On the Hilbert scheme of smooth curves in ${\mathbb{P}}^4$ of degree $d=g+1$ and genus $g$ with negative Brill-Noether number

Abstract

We denote by ${\mathcal{H}}_{d,g,r}$ the Hilbert scheme of smooth curves, which is the union of components whose general point corresponds to a smooth irreducible and non-degenerate curve of degree $d$ and genus $g$ in $\PP^r$. In this article, we show that for low genus $g$ outside the Brill-Noether range, the Hilbert scheme ${\mathcal{H}}_{g+1,g,4}$ is non-empty whenever $g\ge 9$ and irreducible whose only component generically consists of linearly normal curves unless $g=9$ or $g=12$. This complements the validity of the original assertion of Severi regarding the irreducibility of ${\mathcal{H}}_{d,g,r}$ outside the Brill-Nother range for $d=g+1$ and $r=4$.

17 pages.The statement and the proof of Theorem 2.6 were corrected after the referee's comments. Comments are welcome