Bourbaki

Corollary 5

Tag 0E1N, permanent. Cite it as [Bourbaki, Tag 0E1N].

Algebra, chapter X, § 8. Dimension homologique

*Suppose the ring A0A_0 semisimple. If Tor⁡n+1A(A0,A0)=0\operatorname{Tor}_{n+1}^A(A_0, A_0) = 0, one has dpA(M)≤n\mathrm{dp}_A(M) \leq n for every graded AA-module bounded below.

Denote by A∘A^\circ the graded ring opposite to AA: we have (A∘)0=(A0)∘(A^\circ)_0 = (A_0)^\circ, so (A∘)0(A^\circ)_0 is semisimple (VIII, § 5, No. 1, remark 3). Since Tor⁡n+1A∘(A0∘,A0∘)=0\operatorname{Tor}_{n+1}^{A^\circ}(A_0^\circ, A_0^\circ) = 0, one has dpA(A0s∘)≤n\mathrm{dp}_A(A_{0s}^\circ) \leq n by cor. 4: this implies Tor⁡n+1A∘(M0,A0∘)=0\operatorname{Tor}_{n+1}^{A^\circ}(M_0, A_0^\circ) = 0 for every AA-module MM, hence Tor⁡n+1A(A0,M)=0\operatorname{Tor}_{n+1}^A(A_0, M) = 0 and we apply cor. 4.

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

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