The Milnor number of a hypersurface singularity in arbitrary characteristic

The Milnor number of an isolated hypersurface singularity, defined as the codimension $\mu(f)$ of the ideal generated by the partial derivatives of a power series $f$ whose zeros represent locally the hypersurface, is an important topological invariant of the singularity over the complex numbers, but its meaning changes dramatically when the base field is arbitrary. It turns out that if the ground field is of positive characteristic, this number is not even invariant under contact equivalence of the local equation $f$. In this paper we study the variation of the Milnor number in the contact class of $f$, giving necessary and sufficient conditions for its invariance. We also relate, for an isolated singularity, the finiteness of $\mu(f)$ to the smoothness of the generic fiber $f=s$. Finally, we prove that the Milnor number coincides with the conductor of a plane branch when the characteristic does not divide any of the minimal generators of its semigroup of values, showing in particular that this is a sufficient (but not necessary) condition for the invariance of the Milnor number in the whole equisingularity class of $f$.Comment: 20 page

The Milnor Number of Plane Branches With Tame Semigroup of Values

The Milnor number of an isolated hypersurface singularity, defined as the codimension $\mu(f)$ of the ideal generated by the partial derivatives of a power series $f$ that represents locally the hypersurface, is an important topological invariant of the singularity over the complex numbers. However it may loose its significance when the base field is arbitrary. It turns out that if the ground field is of positive characteristic, this number depends upon the equation $f$ representing the hypersurface, hence it is not an invariant of the hypersurface. For a plane branch represented by an irreducible convergent power series $f$ in two indeterminates over the complex numbers, it was shown by Milnor that $\mu(f)$ always coincides with the conductor $c(f)$ of the semigroup of values $S(f)$ of the branch. This is not true anymore if the characteristic of the ground field is positive. In this paper we show that, over algebraically closed fields of arbitrary characteristic, this is true, provided that the semigroup $S(f)$ is tame, that is, the characteristic of the field does not divide any of its minimal generators.Comment: arXiv admin note: substantial text overlap with arXiv:1507.0317