Primary decomposition – Wikipedia
In algebra, expression of an ideal as the intersection of ideals of a specific type
In mathematics, the Lasker–Noether theorem states that every Noetherian ring is a Lasker ring, which means that every ideal can be decomposed as an intersection, called primary decomposition, of finitely many primary ideals (which are related to, but not quite the same as, powers of prime ideals). The theorem was first proven by Emanuel Lasker (1905) for the special case of polynomial rings and convergent power series rings, and was proven in its full generality by Emmy Noether (1921).
The Lasker–Noether theorem is an extension of the fundamental theorem of arithmetic, and more generally the fundamental theorem of finitely generated abelian groups to all Noetherian rings. The theorem plays an important role in algebraic geometry, by asserting that every algebraic set may be uniquely decomposed into a finite union of irreducible components.
It has a straightforward extension to modules stating that every submodule of a finitely generated module over a Noetherian ring is a finite intersection of primary submodules. This contains the case for rings as a special case, considering the ring as a module over itself, so that ideals are submodules. This also generalizes the primary decomposition form of the structure theorem for finitely generated modules over a principal ideal domain, and for the special case of polynomial rings over a field, it generalizes the decomposition of an algebraic set into a finite union of (irreducible) varieties.
The first algorithm for computing primary decompositions for polynomial rings over a field of characteristic 0[Note 1] was published by Noether’s student Grete Hermann (1926).[1][2] The decomposition does not hold in general for non-commutative Noetherian rings. Noether gave an example of a non-commutative Noetherian ring with a right ideal that is not an intersection of primary ideals.
Primary decomposition of an ideal[edit]
Let
be a Noetherian commutative ring. An ideal
of
is called primary if it is a proper ideal and for each pair of elements
and
in
such that
is in
, either
or some power of
is in
; equivalently, every zero-divisor in the quotient
is nilpotent. The radical of a primary ideal
is a prime ideal and
is said to be
-primary for
.
Let
be an ideal in
. Then
has an irredundant primary decomposition into primary ideals:
- .
Irredundancy means:
- Removing any of the changes the intersection, i.e. for each we have: .
- The prime ideals are all distinct.
Moreover, this decomposition is unique in the two ways:
Primary ideals which correspond to non-minimal prime ideals over
are in general not unique (see an example below). For the existence of the decomposition, see #Primary decomposition from associated primes below.
The elements of
are called the prime divisors of
or the primes belonging to
. In the language of module theory, as discussed below, the set
is also the set of associated primes of the
-module
. Explicitly, that means that there exist elements
in
such that
- [3]
By a way of shortcut, some authors call an associated prime of
simply an associated prime of
(note this practice will conflict with the usage in the module theory).
- The minimal elements of are the same as the minimal prime ideals containing and are called isolated primes.
- The non-minimal elements, on the other hand, are called the embedded primes.
In the case of the ring of integers
, the Lasker–Noether theorem is equivalent to the fundamental theorem of arithmetic. If an integer
has prime factorization
, then the primary decomposition of the ideal
generated by
in
, is
Similarly, in a unique factorization domain, if an element has a prime factorization
where
is a unit, then the primary decomposition of the principal ideal generated by
is
Examples[edit]
The examples of the section are designed for illustrating some properties of primary decompositions, which may appear as surprising or counter-intuitive. All examples are ideals in a polynomial ring over a field k.
Intersection vs. product[edit]
The primary decomposition in
of the ideal
is
Because of the generator of degree one, I is not the product of two larger ideals. A similar example is given, in two indeterminates by
Primary vs. prime power[edit]
In
, the ideal
is a primary ideal that has
as associated prime. It is not a power of its associated prime.
Non-uniqueness and embedded prime[edit]
For every positive integer n, a primary decomposition in
of the ideal
is
The associated primes are
Example: Let N = R = k[x, y] for some field k, and let M be the ideal (xy, y2). Then M has two different minimal primary decompositions
M = (y) ∩ (x, y2) = (y) ∩ (x + y, y2).
The minimal prime is (y) and the embedded prime is (x, y).
Non-associated prime between two associated primes[edit]
In
the ideal
has the (non-unique) primary decomposition
The associated prime ideals are
and
is a non associated prime ideal such that
A complicated example[edit]
Unless for very simple examples, a primary decomposition may be hard to compute and may have a very complicated output. The following example has been designed for providing such a complicated output, and, nevertheless, being accessible to hand-written computation.
Let
be two homogeneous polynomials in x, y, whose coefficients
are polynomials in other indeterminates
over a field k. That is, P and Q belong to
and it is in this ring that a primary decomposition of the ideal
is searched. For computing the primary decomposition, we suppose first that 1 is a greatest common divisor of P and Q.
This condition implies that I has no primary component of height one. As I is generated by two elements, this implies that it is a complete intersection (more precisely, it defines an algebraic set, which is a complete intersection), and thus all primary components have height two. Therefore, the associated primes of I are exactly the primes ideals of height two that contain I.
It follows that
is an associated prime of I.
