Bourbaki

Corollary

Tag 0D2I, permanent. Cite it as [Bourbaki, Tag 0D2I].

Commutative Algebra, chapter X, § 2. Modules et anneaux macaulayens

Let AA be a noetherian ring and MM a finitely generated AA-module. The following conditions are equivalent: (i) the AA-module MM is Macaulay; (ii) one has profAp(Mp)=dim⁡Ap(Mp)\mathrm{prof}_{A_p}(M_p) = \dim_{A_p}(M_p) for every p∈Supp(M)p \in \mathrm{Supp}(M); (iii) one has profF(M)=codim(Supp(M)∩F′,Supp(M))\mathrm{prof}_F(M) = \mathrm{codim}(\mathrm{Supp}(M) \cap F', \mathrm{Supp}(M)) for every closed subset FF of Spec(A)\mathrm{Spec}(A); (iv) one has profA(p;M)=dim⁡Ap(Mp)\mathrm{prof}_A(p; M) = \dim_{A_p}(M_p) for every p∈Supp(M)p \in \mathrm{Supp}(M).

(i) ⇒\Rightarrow (ii): this follows from Proposition 1. (ii) ⇒\Rightarrow (iii): by prop. 8 of § 1, No. 5, profF(M)\mathrm{prof}_F(M) is the greatest lower bound of the integers prof(Mp)\mathrm{prof}(M_p) for pp ranging over Supp(M)∩F\mathrm{Supp}(M) \cap F. If MM is Macaulay, then for such an ideal pp one has the equalities prof(Mp)=dim⁡(Mp)=codim(V(p),Supp(M))\mathrm{prof}(M_p) = \dim(M_p) = \mathrm{codim}(V(p), \mathrm{Supp}(M)) (VIII, § 1, No. 4, prop. 9), whence (iii). (iii) ⇒\Rightarrow (iv): it is enough to take F=V(p)F = V(p). (iv) ⇒\Rightarrow (i): this follows from the inequalities profA(p;M)≤prof(Mp)≤dim⁡(Mp)\mathrm{prof}_A(p; M) \leq \mathrm{prof}(M_p) \leq \dim(M_p), valid for every p∈Supp(M)p \in \mathrm{Supp}(M) (§1§ 1, No. 5, remark 3 and No. 4, cor. 2 of th. 2).

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

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