
Lemma 10.127.3. Let $R \to S$, $M$, $\Lambda$, $R_\lambda \to S_\lambda$, $M_\lambda$ be as in Lemma 10.126.13. Assume that $M$ is flat over $R$. Then for some $\lambda \in \Lambda$ the module $M_\lambda$ is flat over $R_\lambda$.

Proof. Pick some $\lambda \in \Lambda$ and consider

$\text{Tor}_1^{R_\lambda }(M_\lambda , R_\lambda /\mathfrak m_\lambda ) = \mathop{\mathrm{Ker}}(\mathfrak m_\lambda \otimes _{R_\lambda } M_\lambda \to M_\lambda ).$

See Remark 10.74.9. The right hand side shows that this is a finitely generated $S_\lambda$-module (because $S_\lambda$ is Noetherian and the modules in question are finite). Let $\xi _1, \ldots , \xi _ n$ be generators. Because $M$ is flat over $R$ we have that $0 = \mathop{\mathrm{Ker}}(\mathfrak m_\lambda R \otimes _ R M \to M)$. Since $\otimes$ commutes with colimits we see there exists a $\lambda ' \geq \lambda$ such that each $\xi _ i$ maps to zero in $\mathfrak m_{\lambda }R_{\lambda '} \otimes _{R_{\lambda '}} M_{\lambda '}$. Hence we see that

$\text{Tor}_1^{R_\lambda }(M_\lambda , R_\lambda /\mathfrak m_\lambda ) \longrightarrow \text{Tor}_1^{R_{\lambda '}}(M_{\lambda '}, R_{\lambda '}/\mathfrak m_{\lambda }R_{\lambda '})$

is zero. Note that $M_\lambda \otimes _{R_\lambda } R_\lambda /\mathfrak m_\lambda$ is flat over $R_\lambda /\mathfrak m_\lambda$ because this last ring is a field. Hence we may apply Lemma 10.98.14 to get that $M_{\lambda '}$ is flat over $R_{\lambda '}$. $\square$

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