Singular Curves of Low Degree and Multifiltrations from Osculating Spaces

Abstract

In order to study projections of smooth curves, we introduce multifiltrations obtained by combining flags of osculating spaces. We classify all configurations of singularities occurring for a projection of a smooth curve embedded by a complete linear system away from a projective linear space of dimension at most two. In particular, we determine all configurations of singularities of non-degenerate degree d rational curves in ${\mathbb{P}}^n$ when $d-n\le 3$ and $d<2n$. Along the way, we describe the Schubert cycles giving rise to these projections. We also reprove a special case of the Castelnuovo bound using these multifiltrations: under the assumption $d<2n$, the arithmetic genus of any nondegenerate degree $d$ curve in ${\mathbb{P}}^n$ is at most $d-n$.

34 pages, 11 tables, 2 figures; v2 added references and made minor corrections; v3 more minor revisions, to appear in IMRN