Canonical map of low codimensional subvarieties

Abstract

Fix integers $a\ge 1$, $b$ and $c$. We prove that for certain projective varieties $V\subset {\mathbf{P}}^r$ (e.g. certain possibly singular complete intersections), there are only finitely many components of the Hilbert scheme parametrizing irreducible, smooth, projective, low codimensional subvarieties $X$ of $V$ such that $$ h^0(X,\Cal O_X(aK_X-bH_X)) \leq λd^{ε_1}+c(\sum_{1\leq h < ε_2}p_g(X^{(h)})), $$ where $d$, $K_X$ and $H_X$ denote the degree, the canonical divisor and the general hyperplane section of $X$, $p_g(X^{(h)})$ denotes the geometric genus of the general linear section of $X$ of dimension $h$, and where $\lambda$, ${\epsilon }_1$ and ${\epsilon }_2$ are suitable positive real numbers depending only on the dimension of $X$, on $a$ and on the ambient variety $V$. In particular, except for finitely many families of varieties, the canonical map of any irreducible, smooth, projective, low codimensional subvariety $X$ of $V$, is birational.

31 pages