Nonempty opens preserve irreducible dimension

Given: The objects and hypotheses in the statement.

If $X$ is an irreducible classical variety, then $\dim X={\mathrm{trdeg}}_{k}k(X)<\mathrm{\infty }$. Work over a fixed algebraically closed field $k$, with the Axiom of Choice. Classical varieties are separated and admit finite affine covers; they may be reducible or empty unless irreducibility is specified. Irreducible means nonempty. All fibres and points below are classical closed-point fibres and points. (Dimension equals transcendence degree).

For irreducible classical $X$, the fraction fields of all nonempty affine charts identify canonically; denote the resulting field by $k(X)$. A dominant morphism $f:X\to Y$ between irreducible classical varieties induces an injection $f^{\ast }:k(Y)\hookrightarrow k(X)$. Dominant means that the image is dense. Work over a fixed algebraically closed field $k$, with the Axiom of Choice. Classical varieties are separated and admit finite affine covers; they may be reducible or empty unless irreducibility is specified. Irreducible means nonempty. All fibres and points below are classical closed-point fibres and points. (Function fields and dominant pullbacks on general varieties).

For a classical variety $X$, let $\dim X$ be its chain dimension. If $X_1,\dots ,X_m$ are its irreducible components and $x\in X$ is a closed point, define ${\dim }_{x}X={\max }_{x\in X_i}\dim X_i$. The indexing family is nonempty. Say that $X$ has pure dimension $d$ if every irreducible component has dimension $d$; the condition on components is vacuous for the empty variety, whose dimension is nevertheless $-\mathrm{\infty }$. Work over a fixed algebraically closed field $k$, with the Axiom of Choice. Classical varieties are separated and admit finite affine covers; they may be reducible or empty unless irreducibility is specified. Irreducible means nonempty. All fibres and points below are classical closed-point fibres and points. (Global and local dimension of classical varieties).