Positive Mild Solutions of Periodic Boundary Value Problems for Fractional Evolution Equations
Abstract
The periodic boundary value problem is discussed for a class of fractional evolution equations. The existence and uniqueness results of mild solutions for the associated linear fractional evolution equations are established, and the spectral radius of resolvent operator is accurately estimated. With the aid of the estimation, the existence and uniqueness results of positive mild solutions are obtained by using the monotone iterative technique. As an application that illustrates the abstract results, an example is given.
1. Introduction
The origin of fractional calculus goes back to Newton and Leibnitz in the seventieth century. We observe that 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 of science and engineering. Indeed, we can find numerous applications in viscoelasticity, electrochemistry, electromagnetism, biology, and hydrogeology. For example space-fractional diffusion equations have been used in groundwater hydrology to model the transport of passive tracers carried by fluid flow in a porous medium [1, 2] or to model activator-inhibitor dynamics with anomalous diffusion [3].
In this paper, without the assumptions of lower and upper solutions, by using the monotone iterative technique, we obtain the existence and uniqueness of positive mild solutions for PBVP (1.1). Because in many practical problems such as the reaction diffusion equations, only the positive solution has the significance, we consider the positive mild solutions in this paper. The characteristics of positive operator semigroup play an important role in obtaining the existence of the positive mild solutions. Positive operator semigroup are widely appearing in heat conduction equations, the reaction diffusion equations, and so on (see [19]). It is worth noting that our assumptions are very natural and we have tested them in the practical context. In particular to build intuition and throw some light on the power of our results, we examine sufficient conditions for the existence and uniqueness of positive mild solutions for periodic boundary value problem for fractional parabolic partial differential equations (see Example 4.1).
We now turn to a summary of this work. Section 2 provides the definitions and preliminary results to be used in theorems stated and proved in the paper. In particular to facilitate access to the individual topics, the existence and uniqueness results of mild solutions for the associated linear fractional evolution equations are established and the spectral radius of resolvent operator is accurately estimated. In Section 3, we obtain very general results on the existence and uniqueness of positive mild solutions for PBVP (1.1), when the nonlinear term f satisfies some conditions related to the growth index of the operator semigroup {T(t)} t≥0. The main method is the monotone iterative technique. In Section 4, we give also an example to illustrate the applications of the abstract results.
2. Preliminaries
Let us recall the following known definitions. For more details see [20–23].
Definition 2.1. The fractional integral of order α with the lower limit zero for a function f is defined as:
Definition 2.2. The Riemann-Liouville derivative of order α with the lower limit zero for a function f can be written as:
Definition 2.3. The Caputo fractional derivative of order α for a function f can be written as:
Remark 2.4. (i) If f ∈ Cn[0, ∞), then
(ii) The Caputo derivative of a constant is equal to zero.
(iii) If f is an abstract function with values in X, then the integrals and derivatives which appear in Definitions 2.1–2.3 are taken in Bochner′s sense.
Lemma 2.5 (see [4].)If h satisfies a uniform Hölder condition, with exponent β ∈ (0,1], then the unique solution of the linear initial value problem (LIVP) for the fractional evolution equation,
(ii) see [6, 24], for v ∈ (−1, ∞),
(iii) see [4, 5], the Laplace transform of ζα is given by
(iv) see [24] by (i) and (ii), we can obtain that for p ≥ 0
(v) see [25] for p < 0, 0 < Eα(p) < Eα(0) = 1,
(vi) see [10] if δ > 0 and t > 0, then .
Remark 2.7. See [6, 8], the operators U and V, given by (2.8), have the following properties:
- (i)
For any fixed t ≥ 0, U(t) and V(t) are linear and bounded operators, that is, for any x ∈ X,
(2.12) - (ii)
{U(t)} t≥0 and {V(t)} t≥0 are strongly continuous.
Definition 2.8. If h ∈ C(I, X), by the mild solution of IVP (2.6), we mean that the function u ∈ C(I, X) satisfying the integral (2.7).
Definition 2.9 (see [26].)A C0-semigroup {T(t)} t≥0 is called a compact semigroup if T(t) is compact for t > 0.
Definition 2.10. An analytic semigroup {T(t)} t≥0 is called positive if T(t)x ≥ θ for all x ≥ θ and t ≥ 0.
Definition 2.12. A bounded linear operator K on X is called to be positive if Kx ≥ θ for all x ≥ θ.
