10.120 Factorization
Here are some notions and relations between them that are typically taught in a first year course on algebra at the undergraduate level.
Definition 10.120.1. Let R be a domain.
Elements x, y \in R are called associates if there exists a unit u \in R^* such that x = uy.
An element x \in R is called irreducible if it is nonzero, not a unit and whenever x = yz, y, z \in R, then y is either a unit or an associate of x.
An element x \in R is called prime if the ideal generated by x is a prime ideal.
Lemma 10.120.2. Let R be a domain. Let x, y \in R. Then x, y are associates if and only if (x) = (y).
Proof.
If x = uy for some unit u \in R, then (x) \subset (y) and y = u^{-1}x so also (y) \subset (x). Conversely, suppose that (x) = (y). Then x = fy and y = gx for some f, g \in A. Then x = fg x and since R is a domain fg = 1. Thus x and y are associates.
\square
Lemma 10.120.3. Let R be a domain. Consider the following conditions:
The ring R satisfies the ascending chain condition for principal ideals.
Every nonzero, nonunit element a \in R has a factorization a = b_1 \ldots b_ k with each b_ i an irreducible element of R.
Then (1) implies (2).
Proof.
Let x be a nonzero element, not a unit, which does not have a factorization into irreducibles. Set x_1 = x. We can write x = yz where neither y nor z is irreducible or a unit. Then either y does not have a factorization into irreducibles, in which case we set x_2 = y, or z does not have a factorization into irreducibles, in which case we set x_2 = z. Continuing in this fashion we find a sequence
of elements of R with x_ n/x_{n + 1} not a unit. This gives a strictly increasing sequence of principal ideals (x_1) \subset (x_2) \subset (x_3) \subset \ldots thereby finishing the proof.
\square
Definition 10.120.4. A unique factorization domain, abbreviated UFD, is a domain R such that if x \in R is a nonzero, nonunit, then x has a factorization into irreducibles, and if
x = a_1 \ldots a_ m = b_1 \ldots b_ n
are factorizations into irreducibles then n = m and there exists a permutation \sigma : \{ 1, \ldots , n\} \to \{ 1, \ldots , n\} such that a_ i and b_{\sigma (i)} are associates.
Lemma 10.120.5. Let R be a domain. Assume every nonzero, nonunit factors into irreducibles. Then R is a UFD if and only if every irreducible element is prime.
Proof.
Assume R is a UFD and let x \in R be an irreducible element. Say ab \in (x), i.e., ab = cx. Choose factorizations a = a_1 \ldots a_ n, b = b_1 \ldots b_ m, and c = c_1 \ldots c_ r. By uniqueness of the factorization
a_1 \ldots a_ n b_1 \ldots b_ m = c_1 \ldots c_ r x
we find that x is an associate of one of the elements a_1, \ldots , b_ m. In other words, either a \in (x) or b \in (x) and we conclude that x is prime.
Assume every irreducible element is prime. We have to prove that factorization into irreducibles is unique up to permutation and taking associates. Say a_1 \ldots a_ m = b_1 \ldots b_ n with a_ i and b_ j irreducible. Since a_1 is prime, we see that b_ j \in (a_1) for some j. After renumbering we may assume b_1 \in (a_1). Then b_1 = a_1 u and since b_1 is irreducible we see that u is a unit. Hence a_1 and b_1 are associates and a_2 \ldots a_ n = ub_2\ldots b_ m. By induction on n + m we see that n = m and a_ i associate to b_{\sigma (i)} for i = 2, \ldots , n as desired.
\square
Lemma 10.120.6. Let R be a Noetherian domain. Then R is a UFD if and only if every height 1 prime ideal is principal.
Proof.
Assume R is a UFD and let \mathfrak p be a height 1 prime ideal. Take x \in \mathfrak p nonzero and let x = a_1 \ldots a_ n be a factorization into irreducibles. Since \mathfrak p is prime we see that a_ i \in \mathfrak p for some i. By Lemma 10.120.5 the ideal (a_ i) is prime. Since \mathfrak p has height 1 we conclude that (a_ i) = \mathfrak p.
