Rational components of Hilbert schemes

Abstract

The Gröbner stratum of a monomial ideal $\id{j}$ is an affine variety that parametrizes the family of all ideals having $\id{j}$ as initial ideal (with respect to a fixed term ordering). The Gröbner strata can be equipped in a natural way of a structure of homogeneous variety and are in a close connection with Hilbert schemes of subvarieties in the projective space $\PP^n$. Using properties of the Gröbner strata we prove some sufficient conditions for the rationality of components of $\hilb_{p(z)}^n$. We show for instance that all the smooth, irreducible components in $\hilb_{p(z)}^n$ (or in its support) and the Reeves and Stillman component $H_{RS}$ are rational.

arXiv comment27 pages; Theorem 4.7 and final example strengthened; final version: accepted for publication on Rendiconti del Seminario Matematico dell'Università di Padova