Lemma 10.168.4. Let $A = \mathop{\mathrm{colim}}\nolimits _{i \in I} A_ i$ be a directed colimit of rings. Let $0 \in I$ and $\varphi _0 : B_0 \to C_0$ a map of $A_0$-algebras. Assume
$A \otimes _{A_0} B_0 \to A \otimes _{A_0} C_0$ is surjective,
$C_0$ is of finite type over $B_0$.
Then for some $i \geq 0$ the map $A_ i \otimes _{A_0} B_0 \to A_ i \otimes _{A_0} C_0$ is surjective.
Proof. Let $x_1, \ldots , x_ m$ be generators for $C_0$ over $B_0$. Pick $b_ j \in A \otimes _{A_0} B_0$ mapping to $1 \otimes x_ j$ in $A \otimes _{A_0} C_0$. For some $i \geq 0$ we can find $b_{j, i} \in A_ i \otimes _{A_0} B_0$ mapping to $b_ j$. After increasing $i$ we may assume that $b_{j, i}$ maps to $1 \otimes x_ j$ in $A_ i \otimes _{A_0} C_0$ for all $j = 1, \ldots , m$. Then this $i$ works. $\square$
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