Abstract. For α ∈ R, let Dα denote the scale of Hilbert spaces consisting of Dirichlet series f (s) =The Gordon-Hedenmalm Theorem on composition operators for H 2 = D 0 is extended to the Bergman case α > 0. These composition operators are generated by functions of the form Φ(s) = c 0 s+ϕ(s), where c 0 is a nonnegative integer and ϕ(s) is a Dirichlet series with certain convergence and mapping properties. For the operators with c 0 = 0 a new phenomenon is discovered: If 0 < α < 1, the space Dα is mapped by the composition operator into a smaller space in the same scale. When α > 1, the space Dα is mapped into a larger space in the same scale. Moreover, a partial description of the composition operators on the Dirichlet-Bergman spaces A p for 1 ≤ p < ∞ are obtained, in addition to new partial results for composition operators on the DirichletHardy spaces H p when p is an odd integer.

The Hardy spaces of Dirichlet series denoted by H p (p ≥ 1) have been studied in [12] when p = 2 and in [3] for the general case. In this paper we study some L p -generalizations of spaces of Dirichlet series, particularly two families of Bergman spaces denoted A p and B p . We recover classical properties of spaces of analytic functions: boundedness of point evaluation, embeddings between these spaces and "Littlewood-Paley" formulas when p = 2. We also show that the B p spaces have properties similar to the classical Bergman spaces of the unit disk while the A p spaces have a different behavior.

We study boundedness and compactness of composition operators on weighted Bergman spaces of Dirichlet series. Particularly, we obtain in some specific cases, upper and lower bounds of the essential norm of these operators and a criterion of compactness on classicals weighted Bergman spaces. Moreover, a sufficient condition of compactness is obtained using the notion of Carleson's measure. Mathematics Subject Classification.Primary: 47B33 -Secondary: 46E15.

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We show that a composition operator on weighted Bergman spaces A p µ is invertible if and only if it is Fredholm if and only if it is an isometry.