Bourbaki

Corollary 1

Tag 0CNR, permanent. Cite it as [Bourbaki, Tag 0CNR].

Commutative Algebra, chapter VIII, § 3. Dimension des anneaux noethériens

*Let A be a noetherian ring, M a finitely generated A-module, p1,...,pn,q1,...,qmp_1, ..., p_n, q_1, ..., q_m prime ideals of A and r an integer ⩾1\geqslant 1. Suppose that one has pi⊄qjp_i \not\subset q_j for 1⩽i⩽n1 \leqslant i \leqslant n and 1⩽j⩽m1 \leqslant j \leqslant m, and dim⁡Api(Mpi)⩾r\dim_{A_{p_i}}(M_{p_i}) \geqslant r for 1⩽i⩽n1 \leqslant i \leqslant n. Then there exists a sequence (x1,...,xr)(x_1, ..., x_r) of elements of A belonging to all the pip_i and belonging to none of the qjq_j, such that, for 1⩽i⩽n1 \leqslant i \leqslant n, the images of x1,...,xrx_1, ..., x_r in ApiA_{p_i} form a secant sequence for the module MpiM_{p_i}.

Put α=⋂ipi\alpha = \bigcap_i p_i. One has Mpi≠{0}M_{p_i} \neq \{0\}, hence pi∈Supp⁡(M)p_i \in \operatorname{Supp}(M) for 1⩽i⩽n1 \leqslant i \leqslant n, whence V(α)⊂Supp⁡(M)V(\alpha) \subset \operatorname{Supp}(M). One has

codim⁡(V(α)∩Supp⁡(M),Supp⁡(M))=codim⁡(V(α),Supp⁡(M))==inf⁡i(codim⁡(V(pi),Supp⁡(M)))=inf⁡idim⁡(Mpi)⩾r \operatorname{codim}(V(\alpha) \cap \operatorname{Supp}(M), \operatorname{Supp}(M)) = \operatorname{codim}(V(\alpha), \operatorname{Supp}(M)) = = \inf_i (\operatorname{codim}(V(p_i), \operatorname{Supp}(M))) = \inf_i \dim(M_{p_i}) \geqslant r

(§ 1, No. 4, Prop. 9), and Proposition 9 is applied.

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

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