Non-trivial Linear Systems on Smooth Plane Curves

Abstract

Let $C$ be a smooth plane curve of degree $d$ defined over an algebraically closed field $k$. A base point free complete very special linear system $g_n^r$ on $C$ is trivial if there exists an integer $m\ge 0$ and an effective divisor $E$ on $C$ of degree $md-n$ such that $g_n^r=|m{g}_d^2-E|$ and $r=(m^2+3m)/2-(md-n)$. In this paper, we prove the following: Theorem Let $g_n^r$ be a base point free very special non-trivial complete linear system on $C$. Write $r=(x+1)(x+2)/2-b$ with $x,b$ integers satisfying $x\ge 1,0\le b\le x$. Then $n\ge n(r):=(d-3)(x+3)-b$. Moreover, this inequality is best possible.

15 pages, LaTeX 2.09