Perfect complex

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 $D(A)$ 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 ${𝒪}_X$ 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 $(X,{𝒪}_X)$, an ${𝒪}_X$-module is called pseudo-coherent if for every integer $n\ge 0$, locally, there is a free presentation of finite type of length n; i.e.,

$L_n\to L_{n-1}\to \cdots \to L_0\to F\to 0$.

A complex F of ${𝒪}_X$-modules is called pseudo-coherent if, for every integer n, there is locally a quasi-isomorphism $L\to F$ where L has degree bounded above and consists of finite free modules in degree $\ge n$. 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

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