The coordinate cross has two one-dimensional components

Given: The objects and hypotheses in the statement.

If a Noetherian space $T$ is a finite union of closed subsets $T_1,\dots ,T_m$, then $\dim T={\max }_i\dim T_i$. For $m=0$ both sides are $-\mathrm{\infty }$. (Dimension of a finite closed union).

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).

For every integer $n\ge 0$, $\dim {\mathbf{A}}_k^n=\dim {\mathbf{P}}_k^n=n$. 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. (Affine and projective n-space have dimension n).