On the Brieskorn (a,b)-module of an hypersurface singularity

Abstract

We show in this note that for a germ $g$ of holomorphic function with an isolated singularity at the origin of $\mathbb{C}^n$ there is a pole for the meromorphic extension of the distribution \begin{equation*} \frac{1}{\Gamma(\lambda)} \int_X | g |^{2\lambda}\bar{g}^{-n} \square \tag{*} \end{equation*} at $- n - \alpha$ when $ \alpha$ is the smallest root in its class modulo $\mathbb{Z}$ of the reduce Bernstein-Sato polynomial of $g$. This is rather unexpected result comes from the fact that the self-duality of the Brieskorn (a,b)-module $E_g$ associated to $g$ exchanges the biggest simple pole sub-(a,b)-module of $E_g$ with the saturation of $E_g$ by $b^{-1}a$. In the first part of this note, we prove that the biggest simple pole sub-(a,b)-module of the Briekorn (a,b)-module $E$ of $g$ is "geometric" in the sense that it depends only on the hypersurface germ $\{g = 0 \}$ at the origin in $\mathbb{C}^n$ and not on the precise choice of the reduced equation $g$, as the poles of (*). By duality, we deduce the same property for the saturation $\tilde{E}$ of $E$. This duality gives also the relation between the "dual" Bernstein-Sato polynomial and the usual one, which is the key of the proof of the theorem.