Monotone Iterative Technique for Fractional Evolution Equations in Banach Spaces
Abstract
We investigate the initial value problem for a class of fractional evolution equations in a Banach space. Under some monotone conditions and noncompactness measure conditions of the nonlinearity, the well-known monotone iterative technique is then extended for fractional evolution equations which provides computable monotone sequences that converge to the extremal solutions in a sector generated by upper and lower solutions. An example to illustrate the applications of the main results is given.
1. Introduction
The origin of fractional calculus (i.e., calculus of integrals and derivatives of any arbitrary real or complex order) goes back to Newton and Leibnitz in the seventieth century. We observe that the fractional order can be complex in viewpoint of pure mathematics, and there is much interest in developing the theoretical analysis and numerical methods to fractional equations, because they have recently proved to be valuable in various fields such as physics, chemistry, aerodynamics, viscoelasticity, porous media, electrodynamics of complex medium, electrochemistry, control, and electromagnetic. For instance, fractional calculus concepts have been used in the modeling of transmission lines [1], neurons [2], viscoelastic materials [3], and electrical capacitors [4–6]. References [5, 6] used modified Riemann-Liouville fractional derivatives (Jumarie′s fractional derivatives) and proposed the method of fractional complex transform to find exact solutions which are much needed in engineering applications. Other examples from fractional-order dynamics can be found in [7, 8] and the references therein.
It is well known that the method of monotone iterative technique has been proved to be an effective and a flexible mechanism. It yields monotone sequences of lower and upper approximate solutions that converge to the minimal and maximal solutions between the lower and upper solutions. Early on, Du and Lakshmikantham [27] established a monotone iterative method for an initial value problem for ordinary differential equation. Later on, many papers used the monotone iterative technique to establish existence and comparison results for nonlinear problems. For evolution equations of integer order (α = 1), Li [28–32] and Yang [33] used this method, in which positive C0-semigroup plays an important role. Recently, there have been some papers which deal with the existence of the solutions of initial value problems or boundary value problems for fractional ordinary differential equations by using this method, see [34–43].
However, when many partial differential equations involving time-variable t turn to evolution equations in Banach spaces, they always generate an unbounded closed operator term A, such as (1.2). A is corresponding to linear partial differential operator with certain boundary conditions. In this case, the results in [34–43] are not suitable. To the best of the authors′ knowledge, no results yet exist for the fractional evolution equations involving a closed operator term by using the monotone iterative technique. The approach via fractional differential inequalities is clearly better suited as in the case of classical results of differential equations, and therefore this paper choose to proceed in that setup.
If −A is the infinitesimal generator of an analytic semigroup in a Banach space, then −(A + qI) generates a uniformly bounded analytic semigroup for q > 0 large enough. This allows us to reduce the general case in which −A is the infinitesimal generator of an analytic semigroup to the case in which the semigroup is uniformly bounded. Hence, for convenience, throughout this paper, we suppose that −A is the infinitesimal generator of an analytic semigroup of uniformly bounded linear operators T(t) (t ≥ 0).
Our contribution in this work is to establish the monotone iterative technique for the fractional evolution (1.1). Under some monotone conditions and noncompactness measure conditions of nonlinearity f, which are analogous to those in Li and liu [44], Li [28–32], Chen and li [45], Chen [46], and Yang [33, 47], we obtain results on the existence and uniqueness of mild solutions of the problem (1.1). In this paper, positive semigroup also plays an important role. At last, to illustrate our main results, we examine sufficient conditions for the main results to a fractional partial differential diffusion equation.
2. Preliminaries
In this section, we introduce notations, definitions, and preliminary facts which are used throughout this paper.
