Lemma 31.19.4. Let

\[ \xymatrix{ Z \ar[r]_ i \ar[d]_ f & X \ar[d]^ g \\ Z' \ar[r]^{i'} & X' } \]

be a fibre product diagram in the category of schemes with $i$, $i'$ immersions. Then the canonical map $f^*\mathcal{C}_{Z'/X', *} \to \mathcal{C}_{Z/X, *}$ of Lemma 31.19.3 is surjective. If $g$ is flat, then it is an isomorphism.

**Proof.**
Let $R' \to R$ be a ring map, and $I' \subset R'$ an ideal. Set $I = I'R$. Then $(I')^ n/(I')^{n + 1} \otimes _{R'} R \to I^ n/I^{n + 1}$ is surjective. If $R' \to R$ is flat, then $I^ n = (I')^ n \otimes _{R'} R$ and we see the map is an isomorphism.
$\square$

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