Positive Solution of a Nonlinear Fractional Differential Equation Involving Caputo Derivative
Abstract
This paper is concerned with a nonlinear fractional differential equation involving Caputo derivative. By constructing the upper and lower control functions of the nonlinear term without any monotone requirement and applying the method of upper and lower solutions and the Schauder fixed point theorem, the existence and uniqueness of positive solution for the initial value problem are investigated. Moreover, the existence of maximal and minimal solutions is also obtained.
1. Introduction
However, in the previous works, the nonlinear term has to satisfy the monotone or others control conditions. In fact, the fractional differential equations with nonmonotone function can respond better to impersonal law, so it is very important to weaken monotone condition. Considering this, in this paper, we mainly investigate the fractional differential Equation (1.2) without any monotone requirement on nonlinear term by constructing upper and lower control function and exploiting the method of upper and lower solutions and the Schauder fixed point theorem. The existence and uniqueness of positive solution for (1.2) are obtained. Some properties concerning the maximal and minimal solutions are also given. This work is motivated by the above references and my previous work [16, 17]. Other related results on the fractional differential equations can be found in [18–24].
This paper is organized as follow. In Section 2, we recall briefly some notions of the fractional calculus and the theory of the operators for integration and differentiation of fractional order. Section 3 is devoted to the study of the existence and uniqueness of positive solution for (1.2) utilizing the upper and lower solution method and the Schauder fixed point theorem. The existence of maximal and minimal solutions for (1.2) is given in Section 4.
2. Preliminaries and Notations
First, we give some basic definitions and theorems which are basically used throughout this paper. C[0,1] denotes the space of continuous functions defined on [0,1] and Cn[0,1] denotes the class of all real valued functions defined on [0,1] which have continuous nth order derivative.
Definition 2.1. Let f(x) ∈ C[0,1] and α > 0, then the expression
Definition 2.2. Let n − 1 < α ≤ n, n ∈ N, then the expression
Definition 2.3. Let f(x) ∈ Cn[0,1] and n − 1 < α ≤ n, n ∈ N, then the expression
In further discussion we will denote , , and as , , and , respectively.
Lemma 2.4 (see [25], [26].)Let f(x) ∈ Cn[0,1] and n − 1 < α ≤ n, n ∈ N, then we one has
Lemma 2.5 (see, [10]). If the function f(t, y(t)) is C1[0,1], then the initial value problem (1.2) is equivalent to the Volterra integral equations
Proof. Suppose y(t) satisfies the initial value problem (1.2), then applying Iα to both sides of (1.2) and using Lemma 2.4 (2.7) follows. Conversely, suppose y(t) satisfies (2.7). Then observe that D(m+1)y(t) exists and is integrable, because
Applying on both sides of (2.7), one has
Let X = C[0,1] be the Banach space endowed with the infinity norm and K a nonempty closed subset of X defined as K = {y(t) ∈ X ∣0 < y(t) ≤ l, 0 < t ≤ 1, y(k)(0) = ck, 1 ≤ k ≤ m, 0 ≤ ck}. The positive solution which we consider in this paper is a function such that y(t) ∈ K.
Lemma 2.6. Let f : [0,1]×[0, l] → R+ a given continuous function. Then the operator T : K → K is completely continuous.
Proof. Let M ⊂ K be bounded, that is, there exists a positive constant l* such that ∥y∥∞ ≤ l* for any y(t) ∈ M. Since f(t, y(t)) is a given continuous function, we have
Let L = max(t,y)∈Df(t, y), then for any y(t) ∈ M, we have
Now, we prove that T : K → K is continuous. Since f(t, y(t)) is continuous function in a compact set [0,1]×[0, l], then it is uniformly continuous there. Thus given ε > 0, we can find μ > 0 such that ∥f(t, y) − f(t, z)∥ < ε* whenever ∥y − z∥ < μ, where ε* = εΓ(α + 1). Then
Now, we will prove that the operator T : K → K is equicontinuous. For each y(t) ∈ M, any ε > 0, t1, t2 ∈ [0,1] and t1 < t2. Let , then when |t2 − t1| < δ, we have
Lemma 2.7. If the operator A : X → X is the contraction mapping, where X is the Banach space, then A has a unique fixed point in X.
