The maximal genus of space curves in the range A

Abstract

Fix positive integers $d, m$ such that $\frac{m^2+4m+6}{6} \leq d \lt \frac{m^2+4m+6}{3}$ (the so-called Range A for space curves). Let $G(d, m)$ be the maximal genus of a smooth and connected degree $d$ curve $C \subset \mathbb P^3$ such that $h^0(\mathcal I_C(m-1)) = 0$. Here we prove that $G(d, m) = 1+(m-1)d -\binom{m+2}{3}$ if $m\ge 13.8\cdot 10^5$. The case $\frac{m^2+4m+6}{4} \leq d \lt \frac{m^2+4m+6}{3}$ was known by work of Fløystad [14, 15] and joint work of Ballico, Bolondi, Ellia, Mirò-Roig; see [2]. To prove the case $\frac{m^2+4m+6}{6} \leq d \lt \frac{m^2+4m+6}{4}$ we show that in this range for large $d$ every integer between $0$ and $1+(m-1)d -\binom{m+2}{3}$ is the genus of some degree $d$ smooth and connected curve $C \subset \mathbb P^3$ such that $h^0(\mathcal I_C(m-1)) = 0$.