# mapToPunctualHilbertScheme -- maps an ideal into a point in a certain punctual Hilbert scheme

## Synopsis

• Usage:
I = mapToPunctualHilbertScheme(Q)
• Inputs:
• Q, an ideal, a primary ideal
• Outputs:
• I, an ideal, an ideal that parametrizes Q in a punctual Hilbert scheme

## Description

This method maps a P-primary ideal Q into a point in a punctual Hilbert scheme. Let $\mathbb{K}$ be a characteristic zero and a prime ideal $P$ of codimension $c$ in the polynomial ring $R = \mathbb{K}[x_1,\ldots,x_n]$. We write $\mathbb{F}$ for the field of fractions of the integral domain $R/P$. To simplify our notation, perhaps after a linear change of coordinates, we assume that $\{ x_{c+1}, \ldots, x_n \}$ is a maximal independent set of variables module $P$.

The main purpose of this method is to reduce the study of arbitrary $P$-primary ideals in $R = \mathbb{K}[x_1,\ldots,x_n]$ to a zero-dimensional setting over the function field $\mathbb{F}$. This reduction is made by parametrizing $P$-primary ideals with the punctual Hilbert scheme ${\rm Hilb}^m ( \,\mathbb{F}[[y_1,\ldots,y_c]] \,).$ This is a quasiprojective scheme over the function field $\mathbb{F}$. Its classical points are ideals of colength $m$ in the local ring $\mathbb{F}[[y_1,\ldots,y_c]]$.

This method maps a P-primary ideal Q into a unique point in ${\rm Hilb}^m ( \,\mathbb{F}[[y_1,\ldots,y_c]] \,)$ that corresponds with Q. This method can be seen as an implementation of the map $\gamma$ described in Section 2 of Primary ideals and their differential equations.

 i1 : R = QQ[x_1, x_2, x_3] o1 = R o1 : PolynomialRing i2 : Q = ideal(x_1^2, x_2^2, x_1-x_2*x_3) 2 2 o2 = ideal (x , x , - x x + x ) 1 2 2 3 1 o2 : Ideal of R i3 : mapToPunctualHilbertScheme Q 2 o3 = ideal (hx - x hx , hx ) 1 3 2 2 / R \ o3 : Ideal of frac|--------|[hx ..hx ] |(x , x )| 1 2 \ 2 1 /

## Ways to use mapToPunctualHilbertScheme :

• "mapToPunctualHilbertScheme(Ideal)"

## For the programmer

The object mapToPunctualHilbertScheme is .