We have already seen two forms of unique factorization.

- In a UFD, every non-zero element is a unique product of irreducible (also prime) elements.
- In a Dedekind domain, every non-zero ideal is a unique product of maximal ideals.

Here, we will introduce yet another type of factorization, called *primary decomposition*. The main idea is that in a noetherian ring, every ideal (even the zero ideal) is an intersection of *primary ideals*. E.g. in , every non-zero ideal is an intersection of , ideals generated by prime powers.

# Annihilators

Let *A* be a fixed ring and *M* be an *A*-module.

Definition.The

annihilatorof in A is,

an ideal of A. Similarly, the

annihilatorof M in A is,

also an ideal of A. If , we say M is a

faithfulA-module.

Note that if , then *M* can be regarded as a faithful -module.

**Exercise A**

1. Given *A*-submodules , let

.

State and prove the analogue of proposition 2 here for submodules of *M*. Observe that we can write the annihilators as

Conversely, express in terms of annihilators.

2. Prove that if *P *is a finitely generated submodule of *M*.

In particular if *M* is finitely generated then

.

# Support of a Module

Definition.The

supportof an A-module M is:

Geometrically, these are points in Spec *A* at which the module does not vanish.

Proposition 1.If M is a finitely generated A-module, then

In particular, the support of M is a closed subspace of .

**Proof**

Let . For we have is zero if and only if there exists such that , equivalently if there exists . Hence if and only if .

In the general case, let generate *M* as an *A*-module. Then

For the last equivalence, recall that contains if and only if it contains either or . ♦

Proposition 2.If is a short exact sequence of A-modules, then .

If N, P are finitely generated A-modules, then

.

**Note**

Philosophically, if we imagine the first case as *N* + *P* and the second case as *N* × *P*, then the result says if and only if both terms are zero whereas if and only if at least one term is zero.

**Proof**

Let be prime.

For the first claim, we have a short exact sequence and it follows that if and only if .

For the second claim, we have

If or , clearly the RHS is zero. Conversely if both are non-zero, since they are finitely generated -modules, Nakayama’s lemma gives and . But these are vector spaces over so we have

♦

**Exercise B**

1. Let be a homomorphism of finitely generated *A*-modules; prove that is a closed subset of Spec A.

2. Prove: if is multiplicative then

# Associated Primes

Definition.Let be a prime ideal. We say is

associatedto the A-module M if for some .Let be the set of prime ideals of A associated to M.

**Note**

We have: if and only if there is an injective *A*-linear map . Observe that if the annihilator of is , then so is that of any non-zero multiple of *m*; this follows immediately from the definition of prime ideals.

Next suppose *M* is an -module; we can compute both and . There is a bijection

.

In particular, we can compute by considering as an *A*-module or a module over itself. The above shows that there is effectively no difference.

Proposition 3.Suppose A is noetherian. The union of all associated primes of M is its set of zero-divisors

.

In particular, any non-zero A-module M has an associated prime.

**Proof**

Fix an and such that ; we need to show *a* is contained in an associated prime of *M*.

So let be the set of ideals of *A* containing *a* of the form for . Since *A* is noetherian and , there is a maximal . We will show is prime; first write for some .

Pick such that and . Since , we have . And since , by maximality of we have so since we have . ♦

Henceforth, *A* denotes a noetherian ring. *However, we do not assume all modules are finitely generated at first.*

# Properties of Associated Primes

Proposition 4.Let be a short exact sequence of A-modules. Then

**Proof**

We may assume is a submodule and . The first containment in the claim is obvious. For the second, suppose we have an *A*-linear map .

If , then any non-zero has annihilator so . If , then composing is still injective, so . ♦

Corollary 1.We have .

**Proof**

Follows immediately from proposition 4. ♦

Next, we show that taking the set of associated primes commutes with localization.

Proposition 5.Let be a multiplicative subset. Then

**Proof**

(⊇) If and , then localization at *S* gives an *A*-linear map .

(⊆) Suppose we have an -linear map where is prime. By theorem 1 here, we can write for some prime such that . Now in the injection we let be the image of 1. It remains to show: there exists such that .

For each , we have in so that in *M* for some , i.e. . Pick a generating set of ; then for each *i* there exists with . Then satisfies .

Conversely if then so lies in the annihilator of , i.e. in . Then ; since we have . ♦

How to conclude that in the last line in proposition 2?

Recall that thus the module is obtained by quotienting the former module (i.e. by tensoring with ). So if the latter is non-zero so is the former.

In statement of proposition 3 , it is mentioned in the end that a y nonzero module has an associated prime. It may happen that M is a faithful A module and A need not be a integral domain. Am i missing something obvious?

In your example, the associated prime would be 0. Thus we get an injection .