Hilbert functions of schemes of double and reduced points

Abstract

It remains an open problem to classify the Hilbert functions of double points in P2. Given a valid Hilbert function H of a zero-dimensional scheme in P2, we show how to construct a set of fat points Z⊆P2 of double and reduced points such that HZ, 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 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.