group scheme of multiplicative units
Let R=ℤ[X,Y]/⟨XY-1⟩. Then SpecR is an affine scheme. The natural homomorphism
ℤ→R makes R into a scheme over Specℤ, i.e. a ℤ-scheme.
What are the ℤ-points of SpecR? Recall that an S-point of a scheme X is a morphism S→X; if we are working in the category of schemes over Y, then the morphism is expected to commute with the structure morphisms. So, here, we seek homomorphisms ℤ[X,Y]/⟨XY-1⟩→ℤ. Such a homomorphism must take X to an invertible element, and it must take Y to its inverse
. Therefore there are two, one taking X to 1 and one taking X to -1. One recognizes these as the multiplicative units of ℤ, and indeed if S is any ring, then the S-points of SpecR are exactly the multiplicative units of S. For this reason, this scheme is often denoted 𝔾m. It is an example of a group scheme.
We can regard any morphism as a family of schemes, one for each fibre.
Since we have a morphism 𝔾m→ℤ, we can ask about the fibres of this morphism. If we select a point x of Specℤ, we have two choices. Such a point must be a prime ideal of ℤ, and there are two kinds: ideals generated by a prime number
, and the zero ideal
. If we select a point x with residue field
k(x), then the fiber of this morphism will be SpecR×Speck(x), which is the same as SpecR⊗k(x). But looking at the definition of R, we see that this is Speck(x)[X,Y]/⟨XY-1⟩, which is just the scheme whose points are the nonzero elements of k(x).
In other words, we have a family of schemes, one in each characteristic. Of course, normally one wants a family to have some additional sort of smoothness condition, but this demonstrates that it is quite possible to have a family of schemes in different characteristics; sometimes one can deduce the behaviour in one characteristic from the behaviour in another. This approach can be useful, for example, when dealing with Hilbert modular varieties.
Title | group scheme of multiplicative units |
---|---|
Canonical name | GroupSchemeOfMultiplicativeUnits |
Date of creation | 2013-03-22 14:09:01 |
Last modified on | 2013-03-22 14:09:01 |
Owner | mathcam (2727) |
Last modified by | mathcam (2727) |
Numerical id | 7 |
Author | mathcam (2727) |
Entry type | Example |
Classification | msc 14A15 |
Synonym | 𝔾m |
Related topic | GroupScheme |
Defines | group scheme of multiplicative units |