Bourbaki

Lemma 1

Tag 053F, permanent. Cite it as [Bourbaki, Tag 053F].

Commutative Algebra, chapter VII, § 4. Modules over integrally closed Noetherian domains

Let pp and p′p' be two prime ideals of AA such that (0) is the only prime ideal of AA contained in p∩p′p \cap p'. Then, for every sub-A-module EE of VV, (Ep)p′=K.E.(E_p)_{p'} = K.E.

Let SS be the multiplicative subset (A−p)(A−p′)(A - p)(A - p') of AA; by Chapter II, §2§ 2, no. 3, Proposition 7, (Ep)p′=S−1E(E_p)_{p'} = S^{-1}E. Further, A⊂S−1A⊂KA \subset S^{-1}A \subset K; the prime ideals of S−1AS^{-1}A correspond to the prime ideals qq of AA such that q∩S=∅q \cap S = \varnothing (Chapter II, §2§ 2, no. 5, Proposition 11) and by hypothesis (0) is the only prime ideal of AA not meeting SS; hence S−1A=KS^{-1}A = K and S−1E=K.E.S^{-1}E = K.E.

We now return to the proof of (ii). If p∈Pp \in P is distinct from the pip_i (1≤i≤h1 \leq i \leq h), Lemma 1 applied to N(pi)N(p_i) gives ((N(pi))pi)p=(K.N(pi))p=V((N(p_i))_{p_i})_p = (K.N(p_i))_p = V, since the pip_i and pp are of height 1. Then

Qp=Mp∩(N(p1))p∩⋯∩(N(ph))p=Mp=N(p) Q_p = M_p \cap (N(p_1))_p \cap \cdots \cap (N(p_h))_p = M_p = N(p)

(Chapter II, §2§ 2, no. 4). On the other hand, if pp is equal to pip_i (1≤i≤h1 \leq i \leq h), then (N(pi))pj=V(N(p_i))_{p_j} = V for i≠ji \neq j by the argument as above and (N(pi))pi=N(pi)(N(p_i))_{p_i} = N(p_i), whence

Qpi=Mpi∩N(pi)=N(pi). Q_{p_i} = M_{p_i} \cap N(p_i) = N(p_i).

We have therefore proved that Qp=N(p)Q_p = N(p) for all p∈Pp \in P. Then

N=Q∗∗=⋂p∈PQp N = Q^{**} = \bigcap_{p \in P} Q_p

is reflexive and satisfies the relations Np=Qp=N(p)N_p = Q_p = N(p) for all p∈Pp \in P; the uniqueness property follows immediately from Theorem 2 of no. 2.

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.