by differential equations of fractional order (see [4-9]).

Recently, boundary value problems (BVPs for short) for fractional differential equations at nonresonance have been studied in many papers (see [10-16]). Moreover, Kosmatov studied the BVPs for fractional differential equations at resonance (see [17]). Motivated by the work above, in this paper, we consider the following BVP of fractional equation at resonance

(1.1)

where denotes the Caputo fractional differential operator of order α, 2 < α ≤ 3. f : [0, 1] × ℝ3 → ×ℝ is continuous.

The rest of this paper is organized as follows. Section 2 contains some necessary notations, definitions, and lemmas. In Section 3, we establish a theorem on existence of solutions for BVP (1.1) under nonlinear growth restriction of f, basing on the coincidence degree theory due to Mawhin (see [18]). Finally, in Section 4, an example is given to illustrate the main result.

### 2 Preliminaries

In this section, we will introduce notations, definitions, and preliminary facts that are used throughout this paper.

Let X and Y be real Banach spaces and let L : domL X Y be a Fredholm operator with index zero, and P : X X, Q : Y Y be projectors such that

It follows that

is invertible. We denote the inverse by KP.

If Ω is an open bounded subset of X, and , the map N : X Y will be called L-compact on if is bounded and is compact. Where I is identity operator.

Lemma 2.1. ([18]) If Ω is an open bounded set, let L : domL X Y be a Fredholm operator of index zero and N : X Y L-compact on . Assume that the following conditions are satisfied

(1) Lx λNx for every (x, λ) ∈ [(domL\KerL)] ∩ ∂Ω × (0, 1);

(2) Nx ∉ ImL for every x ∈ KerL ∩ ∂Ω;

(3) deg(QN|KerL, KerL ∩ Ω, 0) ≠ 0, where Q : Y Y is a projection such that ImL = KerQ.

Then the equation Lx = Nx has at least one solution in .

Definition 2.1. The Riemann-Liouville fractional integral operator of order α > 0 of a function x is given by

provided that the right side integral is pointwise defined on (0, +∞).

Definition 2.2. The Caputo fractional derivative of order α > 0 of a continuous function x is given by

where n is the smallest integer greater than or equal to α, provided that the right side integral is pointwise defined on (0, +∞).

Lemma 2.2. ([19]) For α > 0, the general solution of the Caputo fractional differential equation

is given by

where ci ∈ ℝ, i = 0, 1, 2, . . ., n - 1; here, n is the smallest integer greater than or equal to α.

Lemma 2.3. ([19]) Assume that x C(0, 1) ∩ L(0, 1) with a Caputo fractional derivative of order α > 0 that belongs to C(0, 1) ∩ L(0, 1). Then,

where ci ∈ ℝ, i = 0, 1, 2, . . ., n - 1; here, n is the smallest integer greater than or equal to α.

In this paper, we denote X = C2[0, 1] with the norm ||x||X = max{||x||, ||x'||, ||x"||} and Y = C[0, 1] with the norm ||y||Y = ||y||, where ||x||= maxt∈[0, 1] |x(t)|. Obviously, both X and Y are Banach spaces.

Define the operator L : domL X Y by

(2.1)

where

Let N : X Y be the Nemytski operator

Then, BVP (1.1) is equivalent to the operator equation

### 3 Main result

In this section, a theorem on existence of solutions for BVP (1.1) will be given.

Theorem 3.1. Let f : [0, 1] × ℝ3 → ℝ be continuous. Assume that

(H1) there exist nonnegative functions p, q, r, s C[0, 1] with Γ(α - 1) - q1 - r1 - s1 > 0 such that

where p1 = ||p||, q1 = ||q||, r1 = ||r||, s1 = ||s||.

(H2) there exists a constant B > 0 such that for all u ∈ ℝ with |u| > B either

or

Then, BVP (1.1) has at leat one solution in X.

Now, we begin with some lemmas below.

Lemma 3.1. Let L be defined by (2.1), then

(3.1)

(3.2)

Proof. By Lemma 2.2, has solution

Combining with the boundary value condition of BVP (1.1), one has (3.1) hold.

For y ∈ ImL, there exists x ∈ domL such that y = Lx Y. By Lemma 2.3, we have

Then, we have

and

By conditions of BVP (1.1), we can get that y satisfies

Thus, we get (3.2). On the other hand, suppose y Y and satisfies . Let , then x ∈ domL and . So that, y ∈ ImL. The proof is complete.

