Quantum Projective Planes Finite over their Centers

Abstract: For a $3$-dimensional quantum polynomial algebra $A=\mathcal{A}(E,\sigma)$, Artin-Tate-Van den Bergh showed that $A$ is finite over its center if and only if $|\sigma|<\infty$. Moreover, Artin showed that if $A$ is finite over its center and $E\neq \mathbb{P}^2$, then $A$ has a fat point module, which plays an important role in noncommutative algebraic geometry, however the converse is not true in general. In this paper, we will show that, if $E\neq \mathbb{P}^2$, then $A$ has a fat point module if and only if the quantum projective plane $\mathsf{Proj}_{{\rm nc}} A$ is finite over its center in the sense of this paper if and only if $|\nu^*\sigma^3|<\infty$ where $\nu$ is the Nakayama automorphism of $A$.In particular, we will show that if the second Hessian of $E$ is zero, then $A$ has no fat point module.