Poincaré series of multiplier ideals in two-dimensional local rings with rational singularities

We study the multiplicity of the jumping numbers of an m-primary ideal a in a two-dimensional local ring with a rational singularity. The formula we provide for the multiplicities leads to a very simple and efficient method to detect whether a given rational number is a jumping number. We also give an explicit description of the Poincaré series of multiplier ideals associated to a proving, in particular, that it is a rational function.