By applying fractional integration and derivation to the vector-valued Laplace transform, a functional calculus for -times integrated semigroups is obtained. This functional calculus is related to smooth distribution semigroups. As application, fractional powers of its in®nitesimal generator are de®ned.2000 Mathematics Subject Classi®cation: 47D62, 26A33.
IntroductionOne of the concepts used in the last ®fteen years to study the Abstract Cauchy Problemis the n-times integrated semigroup with n e N [32], [19]. These semigroups are related to other notions: C-semigroups [9], spectral distributions [4], holomorphic semigroups [2]. They have been used with C-semigroups [39], cosine functions [26], [15] or sine functions [23]. In 1991 Hieber [18] introduced -times integrated semigroups with e R . In an informal way, the``formal solution'' of the Cauchy Problem is smoothed integrating it times in the fractional sense of Riemann-Liouville. Some works about these semigroups were published later on, see for instance [29], [8] (about spectral mapping theorem), [37] (in locally convex spaces).In this paper, we investigate the deep relationship between these semigroups and