Remark 2.13. By Remark 2.6(ii), we obtain that U(t) and V(t) are positive for t ≥ 0 if {T(t)} t≥0 is a positive semigroup.
Lemma 2.14. Let X be an ordered Banach space, whose positive cone P is normal. If {T(t)} t≥0 is an exponentially stable analytic semigroup, that is, ν0 = limsup t→+∞(ln ∥T(t)∥/t) < 0. Then the linear periodic boundary value problem (LPBVP),
Proof. For any ν ∈ (0, |ν0|), by there exists M1 such that
Remark 2.15. For sufficient conditions of exponentially stable operator semigroups, one can see [32].
Remark 2.16. If {T(t)} t≥0 is a positive and exponentially stable analytic semigroup generated by −A, by Remark 2.13, then the resolvent operator Q : C(I, X) → C(I, X) is also a positive bounded linear operator.
Remark 2.17. For the applications of Lemma 2.14, it is important to estimate the growth index of {T(t)} t≥0. If T(t) is continuous in the uniform operator topology for t > 0, it is well known that ν0 can be obtained byσ(A): the spectrum of A (see [33])
Corollary 2.18. Let X be an ordered Banach space, whose positive cone P is a regeneration cone. If {T(t)} t≥0 is a compact and positive analytic semigroup, and its first eigenvalue of A is
Proof. By (2.32), we know that the growth index of {T(t)} t≥0 is ν0 = −λ1 < 0, that is, {T(t)} t≥0 is exponentially stable. By Lemma 2.14, Q : C(I, X) → C(I, X) is a bounded linear operator, and the spectral radius r(Q) ≤ 1/λ1. On the other hand, since λ1 has a positive eigenfunction x1, in LPBVP (3.17) we set h(t) = x1, then x1/λ1 is the corresponding mild solution. By the definition of the operator Q, Q(x1) = x1/λ1, that is, 1/λ1 is an eigenvalue of Q. Then r(Q) ≥ 1/λ1. Thus, r(Q) = 1/λ1.
3. Main Results
Theorem 3.1. Let X be an ordered Banach space, whose positive cone P is normal with normal constant N. If {T(t)} t≥0 is a positive analytic semigroup, f(t, θ) ≥ θ for all t ∈ I, and the following conditions are satisfied.
-
(H1) For any R > 0, there exists C = C(R) > 0 such that
(3.1)for any t ∈ I, θ ≤ x1 ≤ x2, ∥x1∥, ∥x2∥≤R. -
(H2) There exists L < −ν0 (ν0 is the growth index of {T(t)} t≥0), such that
(3.2)for any t ∈ I, θ ≤ x1 ≤ x2.
Proof. Let h0(t) = f(t, θ), then h0 ∈ C(I, X), h0 ≥ θ. Consider LPBVP
Set R0 = N∥w0∥ + 1, C = C(R0) is the corresponding constant in (H1). We may suppose C > max {ν0, −L}, otherwise substitute C + |ν0| + |L| for C, (H1) is also satisfied. Then we consider LPBVP
Set F(u) = f(t, u) + Cu, then F : C(I, X) → C(I, X) is continuous, F(θ) = h0 ≥ θ. By (H1), F is an increasing operator on [θ, w0]. Set v0 = θ, we can define the sequences
In the following, we prove that the uniqueness. If u1, u2 are the positive mild solutions of PBVP (1.1). Substitute u1 and u2 for w0, respectively, then wn = Q1 · F(ui) = ui (i = 1,2). By (3.14), we have that
Corollary 3.2. Let X be an ordered Banach space, whose positive cone P is a regeneration cone. If {T(t)} t≥0 is a compact and positive analytic semigroup, f(t, θ) ≥ θ for for all t ∈ I, f satisfies (H1) and the following condition:
-
There exist L < λ1, where λ1 is the first eigenvalue of A, such that
(3.18)for any t ∈ I, θ ≤ x1 ≤ x2.
4. Examples
Example 4.1. Consider the following periodic boundary value problem for fractional parabolic partial differential equations in X:
Theorem 4.2. Assume that f(t, 0) ≥ 0 for t ∈ I, the partial derivative is continuous on any bounded domain and , where λ1 is the first eigenvalue of −Δ under the condition u∣∂Ω = 0. Then the problem (4.1) has a unique positive mild solution.
Acknowledgments
This research was supported by NNSFs of China (10871160, 11061031) and project of NWNU-KJCXGC-3-47.