Asymptotic constructions and invariants of graded linear series

Abstract

Let $X$ be a complete variety of dimension $n$ over an algebraically closed field $\mathbf{K}$. Let $V_{\bullet }$ be a graded linear series associated to a line bundle $L$ on $X$, that is, a collection $\{V_m{\}}_{m\in \mathbb{N}}$ of vector subspaces $V_m\subseteq H^0(X,L^{\otimes m})$ such that $V_0=\mathbf{K}$ and $V_k\cdot V_{\ell }\subseteq V_{k+\ell }$ for all $k,\ell \in \mathbb{N}$. For each $m$ in the semigroup \[ \mathbf{N}(V_\bullet)=\{m\in\mathbb{N}\mid V_m\ne 0\},\] the linear series $V_m$ defines a rational map \[ ϕ_m\colon X\dashrightarrow Y_m\subseteq\mathbb{P}(V_m), \] where $Y_m$ denotes the closure of the image ${\varphi }_m(X)$. We show that for all sufficiently large $m\in \mathbf{N}(V_{\bullet })$, these rational maps ${\varphi }_m:X⇢Y_m$ are birationally equivalent, so in particular $Y_m$ are of the same dimension $\kappa$, and if $\kappa =n$ then ${\varphi }_m:X⇢Y_m$ are generically finite of the same degree. If $\mathbf{N}(V_{\bullet })\ne \{0\}$, we show that the limit \[ \operatorname{vol}_κ(V_\bullet)=\lim_{m\in \mathbf{N}(V_\bullet)}\frac{\dim_\mathbf{K} V_m}{m^κ/κ!}\] exists, and $0<{\mathrm{vol}}_{\kappa }(V_{\bullet })<\infty$. Moreover, if $Z\subseteq X$ is a general closed subvariety of dimension $\kappa$, then the limit \[ (V_\bullet^κ\cdot Z)_\text{mov}=\lim_{m\in \mathbf{N}(V_\bullet)}\frac{\#\bigl((D_{m,1}\cap\cdots\cap D_{m,κ}\cap Z)\setminus \operatorname{Bs}(V_m)\bigr)}{m^κ}\] exists, where $D_{m,1},\dots ,D_{m,\kappa }\in |V_m|$ are general divisors, and \[ (V_\bullet^κ\cdot Z)_\text{mov}=\operatorname{deg}\bigl(ϕ_m|_Z\colon Z\dashrightarrow ϕ_m(Z)\bigr)\operatorname{vol}_κ(V_\bullet) \] for all sufficiently large $m\in \mathbf{N}(V_{\bullet })$.