The aim of this paper is to investigate a class of boundary value problem for fractional differential equations involving nonlinear integral conditions. The main tool used in our considerations is the technique associated with measures of noncompactness.
1. Introduction
The theory of fractional differential equations has been emerging as an important area of investigation in recent years. Let us mention that this theory has many applications in describing numerous events and problems of the real world. For example, fractional differential equations are often applicable in engineering, physics, chemistry, and biology. See Hilfer[1], Glockle and Nonnenmacher [2], Metzler et al. [3], Podlubny [4], Gaul et al. [5], among others. Fractionaldifferential equations are also often an object of mathematical investigations; see the papers of Agarwal et al. [6], Ahmad and Nieto [7], Ahmad and Otero-Espinar [8], Belarbi et al. [9], Belmekki et al [10], Benchohra et al. [11–13], Chang and Nieto [14], Daftardar-Gejji and Bhalekar [15], Figueiredo Camargo et al. [16], and the monographs of Kilbas et al. [17] and Podlubny [4].
Applied problems require definitions of fractional derivatives allowing the utilization
of physically interpretable initial conditions, which contain
, and so forth. the same requirements of boundary conditions. Caputo's fractional
derivative satisfies these demands. For more details on the geometric and physical
interpretation for fractional derivatives of both the Riemann-Liouville and Caputo
types, see [18, 19].
In this paper we investigate the existence of solutions for boundary value problems with fractional order differential equations and nonlinear integral conditions of the form
(11)where
is the Caputo fractional derivative,
,
, and
are given functions satisfying some assumptions that will be specified later, and
is a Banach space with norm
.
Boundary value problems with integral boundary conditions constitute a very interesting and important class of problems. They include two, three, multipoint, and nonlocal boundary value problems as special cases. Integral boundary conditions are often encountered in various applications; it is worthwhile mentioning the applications of those conditions in the study of population dynamics [20] and cellular systems [21].
Moreover, boundary value problems with integral boundary conditions have been studied by a number of authors such as, for instance, Arara and Benchohra [22], Benchohra et al. [23, 24], Infante [25], Peciulyte et al. [26], and the references therein.
In our investigation we apply the method associated with the technique of measures of noncompactness and the fixed point theorem of Mönch type. This technique was mainly initiated in the monograph of Bana and Goebel [27] and subsequently developed and used in many papers; see, for example, Bana and Sadarangoni [28], Guo et al. [29], Lakshmikantham and Leela [30], Mönch [31], and Szufla [32].
2. Preliminaries
In this section, we present some definitions and auxiliary results which will be needed in the sequel.
Denote by
the Banach space of continuous functions
, with the usual supremum norm
(21)Let
be the Banach space of measurable functions
which are Bochner integrable, equipped with the norm
(22)Let
be the Banachspace of measurable functions
which are bounded, equipped with the norm
(23)Let
be the space of functions
, whose first derivative is absolutely continuous.
Moreover, for a given set
of functions
let us denote by
(24)Now let us recall some fundamental facts of the notion of Kuratowski measure of noncompactness.
Definition 2.1 (see [27]).
Let
be a Banach space and
the bounded subsets of
. The Kuratowski measure of noncompactness is the map
defined by
(25)Properties
The Kuratowski measure of noncompactness satisfies some properties (for more details see [27]).
(a)
is compact (
is relatively compact).
(b)
.
(c)
.
(d)
(e)
(f)
.
Here
and
denote the closure and the convex hull of the bounded set
, respectively.
For completeness we recall the definition of Caputo derivative of fractional order.
Definition 2.2 (see [17]).
The fractional order integral of the function
of order
is defined by
(26)where
is the gamma function. When
, we write
where
(27)
for
, and
as
.
Here
is the delta function.
Definition 2.3 (see [17]).
For a function
given on the interval
, the Caputo fractional-order derivative of
, of order
is defined by
(28)Here
and
denotes the integer part of
.
Definition 2.4.
A map
is said to be Carathéodory if
(i)
is measurable for each 
(ii)
is continuous for almost each 
For our purpose we will only need the following fixed point theorem and the important Lemma.
Let
be a bounded, closed and convex subset of a Banach space such that
, and let
be a continuous mapping of
into itself. If the implication
(29)holds for every subset
of
, then
has a fixed point.
Lemma 2.6 (see [32]).
Let
be a bounded, closed, and convex subset of the Banach space
, G a continuous function on
and a function
satisfies the Carathéodory conditions, and there exists
such that for each
and each bounded set
one has
(210)If
is an equicontinuous subset of
, then
(211)3. Existence of Solutions
Let us start by defining what we mean by a solution of the problem (1.1).
Definition 3.1.
A function
is said to be a solution of (1.1) if it satisfies (1.1).
Let
be continuous functions and consider the linear boundary value problem
(31)Lemma 3.2 (see [11]).
Let
and let
be continuous. A function
is a solution of the fractional integral equation
(32)with
(33)
(34)if and only if
is a solution of the fractional boundary value problem (3.1).
Remark 3.3.
It is clear that the function
is continuous on
, and hence is bounded. Let
(35)For the forthcoming analysis, we introduce the following assumptions
(H1)The functions
satisfy the Carathéodory conditions.
(H2)There exist
, such that
(36)(H3)For almost each
and each bounded set
we have
(37)Theorem 3.4.
Assume that assumptions
hold. If
(38)then the boundary value problem (1.1) has at least one solution.
Proof.
We transform the problem (1.1) into a fixed point problem by defining an operator
as
(39)where
(310)and the function
is given by (3.4). Clearly, the fixed points of the operator
are solution of the problem (1.1). Let
and consider the set
(311)Clearly, the subset
is closed, bounded, and convex. We will show that
satisfies the assumptions of Theorem 2.5. The proof will be given in three steps.
Step 1.
is continuous.
Let
be a sequence such that
in
. Then, for each
,
(312)Let
be such that
(313)By (H2) we have
(314)Since
and
are Carathéodory functions, the Lebesgue dominated convergence theorem implies that
(315)Step 2.
maps
into itself.
For each
, by
and (3.8) we have for each 
(316)Step 3.
is bounded and equicontinuous.
By Step 2, it is obvious that
is bounded.
For the equicontinuity of
. Let
,
and
. Then
(317)As
, the right-hand side of the above inequality tends to zero.
Now let
be a subset of
such that
.
is bounded and equicontinuous, and therefore the function
is continuous on
. By (H3), Lemma 2.6, and the properties of the measure
we have for each 
(318)This means that
(319)By (3.8) it follows that
, that is,
for each
, and then
is relatively compact in
. In view of the Ascoli-Arzelà theorem,
is relatively compact in
. Applying now Theorem 2.5 we conclude that
has a fixed point which is a solution of the problem (1.1).
4. An Example
In this section we give an example to illustrate the usefulness of our main results. Let us consider the following fractional boundary value problem:
(41)Set
(42)Clearly, conditions (H1),(H2) hold with
(43)From (3.4) the function
is given by
(44)From (4.4), we have
(45)A simple computation gives
(46)Condition (3.8) is satisfied with
. Indeed
(47)which is satisfied for each
. Then by Theorem 3.4 the problem (4.1) has a solution on
.
Acknowledgments
The authors thank the referees for their remarks. The research of A. Cabada has been partially supported by Ministerio de Educacion y Ciencia and FEDER, project MTM2007-61724, and by Xunta de Galicia and FEDER, project PGIDIT05PXIC20702PN.
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