Definition 2.1 (see [7].)The Riemann-Liouville fractional integral of order α > 0 with the lower limit zero, of function f ∈ L1(ℝ+), is defined as
Definition 2.2 (see [7].)The Caputo fractional derivative of order α > 0 with the lower limit zero, n − 1 < α < n, is defined as
Proposition 2.3. For α, β > 0 and f as a suitable function (for instance, [7]), one has
- (i)
IαIβf(t) = Iα+βf(t)
- (ii)
IαIβf(t) = IβIαf(t)
- (iii)
Iα(f(t) + g(t)) = Iαf(t) + Iαg(t)
- (iv)
IαDαf(t) = f(t) − f(0), 0 < α < 1
- (v)
DαIαf(t) = f(t)
- (vi)
DαDβf(t) ≠ Dα+βf(t)
- (vii)
DαDβf(t) ≠ DβDαf(t),
- (viii)
DαC = 0, C is a constant.
We observe from the above that the Caputo fractional differential operators possess neither semigroup nor commutative properties, which are inherent to the derivatives on integer order. For basic facts about fractional integrals and fractional derivatives, one can refer to the books [7, 48–50].
Definition 2.4. If v0 ∈ Cα,0(I, X)∩C(I, X1) and satisfies
Lemma 2.5 (see [12], [19], [20].)If h satisfies a uniform Hlder condition, with exponent β ∈ (0,1], then the unique solution of the Cauchy problem
Definition 2.7. By the mild solution of the Cauchy problem (2.6), we mean the function u ∈ C(I, X) satisfying the integral equation
Definition 2.8. An operator family S(t) : X → X (t ≥ 0) in X is called to be positive if for any u ∈ P and t ≥ 0 such that S(t)u ≥ θ.
From Definition 2.8, if T(t) (t ≥ 0) is a positive semigroup generated by −A, h ≥ θ, x0 ≥ θ, then the mild solution u ∈ C(I, X) of (2.6) satisfies u ≥ θ. For positive semigroups, one can refer to [28–32].
Now, we recall some properties of the measure of noncompactness will be used later. Let μ(·) denote the Kuratowski measure of noncompactness of the bounded set. For the details of the definition and properties of the measure of noncompactness, see [51]. For any B ⊂ C(I, X) and t ∈ I, set B(t) = {u(t)∣u ∈ B}. If B is bounded in C(I, X), then B(t) is bounded in X, and μ(B(t)) ≤ μ(B).
Lemma 2.9 (see [52].)Let B = {un} ⊂ C(I, X) (n = 1,2, …) be a bounded and countable set, then μ(B(t)) is Lebesgue integral on I,
In order to prove our results, one also needs a generalized Gronwall inequality for fractional differential equation.
Lemma 2.10 (see [53].)Suppose that b ≥ 0, β > 0, and a(t) is a nonnegative function locally integrable on 0 ≤ t < T (some T ≤ +∞), and suppose that u(t) is nonnegative and locally integrable on 0 ≤ t < T with
3. Main Results
Theorem 3.1. Let X be an ordered Banach space, whose positive cone P is normal with normal constant N. Assume that T(t) (t ≥ 0) is positive, the Cauchy problem (1.1) has a lower solution v0 ∈ C(I, X) and an upper solution w0 ∈ C(I, X) with v0 ≤ w0, and the following conditions are satisfied.
-
H1 There exists a constant C ≥ 0 such that
()for any t ∈ I, and v0(t) ≤ x1 ≤ x2 ≤ w0(t), that is, f(t, x) + Cx is increasing in x for x ∈ [v0(t), w0(t)]. -
H2 There exists a constant L ≥ 0 such that
()for any t ∈ I, and increasing or decreasing monotonic sequences {xn}⊂[v0(t), w0(t)],
Proof. It is easy to see that −(A + CI) generates an analytic semigroup S(t) = e−CtT(t), and S(t) (t ≥ 0) is positive. Let . By Remark 2.6, Φ(t) (t ≥ 0) and Ψ(t) (t ≥ 0) are positive. By (2.4) and Remark 2.6, we have that
Let D = [v0, w0], we define a mapping Q : D → C(I, X) by
Remark 3.2. Theorem 3.1 extends [37, Theorem 2.1]. Even if A = 0 and X = ℝ, our results are also new.
