Associated forms: current progress and open problems

Abstract

Let $d\ge 3$, $n\ge 2$. The object of our study is the morphism $\Phi$, introduced in earlier articles by J. Alper, M. Eastwood and the author, that assigns to every homogeneous form of degree $d$ on ${\mathbb C}^n$ for which the discriminant $\Delta$ does not vanish the so-called associated form, which is a form of degree $n(d-2)$ on the dual space. This morphism is ${\mathrm{SL}}_n$-equivariant and is of interest in connection with the well-known Mather-Yau theorem, specifically, with the problem of explicit reconstruction of an isolated hypersurface singularity from its Tjurina algebra. Letting $p$ be the smallest integer such that the product $\Delta^p\Phi$ extends to the entire affine space of degree $d$ forms, one observes that the extended map defines a contravariant. In the present paper we survey known results on the morphism $\Phi$ as well as the contravariant $\Delta^p\Phi$, and state several open problems. Our goal is to draw the attention of complex analysts and geometers to the concept of the associated form and the intriguing connection between complex singularity theory and invariant theory revealed through it.