In mathematics, specifically in algebraic geometry, the fiber product of schemes is a fundamental construction. It has many interpretations and special cases. For example, the fiber product describes how an algebraic variety over one field determines a variety over a bigger field, or the pullback of a family of varieties, or a fiber of a family of varieties. Base change is a closely related notion.
Definition
The category of schemes is a broad setting for algebraic geometry. A fruitful philosophy is that much of algebraic geometry should be developed for a morphism of schemesX → Y, rather than for a single scheme X. For example, rather than simply studying algebraic curves, one can study families of curves over any base scheme Y. Indeed, the two approaches enrich each other. In particular, a scheme over a commutative ringR means a scheme X together with a morphism X → Spec. The older notion of an algebraic variety over a field k is equivalent to a scheme over k with certain properties. In general, a morphism of schemes X → Y can be imagined as a family of schemes parametrized by the points of Y. Given a morphism from some other scheme Z to Y, there should be a "pullback" family of schemes over Z. This is exactly the fiber product X xYZ → Z. Formally: it is a useful property of the category of schemes that the fiber product always exists. That is, for any morphisms of schemes X → Y and Z → Y, there is a scheme X ×YZ with morphisms to X and Z, making the diagram commutative, and which is universal with that property. That is, for any scheme W with morphisms to X and Z whose compositions to Y are equal, there is a unique morphism from W to X ×YZ that makes the diagram commute. As always with universal properties, this condition determines the scheme X ×YZ up to a unique isomorphism, if it exists. The proof that fiber products of schemes always do exist reduces the problem to the tensor product of commutative rings. In particular, when X, Y, and Z are all affine schemes, so X = Spec, Y = Spec, and Z = Spec for some commutative ringsA,B,C, the fiber product is the affine scheme The morphism X xYZ → Z is called the base change or pullback of the morphism X → Y via the morphism Z → Y.
Interpretations and special cases
In the category of schemes over a field k, the productX × Y means the fiber product X ×kY. For example, the product of affine spaces Am and An over a field k is the affine space Am+n over k.
For a scheme X over a field k and any field extensionE of k, the base changeXE means the fiber product X ×Spec Spec. Here XE is a scheme over E. For example, if X is the curve in the projective planeP over the real numbersR defined by the equation xy2 = 7z3, then XC is the complex curve in P defined by the same equation. Many properties of an algebraic variety over a field k can be defined in terms of its base change to the algebraic closure of k, which makes the situation simpler.
Let f: X → Y be a morphism of schemes, and let y be a point in Y. Then there is a morphism Spec → Y with imagey, where k is the residue field of y. The fiber of f over y is defined as the fiber product X ×Y Spec; this is a scheme over the fieldk. This concept helps to justify the rough idea of a morphism of schemes X → Y as a family of schemes parametrized by Y.
Let X, Y, and Z be schemes over a field k, with morphisms X → Y and Z → Y over k. Then the set of k-rational points of the fiber product X xYZ is easy to describe:
If X and Z are closed subschemes of a scheme Y, then the fiber product X xYZ is exactly the intersectionX ∩ Z, with its natural scheme structure. The same goes for open subschemes.
Base change and descent
Some important properties P of morphisms of schemes are preserved under arbitrary base change. That is, if X → Y has property P and Z → Y is any morphism of schemes, then the base change X xYZ → Z has property P. For example, flat morphisms, smooth morphisms, proper morphisms, and many other classes of morphisms are preserved under arbitrary base change. The word descent refers to the reverse question: if the pulled-back morphism X xYZ → Z has some property P, must the original morphism X → Y have property P? Clearly this is impossible in general: for example, Z might be the empty scheme, in which case the pulled-back morphism loses all information about the original morphism. But if the morphism Z → Y is flat and surjective and quasi-compact, then many properties do descend from Z to Y. Properties that descend include flatness, smoothness, properness, and many other classes of morphisms. These results form part of Grothendieck's theory offaithfully flat descent. Example: for any field extension k ⊂ E, the morphism Spec → Spec is faithfully flat and quasi-compact. So the descent results mentioned imply that a scheme X over k is smooth over kif and only if the base change XE is smooth over E. The same goes for properness and many other properties.
