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

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

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

Let F be a functor from the category of k-algebras to the category of sets. Then, for any k-point ${\displaystyle p\in F(k)}$, the fiber of ${\displaystyle \pi :F(k[\epsilon ]/(\epsilon )^{2})\to F(k)}$ over p is called the tangent space to F at p.[2] The tangent space may be given the structure of a vector space over k. If F is a scheme X over k (i.e., ${\displaystyle F=\operatorname {Hom} _{\operatorname {Spec} k}(\operatorname {Spec} -,X)}$), then each v as above may be identified with a derivation at p and this gives the identification of ${\displaystyle \pi ^{-1}(p)}$ 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.[3] Let ${\displaystyle T_{X}=X(k[\epsilon ]/(\epsilon )^{2})}$. Then, for any morphism ${\displaystyle f:X\to Y}$ of schemes over k, one sees ${\displaystyle f^{\#}(\delta _{p}^{v})=\delta _{f(p)}^{df_{p}(v)}}$; this shows that the map ${\displaystyle T_{X}\to T_{Y}}$ that f induces is precisely the differential of f under the above identification.

## References

1. ^ Hartshorne 1977, Exercise II 2.8
2. ^ Eisenbud–Harris 1998, VI.1.3
3. ^ Borel 1991, AG 16.2