Assume every height 1 prime is principal. Since R is Noetherian every nonzero nonunit element x has a factorization into irreducibles, see Lemma 10.120.3. It suffices to prove that an irreducible element x is prime, see Lemma 10.120.5. Let (x) \subset \mathfrak p be a prime minimal over (x). Then \mathfrak p has height 1 by Lemma 10.60.11. By assumption \mathfrak p = (y). Hence x = yz and z is a unit as x is irreducible. Thus (x) = (y) and we see that x is prime.
\square
Lemma 10.120.7 (Nagata's criterion for factoriality).reference Let A be a domain. Let S \subset A be a multiplicative subset generated by prime elements. Let x \in A be irreducible. Then
the image of x in S^{-1}A is irreducible or a unit, and
x is prime if and only if the image of x in S^{-1}A is a unit or a prime element in S^{-1}A.
Moreover, then A is a UFD if and only if every element of A has a factorization into irreducibles and S^{-1}A is a UFD.
Proof.
Say x = \alpha \beta for \alpha , \beta \in S^{-1}A. Then \alpha = a/s and \beta = b/s' for a, b \in A, s, s' \in S. Thus we get ss'x = ab. By assumption we can write ss' = p_1 \ldots p_ r for some prime elements p_ i. For each i the element p_ i divides either a or b. Dividing we find a factorization x = a' b' and a = s'' a', b = s''' b' for some s'', s''' \in S. As x is irreducible, either a' or b' is a unit. Tracing back we find that either \alpha or \beta is a unit. This proves (1).
Suppose x is prime. Then A/(x) is a domain. Hence S^{-1}A/xS^{-1}A = S^{-1}(A/(x)) is a domain or zero. Thus x maps to a prime element or a unit.
Suppose that the image of x in S^{-1}A is a unit. Then y x = s for some s \in S and y \in A. By assumption s = p_1 \ldots p_ r with p_ i a prime element. For each i either p_ i divides y or p_ i divides x. In the second case p_ i and x are associates (as x is irreducible) and we are done. But if the first case happens for all i = 1, \ldots , r, then x is a unit which is a contradiction.
Suppose that the image of x in S^{-1}A is a prime element. Assume a, b \in A and ab \in (x). Then sa = xy or sb = xy for some s \in S and y \in A. Say the first case happens. By assumption s = p_1 \ldots p_ r with p_ i a prime element. For each i either p_ i divides y or p_ i divides x. In the second case p_ i and x are associates (as x is irreducible) and we are done. If the first case happens for all i = 1, \ldots , r, then a \in (x) as desired. This completes the proof of (2).
The final statement of the lemma follows from (1) and (2) and Lemma 10.120.5.
\square
Lemma 10.120.8. A UFD satisfies the ascending chain condition for principal ideals.
Proof.
Consider an ascending chain (a_1) \subset (a_2) \subset (a_3) \subset \ldots of principal ideals in R. Write a_1 = p_1^{e_1} \ldots p_ r^{e_ r} with p_ i prime. Then we see that a_ n is an associate of p_1^{c_1} \ldots p_ r^{c_ r} for some 0 \leq c_ i \leq e_ i. Since there are only finitely many possibilities we conclude.
\square
Lemma 10.120.9. Let R be a domain. Assume R has the ascending chain condition for principal ideals. Then the same property holds for a polynomial ring over R.
Proof.
Consider an ascending chain (f_1) \subset (f_2) \subset (f_3) \subset \ldots of principal ideals in R[x]. Since f_{n + 1} divides f_ n we see that the degrees decrease in the sequence. Thus f_ n has fixed degree d \geq 0 for all n \gg 0. Let a_ n be the leading coefficient of f_ n. The condition f_ n \in (f_{n + 1}) implies that a_{n + 1} divides a_ n for all n. By our assumption on R we see that a_{n + 1} and a_ n are associates for all n large enough (Lemma 10.120.2). Thus for large n we see that f_ n = u f_{n + 1} where u \in R (for reasons of degree) is a unit (as a_ n and a_{n + 1} are associates).