Corollary 3.3. Let X be an ordered Banach space, whose positive cone P is regular. Assume that T(t) (t ≥ 0) is positive, the Cauchy problem (1.1) has a lower solution v0 ∈ C(I, X) and an upper solution w0 ∈ C(I, X) with v0 ≤ w0, and (H1) holds, then the Cauchy problem (1.1) has the minimal and maximal mild solutions between v0 and w0, which can be obtained by a monotone iterative procedure starting from v0 and w0, respectively.
Proof. Since (H1) is satisfied, then (3.9) holds. In regular positive cone P, any monotonic and ordered-bounded sequence is convergent, then there exist , , and , . Then by the proof of Theorem 3.1, the proof is then complete.
Corollary 3.4. Let X be an ordered and weakly sequentially complete Banach space, whose positive cone P is normal with normal constant N. Assume that T(t) (t ≥ 0) is positive, the Cauchy problem (1.1) has a lower solution v0 ∈ C(I, X) and an upper solution w0 ∈ C(I, X) with v0 ≤ w0, and (H1) holds, then the Cauchy problem (1.1) has the minimal and maximal mild solutions between v0 and w0, which can be obtained by a monotone iterative procedure starting from v0 and w0, respectively.
Proof. Since X is an ordered and weakly sequentially complete Banach space, then the assumption (H2) holds. In fact, by [54, Theorem 2.2], any monotonic and ordered bounded sequence is precompact. Let xn be an increasing or decreasing sequence. By (H1), {f(t, xn) + Cxn} is a monotonic and ordered bounded sequence. Then, by the properties of the measure of noncompactness, we have
Theorem 3.5. Let X be an ordered Banach space, whose positive cone P is normal with normal constant N. Assume that T(t) (t ≥ 0) is positive, the Cauchy problem (1.1) has a lower solution v0 ∈ C(I, X) and an upper solution w0 ∈ C(I, X) with v0 ≤ w0, (H1) holds, and the following condition is satisfied:
-
H 3 there is constant S ≥ 0 such that
()for any t ∈ I, v0(t) ≤ x1 ≤ x2 ≤ w0(t).
Proof. We can find that (H1) and (H3) imply (H2). In fact, for t ∈ I, let {xn}⊂[v0(t), w0(t)] be an increasing sequence. For m, n = 1,2, … with m > n, by (H1) and (H3), we have that
Therefore, by Theorem 3.1, the Cauchy problem (1.1) has the minimal mild solution and the maximal mild solution on D = [v0, w0]. In view of the proof of Theorem 3.1, we show that . For t ∈ I, by (3.3), (3.4), (3.5), (H3), and the positivity of operator Ψ(t), we have that
By Corollary 3.3, Corollary 3.4, Theorem 3.5, we have the following results.
Corollary 3.6. Assume that T(t) (t ≥ 0) is positive, the Cauchy problem (1.1) has a lower solution v0 ∈ C(I, X) and an upper solution w0 ∈ C(I, X) with v0 ≤ w0, (H1) and (H3) hold, and one of the following conditions is satisfied:
- (i)
X is an ordered Banach space, whose positive cone P is regular,
- (ii)
X is an ordered and weakly sequentially complete Banach space, whose positive cone P is normal with normal constant N,
4. Examples
Example 4.1. In order to illustrate our main results, we consider the fractional partial differential diffusion equation in X,
Let X = L2(Ω), P = {v∣v ∈ L2(Ω), v(y) ≥ 0 a.e. y ∈ Ω}, then X is a Banach space, and P is a normal cone in X. Define the operator A as follows:
-
O1 , 0 ≤ ψ(y) ≤ ψ1(y), g(y, t, 0) ≥ 0, g(y, t, ψ1(y)) ≤ λ1ψ1(y),
-
O2 the partial derivative g′u(y, t, u) is continuous on any bounded domain.
Theorem 4.2. If (O1) and (O2) are satisfied, then the problem (4.1) has the unique mild solution.
Acknowledgments
This research was supported by NNSFs of China (10871160, 11061031) and project of NWNU-KJCXGC-3-47.