Perfectness criterion for flop-functor equivalence

Let XXconX\to X_{\mathrm{con}} be a flopping contraction of 33-folds, with XX having at worst Cohen–Macaulay rational singularities. Let the universal sheaf of the noncommutative deformation functor associated to the contracted curves be the corresponding complex, and let Λ\Lambda and Λcon\Lambda_{\mathrm{con}} be the algebras in the contraction setup. A complex is perfect if it has finite projective dimension; in the stated equivalent formulation this is pdΛΛcon<\operatorname{pd}_{\Lambda}\Lambda_{\mathrm{con}}<\infty. Perfectness criterion for flop-functor equivalence. The flop functor is an equivalence if and only if the universal sheaf is perfect, equivalently, if and only if pdΛΛcon<\operatorname{pd}_{\Lambda}\Lambda_{\mathrm{con}}<\infty. The criterion would recover the known Bridgeland and Chen results for Gorenstein terminal singularities, where the universal sheaf is known to be perfect; the extension to the stated Cohen–Macaulay rational setting is open.

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Primary source

M. Wemyss, “Flops and Clusters in the Homological Minimal Model Program”, arXiv:1411.7189 (2017).

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