The Stacks project

Proof. By Lemma 29.15.5 an open immersion is locally of finite type. A composition of morphisms locally of finite type is locally of finite type (Lemma 29.15.3). Thus it is clear that if $X$ is J-2 then any open and any scheme locally of finite type over $X$ is J-2 as well. This proves the final statement of the lemma.

If $\mathop{\mathrm{Spec}}(R)$ is J-2, then for every finite type $R$-algebra $A$ the regular locus of the scheme $\mathop{\mathrm{Spec}}(A)$ is open. Hence $R$ is J-2, by definition (see More on Algebra, Definition 15.47.1). Combined with the remarks above we conclude that (1) implies (3), and (2) implies (4). Of course (1) $\Rightarrow$ (2) and (3) $\Rightarrow$ (4) trivially.

To finish the proof we show that (4) implies (1). Assume (4) and let $Y \to X$ be a morphism locally of finite type. We can find an affine open covering $Y = \bigcup V_ j$ such that each $V_ j \to X$ maps into one of the $U_ i$. By Lemma 29.15.2 the induced ring map $\mathcal{O}(U_ i) \to \mathcal{O}(V_ j)$ is of finite type. Hence the regular locus of $V_ j = \mathop{\mathrm{Spec}}(\mathcal{O}(V_ j))$ is open. Since $\text{Reg}(Y) \cap V_ j = \text{Reg}(V_ j)$ we conclude that $\text{Reg}(Y)$ is open as desired. $\square$