On Fitting ideals of logarithmic vector fields and Saito's criterion

Abstract

The germ of an analytic set $(X,p)$ in $\mathbb{C}^n$ has an associated $\mathscr{O}_{\mathbb{C}^n,p}$-module $\mathrm{Der}(-\log X)$ of `logarithmic vector fields', the ambient germs of holomorphic vector fields tangent to the smooth locus of $X$. For a module $L\subseteq \mathrm{Der}(-\log X)$ let $I_k(L)$ be the ideal generated by the $k\times k$ minors of a matrix of generators for $L$; these are the Fitting ideals of $\mathrm{Der}_{\mathbb{C}^n,p}/L$. We aim to: (i) find sufficient conditions on $\{I_k(L)\}$ to prove $L=\mathrm{Der}(-\log X)$; (ii) identify $\{I_k(\mathrm{Der}(-\log X))\}$, to provide a necessary condition for equality; and (iii) provide a geometric interpretation of these ideals. Even for $(X,p)$ smooth, an example shows that Fitting ideals alone are insufficient to prove equality, although we give a different criterion. Using (ii) and (iii) in the smooth case, we give partial answers to (ii) and (iii) for arbitrary $(X,p)$. When $(X,p)$ is a hypersurface, we give sufficient algebraic or geometric conditions for the reflexive hull of $L$ to equal $\mathrm{Der}(-\log X)$; for $L$ reflexive, this answers (i) and generalizes criteria of Saito for free divisors and Brion for linear free divisors.