Subsubsection [04UX]
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(2.2.5) Let and be two -models of over . We say that and are crepant birational if there exist a normal proper -model of and morphisms of -models for such that the log pullbacks of and coincide (see [Ko13, 2.23]). Note that we can always assume that is an -model, by taking a log resolution of . The following theorem collects two fundamental results from the Minimal Model Program.
Theorem 2.2.6.
- (1)
The -scheme has a good minimal -model if and only if is semi-ample over .
- (2)
Any two good minimal -models of are crepant birational.
Proof.
(1) The condition that is semi-ample over is obviously necessary, since for every -model of , the divisor is -linearly equivalent to the restriction of to . Conversely, assume that is semi-ample over , and let be a proper -model of . Then applying [HX13, 2.12] to the -pair , we see that has a good minimal -model. Condition (1) of [HX13, 2.12] follows from our assumption, and condition (2) follows from the following observation. Let be a positive integer such that is Cartier. Over a sufficiently small open neighbourhood of in , we have an isomorphism of -algebras
where with the structural morphism. Thus it suffices to show that
is a finitely generated -algebra. If we denote by the maximum of the multiplicities of the components in then we may assume that , so that
is a pair. Hence, the finite generation of follows from [BCHM10].