# Varieties I: affine varieties

This post contains the defintiion of affine varieties, regular functions, coordinate ring, the Nullstellensatz, and the quotient of an affine variety upon action of a finite group.

**Affine varieties**

Let be a field. It is clear what points in , polynomial functions on etc mean.

Definition 1An affine variety is a subset of , that is the common zeros of a set of polynomials.

**Remark**

- The set of polynomials can be taken to be finite. This is because is Noetherian (Hilbert basis theorem), so the ideal generated by a set of polynomials is finitely generated, and their common zero set are the same.
- As a variety, is more often denoted by .

**Some terminologies**

1. **the Zariski topology**

This formalism allows us to talk about varieties more easily. Consider the topology on with the closed sets being affine varieties. It is easy to verify that this is a topology:

- ,
- Union of two closed sets:
- Intersection of closed sets: just put all the equations together.

2. **the ideal of X **

This is the ideal of all the polynomials in that vanish on .

3. **the affine coordinate ring **

Consider as the coordinates of the affine space , and restrict it on . If we allow free multiplication and addition, this is what we will get.

**The Nullstellensatz**

It is clear that when we have a subset , we can define the corresponding ideal exactly like above. On the other hand, given an ideal , we can define to be the common zeroes of all .

Some natural consequences:

- If , then
- If are ideals of , then
- If is an ideal of ,
- If , then (closure in Zariski topology)

Only the proof of is nontrivial. It is clear that . For the other side, suppose that is Zariski closed and . Then , thus . This shows that is the smallest Zariski-closed set containing .

If is an equality, then we can establish a one-one correspondence between the affine varieties in and the ideals of . This is desirable because as in the case manifolds, we want to define “varieties” without an embedding into an affine space. As we shall see, varieties are isomorphic iff their affine coordinate rings are isomorphic as -algebras. Thus once we have this correspondence, we can encode the information of points into the affine coordinate ring as the first step of the definition of “varieties”.

This correspondence exists when is algebraically closed.

Theorem 1 (Hilbert’s Nullstellensatz)Let , where is algebraically closed field. Then .

In other words, if vanishes on the common zeros of an ideal , then for some , .

**Regular functions**

Basically we want to deal with polynomial maps. However, when we restrict on affine varieties, there are some more possibilities. For example, if is an affine variety, and is a polynomial that never vanish on , then makes sense for any polynomial .

However, the following proposition tells us that allowing such division does not enlarge the class of maps concerned, when is algebraically closed.

Proposition 1Suppose that is a polynomial that never vanishes on an affine variety , where is algebraically closed. Then is a unit in .

**Proof**

Let . As never vanishes on , have no common zeros. By the Nullstellensatz, . This means that there exists polynomials such that

Modulo gives the desired result.

Thus maps of the form on can also be represented by , i.e. we are still working with polynomial maps.

Definition 2 (Regular functions)Let be an affine variety. A regular function is a polynomial map from to .

It is clear that the regular functions form a ring, which is exactly , the affine coordinate ring.

**Maps between affine varieties**

Naturally, just as in the case for differentiable functions,

Definition 2For two affine varieties , , a map is calledregularif are regular functions on .

This immediately gives us a notion of **isomorphism**: Two affine varieties and are isomorphic if there exists regular maps and such that and .

**Regular maps and ring of regular functions**

Notice that composition of regular maps is still regular. In particular a regular map induces a -algebra homomorphism .

On the other hand, a -algebra homomorphism has to come from a regular map. For are the coordinates of , and tells you what they are in terms of . Thus consider defined by

Then one can see that . In fact we have

Proposition 2For two affine varieties and , we have anaturalisomorphism

Corollary 1Two affine varieties and are isomorphic if and only if is isomorphic to as -algebra.

**Examples**

1. is an affine variety.

2. is an affine variety.

3. (Let be algebrically closed) The map defined by is regular. It is regular, bijective, yet not an isomorphism, since a polynomial in and can’t possibly give .

