Disconnected multigraded Hilbert schemes on $\mathbb{P}^2\times\mathbb{P}^1$

Abstract

We exhibit an infinite family of disconnected multigraded Hilbert schemes on the biprojective space $\mathbb{P}^2_{\mathbb{k}} \times_\mathbb{k} \mathbb{P}^1_{\mathbb{k}}$. More precisely, for any integer $a \ge 2$ and $p_a(z_1,z_2) = 2az_1+az_2+3a-2a^2 \in \mathbb{Q}[z_1,z_2]$, the multigraded Hilbert scheme ${\rm Hilb}_{p_a}(\mathbb{P}^2_{\mathbb{k}}\times_\mathbb{k} \mathbb{P}^1_{\mathbb{k}})$ is disconnected. As a consequence, there exist infinitely many disconnected Haiman-Sturmfels multigraded Hilbert schemes even for a standard bigrading on a polynomial ring in only five variables.