Let f : [0,1]×[0, l) → R+ be a given function. Take a, b ∈ R+, and a < b < l. For any y ∈ [a, b] one defines the upper-control function H(t, y) = sup a≤η≤yf(t, η), and lower-control function h(t, y) = inf y≤η≤bf(t, η), obviously H(t, y), h(t, y) is monotonous nondecreasing on y and h(t, y) ≤ f(t, y) ≤ H(t, y).
Definition 2.8. Let , and satisfy
3. Existence and Uniqueness of Positive Solution
Now, we give and prove the main results of this paper.
Theorem 3.1. Assume f : [0,1]×[0, l)→[0, +∞) is continuous, and are a pair of order upper and lower solutions of (1.2), then the boundary value problem (1.2) exists one solution y(t) ∈ C[0,1]; moreover,
Proof. Let
For any z(t) ∈ S, we have , then
Corollary 3.2. Assume f : [0,1]×[0, l)→[0, +∞) is continuous, and there exist p2 > p1 ≥ 0, such that
Proof. By assumption (3.4) and the definition of control function, we have
Corollary 3.3. Assume f : [0,1]×[0, +∞)→[c, +∞) is continuous, where c > 0, moreover
Proof. By assumption (3.9), there are positive constants N, R, such that f(t, y) ≤ N whenever u > R. Let M = max 0≤t≤1,0≤y≤R f(t, y), then f(t, y) ≤ N + M, 0 ≤ y < +∞. By the definition of control function, one has H(t, y) ≤ N + M, 0 ≤ t ≤ 1, 0 ≤ y < +∞.
Now, we consider the equation
Corollary 3.4. Assume f : [0,1]×[0, +∞)→[c, +∞) is continuous, where c > 0, moreover
Proof. According to c < lim y→+∞ max 0≤t≤1 (f(t, y)/y) = M < +∞, there exists D > 0, such that for any y(t) ∈ X, we have
Let
Corollary 3.5. Assume f : [0,1]×[0, +∞)→[c, +∞) is continuous and there exists d > 0, e > 0, such that
Proof. By the definition of control function, we have
Theorem 3.6. Let the conditions in Theorem 3.1 hold. Moreover for any y1(t), y2(t) ∈ X, 0 < t < 1, there exists l > 0, such that
Proof. According to Theorem 3.1, if the conditions in Theorem 3.1 hold, then the boundary value problem (1.2) has at least one positive solution in S. Hence we need only to prove that the operator T defined in (2.10) is the contraction mapping in X. In fact, for any y1(t), y2(t) ∈ X, by assumption (3.30), we have
4. Maximal and Minimal Solutions Theorem
In this section, we consider the existence of maximal and minimal solutions for (1.2).
Definition 4.1. Let m(t) be a solution of (1.2) in [0,1], then m(t) is said to be a maximal solution of (1.2), if for every solution y(t) of (1.2) existing on [0,1] the inequality y(t) ≤ m(t), t ∈ [0,1] holds. A minimal solution may be defined similarly by reversing the last inequality.
Theorem 4.2. Let f : [0,1]×[0, +∞)→[0, +∞) be a given continuous and monotone nondecreasing with respect to the second variable. Assume that there exist two positive constants λ, μ (μ > λ) such that
Proof. It is easy to know that and are the upper and lower solutions of (1.2), respectively. Then by using as a pair of coupled initial iterations we construct two sequences from the following linear iteration process:
Acknowledgments
The authors are grateful to the referee for the comments. This work is supported by the Science and Technology Project of Chongqing Municipal Education Committee (Grants nos. KJ110501 and KJ120520) of China, Natural Science Foundation Project of CQ CSTC (Grant no. cstc2012jjA20016) of China, and the NSFC (Grant no. 11101298) of China.