Existence of multiple positive solutions for singular boundary value problems of nonlinear fractional differential equations

In this paper, we consider the properties of the Green’s function for the nonlinear fractional differential equation boundary value problem D0+qu(t)=f(t,u(t)), t∈J:=[0,1], u(0)=u′(1)=0, where 1<q≤2 is a real number, and D0+q is the standard Riemann-Liouville differentiation. As an application of the Green’s function, we give some multiple positive solutions for singular boundary value problems, and we also give the uniqueness of solution for a singular problem by means of the Leray-Schauder nonlinear alternative, a fixed-point theorem on cones, and a mixed monotone method.


Introduction
This paper is mainly concerned with the existence and multiplicity of positive solutions of the nonlinear fractional differential equation boundary value problem (BVP for short) where  < q ≤  is a real number and D q  + is the standard Riemann-Liouville differentiation, and f is a given function satisfying some assumptions that will be specified later, with lim u→ f (·, u) = +∞ (i.e., f is singular at u = ).
In the last few years, fractional differential equations (in short FDEs) have been studied extensively. The motivation for those works stems from both the development of the theory of fractional calculus itself and the applications of such constructions in various sciences such as physics, mechanics, chemistry, engineering, and so on. For an extensive collection of such results, we refer the readers to the monographs by Kilbas  Some basic theory for the initial value problems of FDE involving Riemann-Liouville differential operator has been discussed by Lakshmikantham [-], Babakhani and Daftardar-Gejji [-] and Bai [], and so on. Also, there are some papers which deal with the existence and multiplicity of solutions (or positive solution) for nonlinear FDE ©2014 Zhou et al.; licensee Springer. 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. http://www.advancesindifferenceequations.com/content/2014/1/97 of BVPs by using techniques of nonlinear analysis (fixed-point theorems, Leray-Shauder theory, topological degree theory, etc.); see [-] and the references therein.
Bai and Lü [] studied the following two-point boundary value problem of FDEs: where D q + is the standard Riemann-Liouville fractional derivative. They obtained the existence of positive solutions by means of the Guo-Krasnosel'skii fixed-point theorem and the Leggett-Williams fixed-point theorem.
Zhang [] considered the existence and multiplicity of positive solutions for the nonlinear fractional boundary value problem f is singular at t = ), and c D q + is the standard Caputo's fractional derivative. The authors proved the existence of one positive solution by using the Guo-Krasnosel'skii fixed-point theorem and the nonlinear alternative of Leray-Schauder type in a cone and assuming certain hypotheses on the function f .
Mena et al. [] proved the existence and uniqueness of a positive and nondecreasing solution for the problem (.) by using a fixed-point theorem in partially ordered sets.
From the above works, we can see a fact, although the fractional boundary value problems have been investigated by some authors, singular boundary value problems are seldom considered, in particular, f is singular at u = . Motivated by all the works above, in this paper we discuss the boundary value problem (.)-(.). Using the Leray-Schauder nonlinear alternative theorem and the Guo-Krasnosel'skii fixed-point theorem, we give some new existence criteria for the singular boundary value problem (.)-(.). Finally, we obtain new uniqueness criteria for the singular boundary value problem (.)-(.) by a mixed monotone method.
The plan of this paper is as follows. In Section , we shall give some definitions and lemmas to prove our main results. In Section , we establish the existence of multiple positive solutions for the singular boundary value problem (.)-(.) by the Leray-Schauder nonlinear alternative theorem and the Guo-Krasnosel'skii fixed-point theorem. In Section , by using a mixed monotone method, we obtain some new uniqueness criteria for the singular boundary value problem (.)-(.). http://www.advancesindifferenceequations.com/content/2014/1/97

Preliminaries and lemmas
For the convenience of the reader, we present here the necessary definitions from fractional calculus theory. These definitions can be found in the recent literature such as [, ] and [].
Definition . [, ] The fractional-order integral of the function h ∈ L  ([a, b], R + ) of order α ∈ R + is defined by where is the gamma function. When a = , we write (α) for t > , and ϕ α (t) =  for t ≤ , and ϕ α → δ(t) as α → , where δ is the delta function.  From the definition of the Riemann-Liouville derivative, we can obtain the statement.

Lemma . []
Let α > . If we assume u ∈ C(, ) ∩ L(, ), then differential equation as unique solutions, where N is the smallest integer greater than or equal to α.
where N is the smallest integer greater than or equal to q.
In the following, we present the Green's function of the FDE boundary value problem.

Lemma . Let h(t) ∈ C[, ]
and  < q ≤ , then the unique solution of is given by where G(t, s) is the Green's function given by Proof By Lemma ., we can reduce the equation of problem (.) to an equivalent integral equation: Applying the boundary condition (.), we have Therefore, the unique solution of problem (.)-(.) is which completes the proof.
The following properties of the Green's function form the basis of our main work in this paper.
The function G(t, s) defined by (.) satisfies the following conditions: Proof (i) In the following, we consider (q)G(t, s).
On the other hand, we have On the other hand, we have Thus, On the other hand, we have When s ≥ t, we get Thus, On the other hand, we have Therefore we have (ii). Clearly G(t, s) >  holds trivially. The proof is finished.
Lemma . The function G * (t, s) := t -q G(t, s) has the following properties: Let y(t) = t -q u(t), by u(t) =   G(t, s)h(s) ds, we get The following three theorems are fundamental in the proofs of our main results.

