In algebra, a perfect complex of modules over a commutative ring A is an object in the derived category of A-modules that is quasi-isomorphic to a bounded complex of finite projective A-modules. A perfect module is a module that is perfect when it is viewed as a complex concentrated at degree zero. For example, if A is Noetherian, a module over A is perfect if and only if it is finitely generated and of finite projective dimension.
Other characterizations Perfect complexes are precisely the compact objects in the unbounded derived category D ( A ) {\displaystyle D(A)} of A-modules. They are also precisely the dualizable objects in this category. A compact object in the ∞-category of (say right) module spectra over a ring spectrum is often called perfect; see also module spectrum.
Pseudo-coherent sheaf When the structure sheaf O X {\displaystyle {\mathcal {O}}_{X}} is not coherent, working with coherent sheaves has awkwardness (namely the kernel of a finite presentation can fail to be coherent). Because of this, SGA 6 Expo I introduces the notion of a pseudo-coherent sheaf. By definition, given a ringed space ( X , O X ) {\displaystyle (X,{\mathcal {O}}_{X})} , an O X {\displaystyle {\mathcal {O}}_{X}} -module is called pseudo-coherent if for every integer n ≥ 0 {\displaystyle n\geq 0} , locally, there is a free presentation of finite type of length n; i.e.,
L n → L n − 1 → ⋯ → L 0 → F → 0 {\displaystyle L_{n}\to L_{n-1}\to \cdots \to L_{0}\to F\to 0} . A complex F of O X {\displaystyle {\mathcal {O}}_{X}} -modules is called pseudo-coherent if, for every integer n, there is locally a quasi-isomorphism L → F {\displaystyle L\to F} where L has degree bounded above and consists of finite free modules in degree ≥ n {\displaystyle \geq n} . If the complex consists only of the zero-th degree term, then it is pseudo-coherent if and only if it is so as a module. Roughly speaking, a pseudo-coherent complex may be thought of as a limit of perfect complexes.
See also Hilbert–Burch theorem Elliptic complex (related notion; discussed at SGA 6 Exposé II, Appendix II.)
References
Ben-Zvi, David; Francis, John; Nadler, David (2010), "Integral transforms and Drinfeld centers in derived algebraic geometry", Journal of the American Mathematical Society, 23 (4): 909–966, arXiv:0805.0157, doi:10.1090/S0894-0347-10-00669-7, MR 2669705, S2CID 2202294
Bibliography Berthelot, Pierre; Alexandre Grothendieck; Luc Illusie, eds. (1971). Séminaire de Géométrie Algébrique du Bois Marie - 1966-67 - Théorie des intersections et théorème de Riemann-Roch - (SGA 6) (Lecture notes in mathematics 225). Lecture Notes in Mathematics (in French). Vol. 225. Berlin; New York: Springer-Verlag. xii+700. doi:10.1007/BFb0066283. ISBN 978-3-540-05647-8. MR 0354655. Kerz, Moritz; Strunk, Florian; Tamme, Georg (2018). "Algebraic K-theory and descent for blow-ups". Inventiones Mathematicae. 211 (2): 523–577. arXiv:1611.08466. Bibcode:2018InMat.211..523K. doi:10.1007/s00222-017-0752-2. Lurie, Jacob (2014). "Algebraic K-Theory and Manifold Topology (Math 281), Lecture 19: K-Theory of Ring Spectra" (PDF).
External links "Determinantal identities for perfect complexes". MathOverflow. "An alternative definition of pseudo-coherent complex". MathOverflow. "15.74 Perfect complexes". The Stacks project. "perfect module". ncatlab.org.
