Tangent space to a functor
In algebraic geometry, the tangent space to a functor generalizes the classical construction of a tangent space such as the Zariski tangent space. The construction is based on the following observation.[1] Let X be a scheme over a field k.
- To give a -point of X is the same thing as to give a k-rational point p of X (i.e., the residue field of p is k) together with an element of ; i.e., a tangent vector at p.
(To see this, use the fact that any local homomorphism must be of the form
- )
Let F be a functor from the category of k-algebras to the category of sets. Then, for any k-point , the fiber of over p is called the tangent space to F at p.[2] If the functor F preserves fibered products (e.g. if it is a scheme), the tangent space may be given the structure of a vector space over k. If F is a scheme X over k (i.e., ), then each v as above may be identified with a derivation at p and this gives the identification of with the space of derivations at p and we recover the usual construction.
The construction may be thought of as defining an analog of the tangent bundle in the following way.[3] Let . Then, for any morphism of schemes over k, one sees ; this shows that the map that f induces is precisely the differential of f under the above identification.
References
- ^ Hartshorne 1977, Exercise II 2.8
- ^ Eisenbud & Harris 1998, VI.1.3
- ^ Borel 1991, AG 16.2
- Borel, Armand (1991) [1969], Linear algebraic groups, Graduate Texts in Mathematics, vol. 126 (2nd ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-387-97370-8, MR 1102012
- Eisenbud, David; Harris, Joe (1998). The Geometry of Schemes. Springer-Verlag. ISBN 0-387-98637-5. Zbl 0960.14002.
- Hartshorne, Robin (1977), Algebraic Geometry, Graduate Texts in Mathematics, vol. 52, New York: Springer-Verlag, ISBN 978-0-387-90244-9, MR 0463157