Bourbaki

Corollary

Tag 04L3, permanent. Cite it as [Bourbaki, Tag 04L3].

Commutative Algebra, chapter IV, § 2. Primary decomposition

If M is a module of finite length over a Noetherian ring A, then

long⁡A(M)=∑p∈Ass⁡(M)long⁡Ap(Mp). \operatorname{long}_A(M) = \sum_{p \in \operatorname{Ass}(M)} \operatorname{long}_{A_p}(M_p).

This will follow from Proposition 8 (iv) if we prove that

long⁡A(M/Q(p))=long⁡Ap(Mp). \operatorname{long}_A(M/\mathscr{Q}(p)) = \operatorname{long}_{A_p}(M_p).

Now, it follows from Proposition 1 of no. 1 that for all s∈A−ps \in A - p the homothety with ratio s on M/Q(p)M/\mathscr{Q}(p) is injective; the homothety with ratio s on every submodule R of M/Q(p)M/\mathscr{Q}(p) is therefore injective and, as R is Artinian, it is bijective (Algebra, Chapter VIII, § 2, no. 2, Lemma 3); we conclude that the sub-A-modules of M/Q(p)M/\mathscr{Q}(p) are the images under the bijection f:M,→M/Q(p)f : M, \to M/\mathscr{Q}(p) of the sub-A,-modules of M, (Chapter 11, § 2, no. 3), whence our assertion.

Bourbaki puts the proof directly after the statement, so what is above is the statement and its proof together.

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Cites

Transcribed from the printed volume. Not translated.