Associated Forms and Hypersurface Singularities: The Binary Case

Abstract

In the recent articles by Alper, Eastwood and Isaev, it was conjectured that all rational $GL_n({\mathbb C})$-invariant functions of forms of degree $d\ge 3$ on ${\mathbb C}^n$ can be extracted, in a canonical way, from those of forms of degree $n(d-2)$ by means of assigning every form with nonvanishing discriminant the so-called associated form. While this surprising statement is interesting from the point of view of classical invariant theory, its original motivation was the reconstruction problem for isolated hypersurface singularities, which is the problem of finding a constructive proof of the well-known Mather-Yau theorem. The conjecture was confirmed by Eastwood and Isaev for binary forms of degree $d \le 6$ as well as ternary cubics. Furthermore, a weaker version of it was settled by Alper and Isaev for arbitrary $n$ and $d$. In the present paper, we focus on the case $n=2$ and establish the conjecture, in a rather explicit way, for binary forms of an arbitrary degree. This result allows one to extract a complete system of biholomorphic invariants of homogeneous plane curve singularities from their Milnor algebras.