Fractional calculus and fractional differential equations in nonreflexive Banach spaces

https://doi.org/10.1016/j.cnsns.2013.10.010Get rights and content

Highlights

  • We introduce the notion of pseudo-fractional derivatives.

  • We introduce the notion of fractional Pettis integrals.

  • We discuss functions in non-reflexive Banach spaces equipped with the weak topology.

  • We discuss fractional differential equations in an abstract setting.

Abstract

In this paper we establish an existence result for the fractional differential equationDpαy(t)=f(t,y(t)),y(0)=y0,where Dpαy(·) is a fractional pseudo-derivative of a weakly absolutely continuous and pseudo-differentiable function y(·) : T  E, the function f (t, ·) : T × E  E is weakly–weakly sequentially continuous for every t  T and f (·, y(·)) is Pettis integrable for every weakly absolutely continuous function y(·) : T  E, T is a bounded interval of real numbers and E is a nonreflexive Banach space.

Introduction

The study of first order ordinary differential equations in Banach spaces (reflexive or not) equipped with the weak topology was initiated in the 1950s. Let E be a Banach space and let f (·,·) : [a, b] × E  E be continuous. It is well known that if E is finite dimensional, then for each (t0, y0)  [a, b) × E, there exists a continuous differentiable function y(·) which is a solution of the Cauchy problemy(t)=f(t,y(t)),y(t0)=y0,on some open interval which contains t0. In 1950, Dieudonné [15] showed that when E = c0 the Cauchy problem (1) has no solutions for every continuous function f (·, ·). The notion of the measure of noncompactness was introduced by Kuratowski [29] in 1930. Ambrosetti [1] used the Kuratowski noncompactness measure and Darbo’s fixed point theorem to prove an existence result for (1) in infinite dimensional Banach spaces. Szep [49] first established the existence of weak solutions for (1) if f (·, ·) : [a, b] × E  E is weakly continuous and E is a reflexive Banach space. Chow and Schuur [10] treated the case where E is separable and reflexive and f (·, ·) is a weakly continuous function with bounded range. Kato in [26] showed that if f (·, ·) : [a, b] × BE[y0, r]  E is weakly continuous, then all that is needed to assure the existence of solutions to (1) is the relatively weak compactness of f ([a, b] × BE[y0, r]). Pianigiani [39] showed that in every nonreflexive retractive Banach space there exists a weakly continuous function f (·, ·) such that (1) does not have a weak solution, and Perri [36] showed that this property is true in every nonreflexive Banach space. The measure of weak noncompactness was introduced by De Blasi [11], and it was used by Cramer et al. [9] to obtain an existence result for weak solutions of (1) in nonreflexive Banach spaces. Using the measure of weak noncompactness, Cichoń [6], Cichoń and Kubiaczyk [8], Dutkiewicz and Szufla [17], Gomaa [23], O’Regan [33], [34], improved and generalized previous results in the literature. For a review of this topic we refer the reader to Cichoń [7], Deimling [12], Hashem [24] and Teixeira [51]. In recent years fractional differential equations in Banach spaces were studied. The general literature on fractional differential equations in finite or infinite dimensional Banach spaces is extensive and different topics on the existence and qualitative properties of solutions are considered. Concerning the literature on fractional differential equations we cite the books [30], [43] and the references therein. Only a few papers consider fractional differential equations in reflexive Banach spaces equipped with the weak topology. Salim and El-Sayed [41] were the first authors to discuss the existence of weak solutions for fractional differential equation. Recent progress in this direction can be found in [4], [5], [40] and for a recent review of this topic we refer the reader to [24].

In this paper, we develop fractional calculus for functions with values in a nonreflexive Banach space equipped with the weak topology. Using the Pettis integral, we introduce the notions of fractional Pettis integrals and pseudo-fractional derivatives. Then we present a very general theory for fractional calculus and fractional differential equations in a nonreflexive Banach spaces equipped with the weak topology.

Section snippets

Preliminaries

Let E be a Banach space with the norm ∥ · ∥ and let E be the topological dual of E. If x  E, then its value on an element x  E will be denoted by 〈x, x〉. The space E endowed with the weak topology σ(E, E) will be denoted by Ew. Consider an interval T = [0, b] of R endowed with the Lebesgue σ-algebra L(T) and the Lebesgue measure λ. We will denote by L1(T) the space of all measurable and Lebesgue integrable real functions defined on T, and by L(T) the space of all measurable and essential bounded

Fractional Pettis integral and Abel integral equation

Let us denote by P(T, E) the space of all weakly measurable and Pettis integrable functions x(·) : T  E with the property that 〈x, x(·)〉  L(T) for every x  E. Since for each t  T the real valued function s  (t  s)α−1 is Lebesgue integrable on [0, t] for every α > 0 then, by Proposition 2.4, the fractional Pettis integralIαx(t)0t(t-s)α-1Γ(α)x(s)ds,tTexists for every function x(·)  P(T, E) as a function from T into E. Moreover, we have thatx,Iαx(t)=0t(t-s)α-1Γ(α)x,x(s)ds,tT,for every x  E, and

Fractional pseudo-derivatives

In the following, consider a  (0, 1). If y(·) : T  E is a pseudo-differentiable function with a pseudo-derivative x(·)  P(T, E) on T, then the following fractional Pettis integralI1-αx(t)=0t(t-s)-αΓ(1-α)x(s)dsexists on T. The fractional Pettis integral I1−α x(·) is called a fractional pseudo-derivative of y(·) on T and it will be denoted by Dpαy(·); that is,Dpαy(t)=I1-αx(t),tT.If y(·) : T  E is an a.e. weakly differentiable function with the weak derivative yw(·)P(T,E) on T, thenDwαy(t)I1-αyw(t),t

Fractional differential equations

In this section we establish an existence result for the following fractional differential equationDpαy(t)=f(t,y(t)),y(0)=y0,where Dpαy(·) is a fractional pseudo-derivative of the function y(·) : T  E and f (·, ·) : T × E  E is a given function. Along with the Cauchy problem (18) consider the following integral equationy(t)=y0+0t(t-s)α-1Γ(α)f(s,y(s))ds,tT,where the integral is in the sense of Pettis.

A wAC function (or an AC function, if E is a weakly sequentially complete space) y(·) : T  E is said to be

Conclusions

In this paper, we developed fractional calculus for functions with values in a nonreflexive Banach space equipped with the weak topology. Using the Pettis integral, we introduced and studied the notions of fsractional Pettis integrals and pseudo-fractional derivatives. Then we obtained an existence result for fractional differential equations in a nonreflexive Banach space equipped with the weak topology.

Acknowledgments

The authors wish to thank the referees for their valuable comments and suggestions.

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