1 7 Ju n 20 08 Generating sequences and Poincaré series for a finite set of plane divisorial valuations ∗

Let V be a finite set of divisorial valuations centered at a 2dimensional regular local ring R. In this paper we study its structure by means of the semigroup of values, SV , and the multi-index graded algebra defined by V , grV R. We prove that SV is finitely generated and we compute its minimal set of generators following the study of reduced curve singularities. Moreover, we prove a unique decomposition theorem for the elements of the semigroup. The comparison between valuations in V , the approximation of a reduced plane curve singularity C by families of sets V (k) of divisorial valuations, and the relationship between the value semigroup of C and the semigroups of the sets V (k), allow us to obtain the (finite) minimal generating sequences for C as well as for V . We also analyze the structure of the homogeneous components of grV R. The study of their dimensions allows us to relate the Poincaré series for V and for a general curve C of V . Since the last series coincides with the Alexander polynomial of the singularity, we can deduce a formula of A’Campo type for the Poincaré series of V . Moreover, the Poincaré series of C could be seen as the limit of the series of V (k), k ≥ 0. ∗Math. Subject Class. 14B05, 16W50, 16W70, 13A18 Supported by the Spain Ministry of Education MTM2004-00958 and JCyL VA025A07. Second author also supported by Bancaixa P1-1A2005-08. The authors would like to thank the referee for his/her detailed and helpful comments. Addresses: F. Delgado & A. Núñez: Dto. de Álgebra, Geometŕıa y Topoloǵıa. Universidad de Valladolid. 47005 Valladolid. Spain. e-mail: [email protected], [email protected] C. Galindo: Dto. de Matemáticas (ESTCE), UJI, Campus Riu Sec. 12071 Castellón. Spain. e-mail: [email protected]