In algebraic geometry, a morphism between schemes is said to be quasi-compact if Y can be covered by open affine subschemes such that the pre-images are quasi-compact (as topological space). If f is quasi-compact, then the pre-image of a quasi-compact open subscheme (e.g., open affine subscheme) under f is quasi-compact.
It is not enough that Y admits a covering by quasi-compact open subschemes whose pre-images are quasi-compact. To give an example, let A be a ring that does not satisfy the ascending chain conditions on radical ideals, and put . X contains an open subset U that is not quasi-compact. Let Y be the scheme obtained by gluing two X's along U. X, Y are both quasi-compact. If is the inclusion of one of the copies of X, then the pre-image of the other X, open affine in Y, is U, not quasi-compact. Hence, f is not quasi-compact.
A morphism from a quasi-compact scheme to an affine scheme is quasi-compact.
Let be a quasi-compact morphism between schemes. Then is closed if and only if it is stable under specialization.
The composition of quasi-compact morphisms is quasi-compact. The base change of a quasi-compact morphism is quasi-compact.
An affine scheme is quasi-compact. In fact, a scheme is quasi-compact if and only if it is a finite union of open affine subschemes. Serre’s criterion gives a necessary and sufficient condition for a quasi-compact scheme to be affine.
A quasi-compact scheme has at least one closed point.
- This is the definition in Hartshorne.
- Remark 1.5 in Vistoli
- Schwede, Karl (2005), "Gluing schemes and a scheme without closed points", Recent progress in arithmetic and algebraic geometry, Contemp. Math., 386, Amer. Math. Soc., Providence, RI, pp. 157–172, MR 2182775, doi:10.1090/conm/386/07222. See in particular Proposition 4.1.
- Hartshorne, Algebraic Geometry.
- Angelo Vistoli, "Notes on Grothendieck topologies, fibered categories and descent theory." arXiv:math/0412512
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