Top Qs
Timeline
Chat
Perspective

Prime avoidance lemma

Result concerning ideals of commutative rings From Wikipedia, the free encyclopedia

Remove ads

In algebra, the prime avoidance lemma says that if an ideal I in a commutative ring R is contained in a union of finitely many prime ideals Pi's, then it is contained in Pi for some i.

There are many variations of the lemma (cf. Hochster); for example, if the ring R contains an infinite field or a finite field of sufficiently large cardinality, then the statement follows from a fact in linear algebra that a vector space over an infinite field or a finite field of large cardinality is not a finite union of its proper vector subspaces.[1]

Remove ads

Statement and proof

Summarize
Perspective

The following statement and argument are perhaps the most standard.

Theorem (Prime Avoidance Lemma): Let E be a subset of commutative ring R that is an additive subgroup of R and is multiplicatively closed. (In particular, E could be a subring or ideal of R.) Let be ideals such that are prime ideals for . If E is not contained in any of the , then E is not contained in the union .

Proof by induction on n: The idea is to find an element of R that is in E and not in any of the . The base case is trivial. Next suppose . For each i, choose

,

where each of the sets on the right is nonempty by the inductive hypothesis. We can assume for all i; otherwise, there is some among them that avoids all of the , and we are done. Put

.

Because E is closed under addition and multiplication, z is in E by construction. We claim that z is not in any of the . Indeed, if for some , then , a contradiction. Next suppose . Then . If , this is already a contradiction. If , then, since is a prime ideal, for some , again a contradiction.

Remove ads

E. Davis' prime avoidance

Summarize
Perspective

There is the following variant of prime avoidance due to E. Davis.

Theorem[2] Let A be a ring, prime ideals, x an element of A and J an ideal. For the ideal , if for each i, then there exists some y in J such that for each i.

Proof:[3] We argue by induction on r. Without loss of generality, we can assume there is no inclusion relation between the 's; since otherwise we can use the inductive hypothesis.

Also, if for each i, then we are done; thus, without loss of generality, we can assume . By inductive hypothesis, we find a y in J such that . If is not in , we are done. Otherwise, note that (since ) and since is a prime ideal, we have:

.

Hence, we can choose in that is not in . Then, since , the element has the required property.

Application

Let A be a Noetherian ring, I an ideal generated by n elements and M a finite A-module such that . Also, let = the maximal length of M-regular sequences in I = the length of every maximal M-regular sequence in I. Then ; this estimate can be shown using the above prime avoidance as follows. We argue by induction on n. Let be the set of associated primes of M. If , then for each i. If , then, by prime avoidance, we can choose

for some in such that = the set of zero divisors on M. Now, is an ideal of generated by elements and so, by inductive hypothesis, . The claim now follows.

Remove ads

Notes

References

Loading related searches...

Wikiwand - on

Seamless Wikipedia browsing. On steroids.

Remove ads