Bourbaki

Corollary 2

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

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

Let ρ:A→B\rho : A \to B be a homomorphism of noetherian rings. Then

dim⁡(B)≤dim⁡(A)+sup⁡p∈Spec(A)dim⁡(B⊗Aκ(p)). \dim(B) \leq \dim(A) + \sup_{p \in \mathrm{Spec}(A)} \dim(B \otimes_A \kappa(p)).

Let qq be a prime ideal of BB and p=ρ−1(q)p = \rho^{-1}(q). By Cor. 1, we have dim⁡(Bq)≤dim⁡(Ap)+dim⁡(Bq⊗Aκ(p))\dim(B_q) \leq \dim(A_p) + \dim(B_q \otimes_A \kappa(p)). It follows (Prop. 6, b) of § 1, No. 3) that we have the inequality dim⁡(Bq)≤dim⁡(A)+dim⁡(B⊗Aκ(p))\dim(B_q) \leq \dim(A) + \dim(B \otimes_A \kappa(p)), and one concludes by Prop. 8 of § 1, No. 3.

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

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