Lemma 3.2. Let L be defined by (2.1), then L is a Fredholm operator of index zero, and the linear continuous projector operators P : X X and Q : Y Y can be defined as

Furthermore, the operator KP : ImL → domL ∩ KerP can be written by

Proof. Obviously, ImP = KerL and P2x = Px. It follows from x = (x - Px) + Px that X = KerP + KerL. By simple calculation, we can get that KerL ∩ KerP = {0}. Then, we get

For y Y, we have

Let y = (y - Qy) + Qy, where y - Qy ∈ KerQ = ImL, Qy ∈ ImQ. It follows from KerQ = ImL and Q2y = Qy that ImQ ∩ ImL = {0}. Then, we have

Thus,

This means that L is a Fredholm operator of index zero.

From the definitions of P, KP, it is easy to see that the generalized inverse of L is KP. In fact, for y ∈ ImL, we have

(3.3)

Moreover, for x ∈ domL ∩ KerP, we get x(0) = x'(0) = x"(0) = 0. By Lemma 2.3, we obtain that

which together with x(0) = x'(0) = x"(0) = 0 yields that

(3.4)

Combining (3.3) with (3.4), we know that KP = (L|domL∩KerP)-1. The proof is complete.

Lemma 3.3. Assume Ω ⊂ X is an open bounded subset such that , then N is L-compact on .

Proof. By the continuity of f, we can get that and are bounded. So, in view of the Arzelà -Ascoli theorem, we need only prove that is equicontinuous.

From the continuity of f, there exists constant A > 0 such that |(I - Q)Nx| ≤ A, , t ∈ [0, 1]. Furthermore, denote KP,Q = KP(I - Q)N and for 0 ≤ t1 < t2 ≤ 1, , we have

and

Since tα, tα-1 and tα-2 are uniformly continuous on [0, 1], we can get that , and are equicontinuous. Thus, we get that is compact. The proof is completed.

Lemma 3.4. Suppose (H1), (H2) hold, then the set

is bounded.

Proof. Take x ∈ Ω1, then Nx ∈ ImL. By (3.2), we have

Then, by the integral mean value theorem, there exists a constant ξ ∈ (0, 1) such that f(ξ, x(ξ), x'(ξ), x"(ξ)) = 0. Then from (H2), we have |x(ξ)| ≤ B.

Then, we have

That is

(3.5)

From x ∈ domL, we get x'(0) = 0. Therefore,

That is

(3.6)

By Lx = λNx and x ∈ domL, we have

Then we get

and

From (3.5),(3.6), and (H1), we have

Thus, from Γ(α - 1) - q1 - r1 - s1 > 0, we obtain that

Thus, we get

and

Therefore,

So Ω1 is bounded. The proof is complete.

Lemma 3.5. Suppose (H2) holds, then the set

is bounded.

Proof. For x ∈ Ω2, we have x(t) = c, c ∈ ℝ, and Nx ∈ ImL. Then, we get

which together with (H2) implies |c| ≤ B. Thus, we have

Hence, Ω2 is bounded. The proof is complete.

Lemma 3.6. Suppose the first part of (H2) holds, then the set

is bounded.

Proof. For x ∈ Ω3, we have x(t) = c, c ∈ ℝ, and

(3.7)

If λ = 0, then |c| ≤ B because of the first part of (H2). If λ ∈ (0, 1], we can also obtain |c| ≤ B. Otherwise, if |c| > B, in view of the first part of (H2), one has

Therefore, Ω3 is bounded. The proof is complete.

Remark 3.1. Suppose the second part of (H2) hold, then the set

is bounded.

The proof of Theorem 3.1. Set Ω = {x X | ||x||X < max{M1, B, B + M1} + 1}. It follows from Lemma 3.2 and 3.3 that L is a Fredholm operator of index zero and N is L-compact on . By Lemma 3.4 and 3.5, we get that the following two conditions are satisfied

(1) Lx λNx for every (x, λ) ∈ [(domL\KerL) ∩ ∂Ω] × (0, 1);

(2) Nx ∉ ImL for every x ∈ KerL ∩ ∂Ω.

Take

According to Lemma 3.6 (or Remark 3.1), we know that H(x, λ) ≠ 0 for x ∈ KerL ∩ ∂Ω. Therefore,

So that, the condition (3) of Lemma 2.1 is satisfied. By Lemma 2.1, we can get that Lx = Nx has at least one solution in . Therefore, BVP (1.1) has at least one solution. The proof is complete.

### 4 An example

Example 4.1. Consider the following BVP

(4.1)

where

Choose , , r(t) = 0, s(t) = 0, B = 10. We can get that , r1 = 0, s1 = 0 and

Then, all conditions of Theorem 3.1 hold, so BVP (4.1) has at least one solution.

### Competing interests

The authors declare that they have no competing interests.

### Authors' contributions

All authors typed, read and approved the final manuscript.

### Acknowledgements

The authors would like to thank the referees very much for their helpful comments and suggestions. This research was supported by the Fundamental Research Funds for the Central Universities (2010LKSX09) and the Science Foundation of China University of Mining and Technology (2008A037).

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