Abstract. We propose a general method to obtain the representation of solutions for linear fractional order differential equations based on the theory of (a, k)-regularized families of operators. We illustrate the method in case of the fractional order differential equationwhere A is an unbounded closed operator defined on a Banach space X and f is a X-valued function.
IntroductionWe study in this paper existence of solutions for fractional order differential equations of the form Fractional order differential equations is a subject of increasing interest in different contexts and areas of research, see e. Note that when A = 2∆− 2 ∆ 2 (where ∆ is the Laplace operator) α = 1 and µ = 1/ 2 the above equation was recently considered by Nane [18, Theorem 2.2]. In particular, in case µ = 0, α = 1, A = −∆ 2 and u (0) = − 2 π ∆u 0 with u 0 ∈ D(∆), the equation, t > 0, has been studied in [18, Theorem 2.1] in connection with PDE's and iterated processes. A precise interplay between entire and fractional order differential equations was investigated in reference [10].Observe that one cannot apply semigroup theory directly to solve problem (1.1) in terms of a variation of constant formula. However, our methods based on the theory of (a, k)-regularized families allows us to construct a solution. In fact, we will show that it is possible to give an abstract operator approach to equation (1.1) by defining an ad-hoc family of strongly continuous operators. Then, we are able to show that the solution of equation (1.1) can be written in terms of a kind of variation of constants formula (cf. Theorem 3.1 below). We believe that the method indicated in this paper can be used to handle many classes of linear fractional order differential equations. Our method can be viewed as an extension of the ideas in reference [4] to state the existence of solutions for the abstract fractional order Cauchy problem.Our plan is as follows: In section 2, we introduce some preliminaries on fractional order derivatives, the Mittag-Leffler function and the concept of (α, µ)-regularized families, which give us the necessary 2000 Mathematics Subject Classification. 26A33, 47D06, 45N05.