On the Kodaira dimension of maximal orders

Abstract

Let $\kk$ be an algebraically closed field of characteristic zero and $\KK$ a finitely generated field over $\kk$. Let $Σ$ be a central simple $\KK$-algebra, $X$ a normal projective model of $\KK$ and $Λ$ a sheaf of maximal $\Osh_X$-orders in $Σ$. There is a ramification $\QQ$-divisor $Δ$ on $X$, which is related to the canonical bimodule $ω_Λ$ by an adjunction formula. It only depends on the class of $Σ$ in the Brauer group of $\KK$. When the numerical abundance conjecture holds true, or when $Σ$ is a central simple algebra, we show that the Gelfand-Kirillov dimension (or GK dimension) of the canonical ring of $Λ$ is one more than the Iitaka dimension (or D-dimension) of the log pair $(X,Δ)$. In the case that $Σ$ is a division algebra, we further show that this GK dimension is also one more than the transcendence degree of the division algebra of degree zero fractions of the canonical ring of $Λ$. We prove that these dimensions are birationally invariant when the b-log pair determined by the ramification divisor has b-canonical singularities. In that case we refer to the Iitaka (or D-dimension) of $(X,Δ)$ as the Kodaira dimension of the order $Λ$. For this, we establish birational invariance of the Kodaira dimension of b-log pairs with b-canonical singularities. We also show that the Kodaira dimension can not decrease for an embedding of central simple algebras, finite dimensional over their centres, which induces a Galois extension of their centres, and satisfies a condition on the ramification which we call an effective embedding. For example, this condition holds if the target central simple algebra has the property that its period equals its index.