\square
Lemma 10.120.10. A polynomial ring over a UFD is a UFD. In particular, if k is a field, then k[x_1, \ldots , x_ n] is a UFD.
Proof.
Let R be a UFD. Then R satisfies the ascending chain condition for principal ideals (Lemma 10.120.8), hence R[x] satisfies the ascending chain condition for principal ideals (Lemma 10.120.9), and hence every element of R[x] has a factorization into irreducibles (Lemma 10.120.3). Let S \subset R be the multiplicative subset generated by prime elements. Since every nonunit of R is a product of prime elements we see that K = S^{-1}R is the fraction field of R. Observe that every prime element of R maps to a prime element of R[x] and that S^{-1}(R[x]) = S^{-1}R[x] = K[x] is a UFD (and even a PID). Thus we may apply Lemma 10.120.7 to conclude.
\square
Lemma 10.120.11. A unique factorization domain is normal.
Proof.
Let R be a UFD. Let x be an element of the fraction field of R which is integral over R. Say x^ d - a_1 x^{d - 1} - \ldots - a_ d = 0 with a_ i \in R. We can write x = u p_1^{e_1} \ldots p_ r^{e_ r} with u a unit, e_ i \in \mathbf{Z}, and p_1, \ldots , p_ r irreducible elements which are not associates. To prove the lemma we have to show e_ i \geq 0. If not, say e_1 < 0, then for N \gg 0 we get
u^ d p_2^{de_2 + N} \ldots p_ r^{de_ r + N} = p_1^{-de_1}p_2^ N \ldots p_ r^ N( \sum \nolimits _{i = 1, \ldots , d} a_ i x^{d - i} ) \in (p_1)
which contradicts uniqueness of factorization in R.
\square
Definition 10.120.12. A principal ideal domain, abbreviated PID, is a domain R such that every ideal is a principal ideal.
Lemma 10.120.13. A principal ideal domain is a unique factorization domain.
Proof.
As a PID is Noetherian this follows from Lemma 10.120.6.
\square
Definition 10.120.14. A Dedekind domain is a domain R such that every nonzero ideal I \subset R can be written as a product
I = \mathfrak p_1 \ldots \mathfrak p_ r
of nonzero prime ideals uniquely up to permutation of the \mathfrak p_ i.
Lemma 10.120.15. A PID is a Dedekind domain.
Proof.
Let R be a PID. Since every nonzero ideal of R is principal, and R is a UFD (Lemma 10.120.13), this follows from the fact that every irreducible element in R is prime (Lemma 10.120.5) so that factorizations of elements turn into factorizations into primes.
\square
Lemma 10.120.16.slogan Let A be a ring. Let I and J be nonzero ideals of A such that IJ = (f) for some nonzerodivisor f \in A. Then I and J are finitely generated ideals and finitely locally free of rank 1 as A-modules.
Proof.
It suffices to show that I and J are finite locally free A-modules of rank 1, see Lemma 10.78.2. To do this, write f = \sum _{i = 1, \ldots , n} x_ i y_ i with x_ i \in I and y_ i \in J. We can also write x_ i y_ i = a_ i f for some a_ i \in A. Since f is a nonzerodivisor we see that \sum a_ i = 1. Thus it suffices to show that each I_{a_ i} and J_{a_ i} is free of rank 1 over A_{a_ i}. After replacing A by A_{a_ i} we conclude that f = xy for some x \in I and y \in J. Note that both x and y are nonzerodivisors. We claim that I = (x) and J = (y) which finishes the proof. Namely, if x' \in I, then x'y = af = axy for some a \in A. Hence x' = ax and we win.
