Ringed space

A ringed space (X, O_X) is a topological space X together with a sheaf of rings O_X on X. The sheaf O_X is called the structure sheaf of X.

A locally ringed space is a ringed space (X, O_X) such that all stalks of O_X are local rings (i.e. they have unique maximal ideals). Note that it is not required that O_X(U) be a local ring for every open set U; in fact, this is almost never the case.

An arbitrary topological space X can be considered a locally ringed space by taking O_X to be the sheaf of real-valued (or complex-valued) continuous functions on open subsets of X. The stalk at a point x can be thought of as the set of all germs of continuous functions at x; this is a local ring with the unique maximal ideal consisting of those germs whose value at x is 0.

If X is a manifold with some extra structure, we can also take the sheaf of differentiable, or complex-analytic functions. Both of these give rise to locally ringed spaces.

If X is an algebraic variety carrying the Zariski topology, we can define a locally ringed space by taking O_X(U) to be the ring of rational mappings defined on the Zariski-open set U that do not blow up (become infinite) within U. The important generalization of this example is that of the spectrum of any commutative ring; these spectra are also locally ringed spaces. Schemes are locally ringed spaces obtained by "gluing together" spectra of commutative rings.

A morphism from (X, O_X) to (Y, O_Y) is a pair (f, φ), where f: X → Y is a continuous map between the underlying topological spaces, and φ: O_Y → f_*O_X is a morphism from the structure sheaf of Y to the direct image of the structure sheaf of X. In other words, a morphism from (X, O_X) to (Y, O_Y) is given by the following data:

• a continuous map f : X → Y
• a family of ring homomorphisms φ_V : O_Y(V) → O_X(f ^-1(V)) for every open set V of Y which commute with the restriction maps. That is, if V₁ ⊂ V₂ are two open subsets of Y, then the following diagram must commute (the vertical maps are the restriction homomorphisms):

There is an additional requirement for morphisms between locally ringed spaces:

• the ring homomorphisms induced by φ between the stalks of Y and the stalks of X must be local homomorphisms, i.e. for every x ∈ X the maximal ideal of the local ring (stalk) at f(x) ∈ Y is mapped to the maximal ideal of the local ring at x ∈ X.

Two morphisms can be composed to form a new morphism, and we obtain the category of ringed spaces and the category of locally ringed spaces. Isomorphisms in these categories are defined as usual.

Locally ringed spaces have just enough structure to allow the meaningful definition of tangent spaces. Let X be locally ringed space with structure sheaf O_X; we want to define the tangent space T_x at the point x ∈ X. Take the local ring (stalk) R_x at the point x, with maximal ideal m_x. Then k_x := R_x/m_x is a field and m_x/m_x² is a vector space over that field (the cotangent space). The tangent space T_x is defined as the dual of this vector space.

The idea is the following: a tangent vector at x should tell you how to "differentiate" "functions" at x, i.e. the elements of R_x. Now it is enough to know how to differentiate functions whose value at x is zero, since all other functions differ from these only by a constant, and we know how to differentiate constants. So we only need to consider m_x. Furthermore, if two functions are given with value zero at x, then their product has derivative 0 at x, by the product rule. So we only need to know how to assign "numbers" to the elements of m_x/m_x², and this is what the dual space does.

Given a locally ringed space (X, O_X), certain sheaves of modules on X occur in the applications, the O_X-modules. To define them, consider a sheaf F of abelian groups on X. If F(U) is a module over the ring O_X(U) for every open set U in X, and the restriction maps are compatible with the module structure, then we call F an O_X-module. In this case, the stalk of F at x will be a module over the local ring (stalk) R_x, for every x∈X.

A morphism between two such O_X-modules is a morphism of sheaves which is compatible with the given module structures. The category of O_X-modules over a fixed locally ringed space (X, O_X) is an abelian category.

An important subcategory of the category of O_X-modules is the category of quasi-coherent sheaves on X. A sheaf of O_X-modules is called quasi-coherent if it is, locally, isomorphic to the cokernel of a map between free O_X-modules. A coherent sheaf F is a quasi-coherent sheaf which is, locally, of finite type and for every open subset U of X the kernel of any morphism from a free O_U-modules of finite rank to F_U is also of finite type.

• Section 0.4 of Grothendieck, Alexandre; Dieudonné, Jean (1960). "Éléments de géométrie algébrique: I. Le langage des schémas". Publications Mathématiques de l'IHÉS. 4. doi:10.1007/bf02684778. MR 0217083.
• Hartshorne, Robin (1977), Algebraic Geometry, Graduate Texts in Mathematics, 52, New York: Springer-Verlag, ISBN 978-0-387-90244-9, MR 0463157

• Onishchik, A.L. (2001) [1994], "Ringed space", Encyclopedia of Mathematics, EMS Press