Let
be the homogeneous resultant in x, y of P and Q. As the greatest common divisor of P and Q is a constant, the resultant D is not zero, and resultant theory implies that I contains all products of D by a monomial in x, y of degree m + n – 1. As
all these monomials belong to the primary component contained in
This primary component contains P and Q, and the behavior of primary decompositions under localization shows that this primary component is
In short, we have a primary component, with the very simple associated prime
such all its generating sets involve all indeterminates.
The other primary component contains D. One may prove that if P and Q are sufficiently generic (for example if the coefficients of P and Q are distinct indeterminates), then there is only another primary component, which is a prime ideal, and is generated by P, Q and D.
Geometric interpretation[edit]
In algebraic geometry, an affine algebraic set V(I) is defined as the set of the common zeros of an ideal I of a polynomial ring
An irredundant primary decomposition
of I defines a decomposition of V(I) into a union of algebraic sets V(Qi), which are irreducible, as not being the union of two smaller algebraic sets.
If
is the associated prime of
, then
and Lasker–Noether theorem shows that V(I) has a unique irredundant decomposition into irreducible algebraic varieties
where the union is restricted to minimal associated primes. These minimal associated primes are the primary components of the radical of I. For this reason, the primary decomposition of the radical of I is sometimes called the prime decomposition of I.
The components of a primary decomposition (as well as of the algebraic set decomposition) corresponding to minimal primes are said isolated, and the others are said embedded.
For the decomposition of algebraic varieties, only the minimal primes are interesting, but in intersection theory, and, more generally in scheme theory, the complete primary decomposition has a geometric meaning.
Primary decomposition from associated primes[edit]
Nowadays, it is common to do primary decomposition of ideals and modules within the theory of associated primes. Bourbaki’s influential textbook Algèbre commutative, in particular, takes this approach.
Let R be a ring and M a module over it. By definition, an associated prime is a prime ideal which is the annihilator of a nonzero element of M; that is,
for some
(this implies
). Equivalently, a prime ideal
is an associated prime of M if there is an injection of R-modules
.
A maximal element of the set of annihilators of nonzero elements of M can be shown to be a prime ideal and thus, when R is a Noetherian ring, there exists an associated prime of M if and only if M is nonzero.
The set of associated primes of M is denoted by
or
. Directly from the definition,
If M is a finitely generated module over R, then there is a finite ascending sequence of submodules
such that each quotient Mi /Mi−1 is isomorphic to
for some prime ideals
, each of which is necessarily in the support of M.[6] Moreover every associated prime of M occurs among the set of primes
; i.e.,
- .[7]
(In general, these inclusions are not the equalities.) In particular,
is a finite set when M is finitely generated.
Let
be a finitely generated module over a Noetherian ring R and N a submodule of M. Given
, the set of associated primes of
, there exist submodules
such that
and
- [8][9]
A submodule N of M is called
-primary if
. A submodule of the R-module R is
-primary as a submodule if and only if it is a
-primary ideal; thus, when
, the above decomposition is precisely a primary decomposition of an ideal.
Taking
, the above decomposition says the set of associated primes of a finitely generated module M is the same as
when
(without finite generation, there can be infinitely many associated primes.)
Properties of associated primes[edit]
Let
be a Noetherian ring. Then
- The set of zero-divisors on R is the same as the union of the associated primes of R (this is because the set of zerodivisors of R is the union of the set of annihilators of nonzero elements, the maximal elements of which are associated primes).[10]
- For the same reason, the union of the associated primes of an R-module M is exactly the set of zero-divisors on M, that is, an element r such that the endomorphism is not injective.[11]
- Given a subset , M an R-module , there exists a submodule such that and .[12]
- Let be a multiplicative subset, an -module and the set of all prime ideals of not intersecting . Then
is a bijection.[13] Also,
.[14]
- Any prime ideal minimal with respect to containing an ideal J is in These primes are precisely the isolated primes.
- A module M over R has finite length if and only if M is finitely generated and consists of maximal ideals.[15]
- Let be a ring homomorphism between Noetherian rings and F a B-module that is flat over A. Then, for each A-module E,
- .[16]
Non-Noetherian case[edit]
The next theorem gives necessary and sufficient conditions for a ring to have primary decompositions for its ideals.
Theorem — Let R be a commutative ring. Then the following are equivalent.
- Every ideal in R has a primary decomposition.
- R has the following properties:
- (L1) For every proper ideal I and a prime ideal P, there exists an x in R – P such that (I : x) is the pre-image of I RP under the localization map R → RP.
- (L2) For every ideal I, the set of all pre-images of I S−1R under the localization map R → S−1R, S running over all multiplicatively closed subsets of R, is finite.
The proof is given at Chapter 4 of Atiyah–MacDonald as a series of exercises.[17]
There is the following uniqueness theorem for an ideal having a primary decomposition.
Theorem — Let R be a commutative ring and I an ideal. Suppose I has a minimal primary decomposition
(note: “minimal” implies
are distinct.) Then
- The set is the set of all prime ideals in the set .
- The set of minimal elements of E is the same as the set of minimal prime ideals over I. Moreover, the primary ideal corresponding to a minimal prime P is the pre-image of I RP and thus is uniquely determined by I.
