## 15.17 Flattening over an Artinian ring

A universal flattening exists when the base ring is an Artinian local ring. It exists for an arbitrary module. Hence, as we will see later, a flatting stratification exists when the base scheme is the spectrum of an Artinian local ring.

Lemma 15.17.1. Let $R$ be an Artinian ring. Let $M$ be an $R$-module. Then there exists a smallest ideal $I \subset R$ such that $M/IM$ is flat over $R/I$.

Proof. This follows directly from Lemma 15.16.1 and the Artinian property. $\square$

This ideal has the following universal property.

Lemma 15.17.2. Let $R$ be an Artinian ring. Let $M$ be an $R$-module. Let $I \subset R$ be the smallest ideal $I \subset R$ such that $M/IM$ is flat over $R/I$. Then $I$ has the following universal property: For every ring map $\varphi : R \to R'$ we have

$R' \otimes _ R M\text{ is flat over }R' \Leftrightarrow \text{we have }\varphi (I) = 0.$

Proof. Note that $I$ exists by Lemma 15.17.1. The implication $\Rightarrow$ follows from Algebra, Lemma 10.39.7. Let $\varphi : R \to R'$ be such that $M \otimes _ R R'$ is flat over $R'$. Let $J = \mathop{\mathrm{Ker}}(\varphi )$. By Algebra, Lemma 10.101.7 and as $R' \otimes _ R M = R' \otimes _{R/J} M/JM$ is flat over $R'$ we conclude that $M/JM$ is flat over $R/J$. Hence $I \subset J$ as desired. $\square$

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