The differential Hilbert series of a local algebra

Abstract.

The differential Hilbert series of a commutative local algebra R/R ₀ which is essentially of finite type is the generating function of the numerical function which associates with each $n\in \Bbb N$ the minimal number of generators of the algebra $P^n_{R/R_0}$ of principal parts of order n, considered as an R-module. It can be expressed as a rational function over the integers. We wish to compute this rational function in terms of other invariants of the local algebra or at least give estimates of it. We obtain formulas which generalize wellknown facts about the minimal number of generators of the module of Kähler differentials.