Now, for any commutative ring R, an ideal I and a minimal prime P over I, the pre-image of I RP under the localization map is the smallest P-primary ideal containing I.[18] Thus, in the setting of preceding theorem, the primary ideal Q corresponding to a minimal prime P is also the smallest P-primary ideal containing I and is called the P-primary component of I.
For example, if the power Pn of a prime P has a primary decomposition, then its P-primary component is the n-th symbolic power of P.
Additive theory of ideals[edit]
This result is the first in an area now known as the additive theory of ideals, which studies the ways of representing an ideal as the intersection of a special class of ideals. The decision on the “special class”, e.g., primary ideals, is a problem in itself. In the case of non-commutative rings, the class of tertiary ideals is a useful substitute for the class of primary ideals.
- ^ Primary decomposition requires testing irreducibility of polynomials, which is not always algorithmically possible in nonzero characteristic.
- ^ Ciliberto, Ciro; Hirzebruch, Friedrich; Miranda, Rick; Teicher, Mina, eds. (2001). Applications of Algebraic Geometry to Coding Theory, Physics and Computation. Dordrecht: Springer Netherlands. ISBN 978-94-010-1011-5.
- ^ Hermann, G. (1926). “Die Frage der endlich vielen Schritte in der Theorie der Polynomideale”. Mathematische Annalen (in German). 95: 736–788. doi:10.1007/BF01206635.
- ^ In other words, is the ideal quotient.
- ^ Bourbaki, Ch. IV, § 1, no 1, Proposition 3.
- ^ a b Bourbaki, Ch. IV, § 1, no 3, Corollaire 1.
- ^ Bourbaki, Ch. IV, § 1, no 4, Théorème 1.
- ^ Bourbaki, Ch. IV, § 1, no 4, Théorème 2.
- ^ Bourbaki, Ch. IV, § 2, no. 2. Theorem 1.
- ^ Here is the proof of the existence of the decomposition (following Bourbaki). Let M be a finitely generated module over a Noetherian ring R and N a submodule. To show N admits a primary decomposition, by replacing M by , it is enough to show that when . Now,
where
are primary submodules of M. In other words, 0 has a primary decomposition if, for each associated prime P of M, there is a primary submodule Q such that . Now, consider the set (which is nonempty since zero is in it). The set has a maximal element Q since M is a Noetherian module. If Q is not P-primary, say, is associated with , then for some submodule Q’, contradicting the maximality. Thus, Q is primary and the proof is complete.Remark: The same proof shows that if R, M, N are all graded, then
in the decomposition may be taken to be graded as well.
- ^ Bourbaki, Ch. IV, § 1, Corollary 3.
- ^ Bourbaki, Ch. IV, § 1, Corollary 2.
- ^ Bourbaki, Ch. IV, § 1, Proposition 4.
- ^ Bourbaki, Ch. IV, § 1, no. 2, Proposition 5.
- ^ Matsumura 1970, 7.C Lemma
- ^ Cohn, P. M. (2003), Basic Algebra, Springer, Exercise 10.9.7, p. 391, ISBN 9780857294289.
- ^ Bourbaki, Ch. IV, § 2. Theorem 2.
- ^ Atiyah–MacDonald 1969
- ^ Atiyah–MacDonald 1969, Ch. 4. Exercise 11
References[edit]
- M. Atiyah, I.G. Macdonald, Introduction to Commutative Algebra, Addison–Wesley, 1994. ISBN 0-201-40751-5
- Bourbaki, Algèbre commutative.
- Danilov, V.I. (2001) [1994], “Lasker ring”, Encyclopedia of Mathematics, EMS Press
- Eisenbud, David (1995), Commutative algebra, Graduate Texts in Mathematics, vol. 150, Berlin, New York: Springer-Verlag, doi:10.1007/978-1-4612-5350-1, ISBN 978-0-387-94268-1, MR 1322960, esp. section 3.3.
- Hermann, Grete (1926), “Die Frage der endlich vielen Schritte in der Theorie der Polynomideale”, Mathematische Annalen (in German), 95: 736–788, doi:10.1007/BF01206635. English translation in Communications in Computer Algebra 32/3 (1998): 8–30.
- Lasker, E. (1905), “Zur Theorie der Moduln und Ideale”, Math. Ann., 60: 19–116, doi:10.1007/BF01447495
- Markov, V.T. (2001) [1994], “Primary decomposition”, Encyclopedia of Mathematics, EMS Press
- Matsumura, Hideyuki (1970), Commutative algebra
- Noether, Emmy (1921), “Idealtheorie in Ringbereichen”, Mathematische Annalen, 83 (1): 24–66, doi:10.1007/BF01464225
- Curtis, Charles (1952), “On Additive Ideal Theory in General Rings”, American Journal of Mathematics, The Johns Hopkins University Press, 74 (3): 687–700, doi:10.2307/2372273, JSTOR 2372273
- Krull, Wolfgang (1928), “Zur Theorie der zweiseitigen Ideale in nichtkommutativen Bereichen”, Mathematische Zeitschrift, 28 (1): 481–503, doi:10.1007/BF01181179
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