Tangent space to a functor
# 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. Let X be a scheme over a field k.

    To give a $k[\epsilon ]/(\epsilon )^{2}$-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 $({\mathfrak {m}}_{X,p}/{\mathfrak {m}}_{X,p}^{2})^{*}$; i.e., a tangent vector at p.

(To see this, use the fact that any local homomorphism ${\mathcal {O}}_{p}\to k[\epsilon ]/(\epsilon )^{2}$ must be of the form

    $\delta _{p}^{v}:u\mapsto u(p)+\epsilon v(u),\quad v\in {\mathcal {O}}_{p}^{*}.$)

Let F be a functor from the category of k-algebras to the category of sets. Then, for any k-point $p\in F(k)$, the fiber of $\pi :F(k[\epsilon ]/(\epsilon )^{2})\to F(k)$ over p is called the tangent space to F at p. 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., $F=\operatorname {Hom} _{\operatorname {Spec} k}(\operatorname {Spec} -,X)$), then each v as above may be identified with a derivation at p and this gives the identification of $\pi ^{-1}(p)$ 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. Let $T_{X}=X(k[\epsilon ]/(\epsilon )^{2})$. Then, for any morphism $f:X\to Y$ of schemes over k, one sees $f^{\#}(\delta _{p}^{v})=\delta _{f(p)}^{df_{p}(v)}$; this shows that the map $T_{X}\to T_{Y}$ that f induces is precisely the differential of f under the above identification.

