On the Hilbert Schemes of Canonically-Embedded Curves of Genus 5 and 6

Abstract

A canonically-embedded curve of genus $g$ is a pure 1-dimensional, non-degenerate subscheme $C$ of ${\bf P}^{g-1}$ over an algebraically closed field $k$, for which ${\cal O}_C(1) \cong ω_C$, (the dualizing sheaf)$ and $h^0(C, {\cal O}_C) = 1$, $h^0(C, ω_C) = g$. The singularities of $C$ (if any) are Gorenstein, and $C$ is connected of degree $2g-2$ and arithmetic genus $g$. In a recent paper, Schreyer has proved that Petri's normalization of the homogeneous ideal $I(C)$ of a smooth canonically-embedded curve can be also carried out for singular curves, provided that the curve has a simple $(g-2)$-secant (a linear ${\bf P}^{g-3}$ intersecting $C$ transversely at exactly $g-2$ (smooth) points). We use the Petri normalization to study the Hilbert scheme of curves of degree $2g-2$ and arithmetic genus $g$ in ${\bf P}^{g-1}$ in the low-genus cases $g = 5,6$. The main results are that the Hilbert points of all curves for which Petri's approach applies lie on one irreducible component of the Hilbert scheme in these cases. (The same is not true for $g$ sufficiently large.)