Lemma 47.16.7. Let (A, \mathfrak m, \kappa ) be a Noetherian local ring with normalized dualizing complex \omega _ A^\bullet . Let M be a finite A-module. The following are equivalent
M is Cohen-Macaulay,
\mathop{\mathrm{Ext}}\nolimits ^ i_ A(M, \omega _ A^\bullet ) is nonzero for at most one i,
\mathop{\mathrm{Ext}}\nolimits ^{-i}_ A(M, \omega _ A^\bullet ) is zero for i \not= \dim (\text{Supp}(M)).
Denote CM_ d the category of finite Cohen-Macaulay A-modules of depth d. Then M \mapsto \mathop{\mathrm{Ext}}\nolimits ^{-d}_ A(M, \omega _ A^\bullet ) defines an anti-auto-equivalence of CM_ d.
Proof.
We will use the results of Lemma 47.16.5 without further mention. Fix a finite module M. If M is Cohen-Macaulay, then only \mathop{\mathrm{Ext}}\nolimits ^{-d}_ A(M, \omega _ A^\bullet ) can be nonzero, hence (1) \Rightarrow (3). The implication (3) \Rightarrow (2) is immediate. Assume (2) and let N = \mathop{\mathrm{Ext}}\nolimits ^{-\delta }_ A(M, \omega _ A^\bullet ) be the nonzero \mathop{\mathrm{Ext}}\nolimits where \delta = \text{depth}(M). Then, since
M[0] = R\mathop{\mathrm{Hom}}\nolimits _ A(R\mathop{\mathrm{Hom}}\nolimits _ A(M, \omega _ A^\bullet ), \omega _ A^\bullet ) = R\mathop{\mathrm{Hom}}\nolimits _ A(N[\delta ], \omega _ A^\bullet )
(Lemma 47.15.3) we conclude that M = \mathop{\mathrm{Ext}}\nolimits _ A^{-\delta }(N, \omega _ A^\bullet ). Thus \delta \geq \dim (\text{Supp}(M)). However, since we also know that \delta \leq \dim (\text{Supp}(M)) (Algebra, Lemma 10.72.3) we conclude that M is Cohen-Macaulay.
To prove the final statement, it suffices to show that N = \mathop{\mathrm{Ext}}\nolimits ^{-d}_ A(M, \omega _ A^\bullet ) is in CM_ d for M in CM_ d. Above we have seen that M[0] = R\mathop{\mathrm{Hom}}\nolimits _ A(N[d], \omega _ A^\bullet ) and this proves the desired result by the equivalence of (1) and (3).
\square
Comments (5)
Comment #3585 by Kestutis Cesnavicius on
Comment #3709 by Johan on
Comment #8491 by Haohao Liu on
Comment #8492 by Haohao Liu on
Comment #9103 by Stacks project on