# Perfect complex

(Redirected from Draft:Perfect module)

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 has finite projective dimension.

## Other characterizations

Perfect complexes are precisely the compact objects in the unbounded derived category ${\displaystyle D(A)}$ of A-modules.[1] They are also precisely the dualizable objects in this category.[2]

A compact object in the ∞-category of (say right) module spectra over a ring spectrum is often called perfect; [3] see also module spectrum.

## Pseudo-coherent sheaf

When the structure sheaf ${\displaystyle {\mathcal {O}}_{X}}$ 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 ${\displaystyle (X,{\mathcal {O}}_{X})}$, an ${\displaystyle {\mathcal {O}}_{X}}$-module is called pseudo-coherent if for every integer ${\displaystyle n\geq 0}$, locally, there is a free presentation of finite type of length n; i.e.,

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

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