\square
Lemma 10.120.17. Let R be a ring. The following are equivalent
R is a Dedekind domain,
R is a Noetherian domain and for every nonzero maximal ideal \mathfrak m the local ring R_{\mathfrak m} is a discrete valuation ring, and
R is a Noetherian, normal domain, and \dim (R) \leq 1.
Proof.
Assume (1). The argument is nontrivial because we did not assume that R was Noetherian in our definition of a Dedekind domain. Let \mathfrak p \subset R be a prime ideal. Observe that \mathfrak p \not= \mathfrak p^2 by uniqueness of the factorizations in the definition. Pick x \in \mathfrak p with x \not\in \mathfrak p^2. Let y \in \mathfrak p be a second element (for example y = 0). Write (x, y) = \mathfrak p_1 \ldots \mathfrak p_ r. Since (x, y) \subset \mathfrak p at least one of the primes \mathfrak p_ i is contained in \mathfrak p. But as x \not\in \mathfrak p^2 there is at most one. Thus exactly one of \mathfrak p_1, \ldots , \mathfrak p_ r is contained in \mathfrak p, say \mathfrak p_1 \subset \mathfrak p. We conclude that (x, y)R_\mathfrak p = \mathfrak p_1R_\mathfrak p is prime for every choice of y. We claim that (x)R_\mathfrak p = \mathfrak pR_\mathfrak p. Namely, pick y \in \mathfrak p. By the above applied with y^2 we see that (x, y^2)R_\mathfrak p is prime. Hence y \in (x, y^2)R_\mathfrak p, i.e., y = ax + by^2 in R_\mathfrak p. Thus (1 - by)y = ax \in (x)R_\mathfrak p, i.e., y \in (x)R_\mathfrak p as desired.
Writing (x) = \mathfrak p_1 \ldots \mathfrak p_ r anew with \mathfrak p_1 \subset \mathfrak p we conclude that \mathfrak p_1 R_\mathfrak p = \mathfrak p R_\mathfrak p, i.e., \mathfrak p_1 = \mathfrak p. Moreover, \mathfrak p_1 = \mathfrak p is a finitely generated ideal of R by Lemma 10.120.16. We conclude that R is Noetherian by Lemma 10.28.10. Moreover, it follows that R_\mathfrak m is a discrete valuation ring for every prime ideal \mathfrak p, see Lemma 10.119.7.
The equivalence of (2) and (3) follows from Lemmas 10.37.10 and 10.119.7. Assume (2) and (3) are satisfied. Let I \subset R be an ideal. We will construct a factorization of I. If I is prime, then there is nothing to prove. If not, pick I \subset \mathfrak p with \mathfrak p \subset R maximal. Let J = \{ x \in R \mid x \mathfrak p \subset I\} . We claim J \mathfrak p = I. It suffices to check this after localization at the maximal ideals \mathfrak m of R (the formation of J commutes with localization and we use Lemma 10.23.1). Then either \mathfrak p R_\mathfrak m = R_\mathfrak m and the result is clear, or \mathfrak p R_\mathfrak m = \mathfrak m R_\mathfrak m. In the last case \mathfrak p R_\mathfrak m = (\pi ) and the case where \mathfrak p is principal is immediate. By Noetherian induction the ideal J has a factorization and we obtain the desired factorization of I. We omit the proof of uniqueness of the factorization.
\square
The following is a variant of the Krull-Akizuki lemma.
Lemma 10.120.18. Let A be a Noetherian domain of dimension 1 with fraction field K. Let L/K be a finite extension. Let B be the integral closure of A in L. Then B is a Dedekind domain and \mathop{\mathrm{Spec}}(B) \to \mathop{\mathrm{Spec}}(A) is surjective, has finite fibres, and induces finite residue field extensions.
Proof.
By Krull-Akizuki (Lemma 10.119.12) the ring B is Noetherian. By Lemma 10.112.4 \dim (B) = 1. Thus B is a Dedekind domain by Lemma 10.120.17. Surjectivity of the map on spectra follows from Lemma 10.36.17. The last two statements follow from Lemma 10.119.10.
\square
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