**What algebras are coordinate rings?**

This is answered by the following corollary of the Nullstellensatz.

Corollary 2Let be algebraically closed. The association is a 1-1 correspondence betweenaffine subvarieties ofandradical ideals of. Moreover, is irreducible iff is prime.

Recall that a topological space is irreducible if it cannot be written as a union of two proper closed subsets.

**Proof**

The 1-1 correspondence was established already.

Suppose that is reducible, i.e. , being closed and proper. Then . If is prime, then or , meaning or , contradiction. Therefore if is reducible, is not prime.

If is not prime, let but . (i.e. vanishes entirely on , but neither or do). Then , and both are proper, closed subsets. So if is not prime, is reducible.

Corollary 3Let be algebraically closed. Then an -algebra is an affine coordinate ring if and only if it is finitely generated over and admits no nilpotents.

**Quotient by group action**

(*Assumption: * *is an algebraically closed field*)

Let be an affine variety, and be a finite group of automorphisms of . can also be realized as a group of -automorphisms of . We want to give the -orbits a variety structure.

What is the natural definition of this? Well, mimicking quotient topology, we would want a natural regular map if is the variety representing the -orbits. As we already know that the affine coordinate ring is an invariant, we can first study the functions on , and try to see if there is any nice -algebra homomorphism .

The natural class of functions on should be those functions on , who have consistent value on each -orbit. This is exactly , the -invariants of .

The first question is, can possibly be an affine coordinate ring? It is clear that has no nilpotents, being a subalgebra of .

Proposition 3Let be the order of . Suppose that , then is a finitely generated -algebra.

We have a natural inclusion map , which tells us that if is an affine variety such that , then we have a map induced by the above inclusion. We then want to see if really represents the orbits.

Proposition 4are in the same -orbit if and only if . Furthermore, is onto.

**Proof of proposition 3**

Define the averaging operator

for .This is a map from to .

Let be the generators of , this means that for arbitrary , we have an expression

where . Since are not fixed by , we hope that we can get something after averaging. Yet does NOT behave well with respect to multiplication.

So first let us deal with the simple case, WLOG, . By definition,

So this is like a power sum. Then by Newton’s identity, we would want to consider the polynomial . The coeffcients of the polynomial lie in , and can be expressed as a sum of (coefficients of * ), where , the order of .

Learning from the monomial case, we tackle the case of a general multinomial in the same fashion. WLOG consider , with . Using the same idea, we see that it can be expressed as the sum of (coefficients of * ), where , the order of .

This shows that in general, an arbitrary monomial can be written as the sum of (coefficients of * ), where , the order of . Notice that is additive, and that there are only finitely many elements in to be of the form (coefficients of * ), where , we see that after taking , these are the desired generators of .

**Proof of proposition 4**

(I can’t find a good proof for surjectivity)

If are in the same -orbit,

recall that how is constructed. We take a set of generators of , embed it into , and use these regular functions to define . This means that the coordinate functions are all -invariant. Therefore for are in the same orbit.

If are not in the same -orbit,

we want to show that for some set of generators of , one of them would have different values at and . This is equivalent to finding a -invariant polynomial that has different values at and .

Construct a polynomial such that and for all . (This can be done using Lagrange interpolation, analogous to the one-variable case) Then we symmetrize it by considering

giving us one of the desired polynomials that separate and .

For surjectivity, one can proceed using commutative algebra, but I still can’t see conceptually why it is onto.

The inclusion is integral, because for any , the polynomial is one integral polynomial for with coefficients in . It is not difficult to show that is a closed map. Since is an inclusion, the Spec map is dense, thus onto.

For another proof of Proposition 3, which uses integrality (a fact you observe at the end of the proof of Prop. 4), see e.g. my post at http://deltaepsilons.wordpress.com/2009/08/11/integrality-invariant-theory-for-finite-groups-and-more-tools-for-noetherian-testing/.

Cool, thanks!