Lemma . []
Let X be a Banach space, and let P ⊂ X be a cone in X. Assume  ,  are open subsets of X with θ ∈  ⊂  ⊂  , and let A : P → P be a completely continuous operator such that either Then A has a fixed point in P ∩ (  \  ).
Let P be a normal cone of a Banach space E, and e ∈ P with e ≤ , e = θ . Define  A(x, y), ∀x, y ∈ Q e ,  < t < .
Then A has a unique fixed point x * ∈ Q e .

Lemma . []
Assume is a relative subset of a convex set K in a normed space X. Let A : → K be a compact map with  ∈ . Then either (A) A has a fixed point in , or (A) there is a x ∈ ∂ and a λ <  such that x = λA(x).

Positive solutions of a singular problem
In this section, we establish some new existence results for the singular fractional differential equation where λ ∈ [, ] and We claim that any solution u of (.) for any λ ∈ [, ] must satisfy u = r. Otherwise, assume that u is a solution of (.) for some λ ∈ [, ] such that u = r. Then u(t) ≥  n for t ∈ [, ]. Note that Hence, for all t ∈ [, ], we have Thus we have from condition (H), for all t ∈ [, ], This is a contradiction and the claim is proved. Now the Leray-Schauder nonlinear alternative guarantees that the integral equation has a solution, denoted by u n , in B r = {x ∈ C(J) : u ≤ r}.
Next we claim that u n (t) has a uniform sharper lower bound, i.e., there exists a function ρ ∈ C([, ]) that is unrelated to n such that ρ(t) >  for a.e. t ∈ [, ] and for any n ∈ N  , (.) http://www.advancesindifferenceequations.com/content/2014/1/97 By (H), there exists a continuous function φ r  such that f (t, x) ≥ φ r (t) for all t ∈ [, ] and x ≤ r. In view of u n (t) ≤ r, so we have We choose ρ(t) = t q-   g(s)φ r (s) ds. Then (.) holds. In order to pass from the solutions u n of the truncation equation (.) to that of the original equation (.)-(.), we need the following fact: In fact, for any t  , t  ∈ [, ], we have By continuity of G(t, ·) and the mean value theorem for integrals, there exists a ξ ∈ (, ) such that By the continuity of G(·, s) and (H), then (.) holds. By the Arzela-Ascoli theorem, there exist a subsequence N  of N  and u ∈ C([, ]) such that {u n } n∈N  is uniformly convergent to u and u satisfies ρ(t) ≤ u(t) ≤ r for any t ∈ [, ]. In view of u n (t) =   G(t, s)f n (s, u n (s)) ds, by the Lebesgue dominated convergence theorem, we have u(t) =   G(t, s)f (s, u(s)) ds. Therefore, (.)-(.) have one positive solution u with  < u < r. This completes the proof.
Proof To show the existence of u, we will use Lemma .. Define this implies that (Ay)(t) ≥ t Ay , i.e. Ay ∈ K . Next, we show that A is equicontinuous. The proof will be given in several steps.
Step : We will prove that the operator A : and for t, t ∈ [, ], we have By continuity of G(t, ·) and the mean value theorem for integrals, there exists a η ∈ (, ) such that By continuity of G(·, s), using condition (H), and the Arzela-Ascoli theorem guarantees that A : K ∩ (  \  ) → K is compact. Now we prove that In fact, for any x ∈ K ∩ ∂  , we have for t ∈ [, ], In fact, for any x ∈ K ∩ ∂  , we have for t ∈ [, ], This implies (.) holds. It follows from Lemma ., (.), and (.) that A has a fixed point K ∩ (  \  ). Clearly, this fixed point is a positive solution of (.)-(.) satisfying r < u ≤ R. This completes the proof.

Uniqueness of solution for a singular problem
Throughout this section we assume that and for any t ∈ (, ) and x > , and q  ∈ C((, ), (, ∞)) satisfies Similarly, from (.), one has g  (tx) ≥ t β g  (x), g  (t) ≥ t β g  (), t ∈ (, ), x > . Thus A λ is well defined and A λ (Q e × Q e ) ⊆ Q e . Next, for any l ∈ (, ) and x, y ∈ Q e we have Thus the conditions of Lemma . hold. Therefore there exists a unique x * ∈ Q e such that A λ (x * , x * ) = x * . This completes the proof. where  < a, b < , ν ≥ . We let β = max{a, b} < , q  (t) = , g  (x) = νx b , h  (x) = x -a .

Example  Consider the boundary value problem
Thus, we have For any t ∈ (, ) and x > , and    (q) n(s)a -β (s)q  (s) ds < +∞.
Since β < , and  < q ≤ , thus all conditions in Theorem . are satisfied. Applying Theorem ., we can find that (.)-(.) has a unique positive solution x * (t).