Twisted rings and moduli stacks of “fat” point modules in non-commutative projective geometry

The Hilbert scheme of point modules was introduced by Artin-Tate-Van den Bergh to study non-commutative graded algebras. The key tool is the construction of a map from the algebra to a twisted ring on this Hilbert scheme. In this paper, we study moduli stacks of more general “fat” point modules, and show that there is a similar map to a twisted ring associated to the stack. This is used to provide a sufficient criterion for a non-commutative projective surface to be birationally PI. It is hoped that such a criterion will be useful in understanding Mike Artin’s conjecture on the birational classification of non-commutative surfaces. Throughout, all objects and maps are assumed to be defined over some algebraically closed base field k. Also, the unadorned tensor product symbol ⊗ will be used for tensor products over k.