Hilbert functions of schemes of double and reduced points

Abstract

It remains an open problem to classify the Hilbert functions of double points in ${\mathbb{P}}^2$. Given a valid Hilbert function $H$ of a zero-dimensional scheme in ${\mathbb{P}}^2$, we show how to construct a set of fat points $Z\subseteq {\mathbb{P}}^2$ of double and reduced points such that $H_Z$, the Hilbert function of $Z$, is the same as $H$. In other words, we show that any valid Hilbert function $H$ of a zero-dimensional scheme is the Hilbert function of a set of a positive number of double points and some reduced points. For some families of valid Hilbert functions, we are also able to show that $H$ is the Hilbert function of only double points. In addition, we give necessary and sufficient conditions for the Hilbert function of a scheme of a double points, or double points plus one additional reduced point, to be the Hilbert function of points with support on a star configuration of lines.

22 pages; more detail added and small corrections throughout. This version to appear in Journal of Pure and Applied Algebra