Lemma 15.41.3 (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 10.39.7. Consider a prime $\mathfrak p' \subset R'$ lying over $\mathfrak p \subset R$. The residue field extension $\kappa (\mathfrak p')/\kappa (\mathfrak p)$ is finitely generated as $R'$ is of finite type over $R$. Hence the fibre ring

is Noetherian by Algebra, Lemma 10.31.8 and the assumption on the fibre rings of $R \to \Lambda $. Geometric regularity of the fibres is preserved by Algebra, Lemma 10.166.1. $\square$

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