Hilbert schemes of 8 points

TL;DR

This paper investigates the structure of Hilbert schemes of points in affine space, revealing reducibility conditions for up to 8 points and providing explicit equations for certain components.

Contribution

It establishes the reducibility of Hilbert schemes of 8 points in dimensions four and higher, and describes the components of related multigraded Hilbert schemes for small colengths.

Findings

Hilbert scheme H^d_8 is reducible iff d >= 4.

Explicit defining equation for the component R^4_8.

Minimal reducible example for multigraded Hilbert schemes with Hilbert function (1,3,2,1).

Abstract

The Hilbert scheme H^d_n of n points in A^d contains an irreducible component R^d_n which generically represents n distinct points in A^d. We show that when n is at most 8, the Hilbert scheme H^d_n is reducible if and only if n = 8 and d >= 4. In the simplest case of reducibility, the component R^4_8 \subset H^4_8 is defined by a single explicit equation which serves as a criterion for deciding whether a given ideal is a limit of distinct points. To understand the components of the Hilbert scheme, we study the closed subschemes of H_n^d which parametrize those ideals which are homogeneous and have a fixed Hilbert function. These subschemes are a special case of multigraded Hilbert schemes, and we describe their components when the colength is at most 8. In particular, we show that the scheme corresponding to the Hilbert function (1,3,2,1) is the minimal reducible example.