Lemma 15.40.3 (Regular maps and base change). Let $R \to \Lambda $ be a regular ring map. For any finite type ring map $R \to R'$ the base change $R' \to \Lambda \otimes _ R R'$ is regular too.
Proof. Flatness is preserved under any base change, see Algebra, Lemma 10.38.7. Consider a prime $\mathfrak p' \subset R'$ lying over $\mathfrak p \subset R$. The residue field extension $\kappa (\mathfrak p) \subset \kappa (\mathfrak p')$ is finitely generated as $R'$ is of finite type over $R$. Hence the fibre ring
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