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Concept
Fiber product of schemes
Summary
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.
The of schemes is a broad setting for algebraic geometry. A fruitful philosophy (known as Grothendieck's relative point of view) is that much of algebraic geometry should be developed for a morphism of schemes X → Y (called a scheme X over 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 ring R means a scheme X together with a morphism X → Spec(R). The older notion of an algebraic variety over a field k is equivalent to a scheme over k with certain properties. (There are different conventions for exactly which schemes should be called "varieties". One standard choice is that a variety over a field k means an integral separated scheme of finite type over k.)
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 ×Y Z → Z.
Formally: it is a useful property of the category of schemes that the always exists. That is, for any morphisms of schemes X → Y and Z → Y, there is a scheme X ×Y Z 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 ×Y Z that makes the diagram commute. As always with universal properties, this condition determines the scheme X ×Y Z up to a unique isomorphism, if it exists.
Official source
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