nLab algebraic space




An algebraic space is an object in the sheaf topos over the fppf-site, that has representable diagonal and an étale cover (atlas) by a scheme.

In algebraic geometry one can glue affine schemes in various topologies; this way one obtains various kinds of locally affine ringed spaces. For example, schemes locally affine in Zariski topology. Étale topology is finer than Zariski, hence the category of locally affine (ringed) spaces in étale topology is larger than the category of schemes. Algebraic spaces make a category which includes the category of all schemes and is close to the category of locally affine spaces in étale topology, namely it consists of those ringed spaces which may be obtained as a quotient of a scheme SS by an equivalence relation RS×SR\subset S\times S which is a closed subscheme, and whose projections p 1,p 2:RSp_1,p_2: R\to S are étale morphisms of schemes.


Write C fppfC_{fppf} for the fppf-site (over some scheme, as desired).


An algebraic space is

  • an object XSh(C fppf)X \in Sh(C_{fppf}) in the sheaf topos;

  • whose diagonal morphism XX×XX \to X \times X is representable;

  • and for which there exists UCU \in C and a morphism UXU \to X which is

In this form this appears as de Jong, def. 35.6.1.


Characterization as presheaf on affine schemes


Algebraic spaces are the topic of part 4 (tag 0ELT) in


Lecture notes:

  • James Milne, section 7 of Lectures on Étale Cohomology

  • G. B. Winters, An elementary lecture on algebraic spaces, in: P. Salmon (eds) Categories and Commutative Algebra. C.I.M.E. Summer Schools 58 (Varena 1971), C.I.M.E., Ed. Cremonese, Roma 1973; reprint Springer 2010, doi 2010

Definition in E-∞ geometry is in

Some related MO questions:

Last revised on April 8, 2023 at 10:24:54. See the history of this page for a list of all contributions to it.