Dimension theory (algebra)

In mathematics, dimension theory is the study in terms of commutative algebra of the notion dimension of an algebraic variety (and by extension that of a scheme). The need of a theory for such an apparently simple notion results from the existence of many definitions of dimension that are equivalent only in the most regular cases (see Dimension of an algebraic variety). A large part of dimension theory consists in studying the conditions under which several dimensions are equal, and many important classes of commutative rings may be defined as the rings such that two dimensions are equal; for example, a regular ring is a commutative ring such that the homological dimension is equal to the Krull dimension.

The theory is simpler for commutative rings that are finitely generated algebras over a field, which are also quotient rings of polynomial rings in a finite number of indeterminates over a field. In this case, which is the algebraic counterpart of the case of affine algebraic sets, most of the definitions of the dimension are equivalent. For general commutative rings, the lack of geometric interpretation is an obstacle to the development of the theory; in particular, very little is known for non-noetherian rings. (Kaplansky's Commutative rings gives a good account of the non-noetherian case.)

Throughout the article, denotes Krull dimension of a ring and the height of a prime ideal (i.e., the Krull dimension of the localization at that prime ideal.) Rings are assumed to be commutative except in the last section on dimensions of non-commutative rings.

Basic resultsEdit

Let R be a noetherian ring or valuation ring. Then

 
If R is noetherian, this follows from the fundamental theorem below (in particular, Krull's principal ideal theorem), but it is also a consequence of a more precise result. For any prime ideal   in R,
 
 
for any prime ideal   in   that contracts to  . This can be shown within basic ring theory (cf. Kaplansky, commutative rings). In addition, in each fiber of  , one cannot have a chain of primes ideals of length  .

Since an artinian ring (e.g., a field) has dimension zero, by induction one gets a formula: for an artinian ring R,

 

Local ringsEdit

Fundamental theoremEdit

Let   be a noetherian local ring and I a  -primary ideal (i.e., it sits between some power of   and  ). Let   be the Poincaré series of the associated graded ring  . That is,

 
where   refers to the length of a module (over an artinian ring  ). If   generate I, then their image in   have degree 1 and generate   as  -algebra. By the Hilbert–Serre theorem, F is a rational function with exactly one pole at   of order  . Since
 
we find that the coefficient of   in   is of the form
 
That is to say,   is a polynomial   in n of degree  . P is called the Hilbert polynomial of  .

We set  . We also set   to be the minimum number of elements of R that can generate an  -primary ideal of R. Our ambition is to prove the fundamental theorem:

 
Since we can take s to be  , we already have   from the above. Next we prove   by induction on  . Let   be a chain of prime ideals in R. Let   and x a nonzero nonunit element in D. Since x is not a zero-divisor, we have the exact sequence
 
The degree bound of the Hilbert-Samuel polynomial now implies that  . (This essentially follows from the Artin-Rees lemma; see Hilbert-Samuel function for the statement and the proof.) In  , the chain   becomes a chain of length   and so, by inductive hypothesis and again by the degree estimate,
 
The claim follows. It now remains to show   More precisely, we shall show:

Lemma — The maximal ideal   contains elements  , d = Krull dimension of R, such that, for any i, any prime ideal containing   has height  .

(Notice:   is then  -primary.) The proof is omitted. It appears, for example, in Atiyah–MacDonald. But it can also be supplied privately; the idea is to use prime avoidance.

Consequences of the fundamental theoremEdit

Let   be a noetherian local ring and put  . Then

  •  , since a basis of   lifts to a generating set of   by Nakayama. If the equality holds, then R is called a regular local ring.
  •  , since  .
  • (Krull's principal ideal theorem) The height of the ideal generated by elements   in a noetherian ring is at most s. Conversely, a prime ideal of height s is minimal over an ideal generated by s elements. (Proof: Let   be a prime ideal minimal over such an ideal. Then  . The converse was shown in the course of the proof of the fundamental theorem.)

Theorem — If   is a morphism of noetherian local rings, then[1]

 
The equality holds if   is flat or more generally if it has the going-down property.

Proof: Let   generate a  -primary ideal and   be such that their images generate a  -primary ideal. Then   for some s. Raising both sides to higher powers, we see some power of   is contained in  ; i.e., the latter ideal is  -primary; thus,  . The equality is a straightforward application of the going-down property. Q.E.D.

Proposition — If R is a noetherian ring, then

 

Proof: If   are a chain of prime ideals in R, then   are a chain of prime ideals in   while   is not a maximal ideal. Thus,  . For the reverse inequality, let   be a maximal ideal of   and  . Clearly,  . Since   is then a localization of a principal ideal domain and has dimension at most one, we get   by the previous inequality. Since   is arbitrary, it follows  . Q.E.D.

Nagata's altitude formulaEdit

Theorem — Let   be integral domains,   be a prime ideal and  . If R is a Noetherian ring, then

 
where the equality holds if either (a) R is universally catenary and R' is finitely generated R-algebra or (b) R' is a polynomial ring over R.

Proof:[2] First suppose   is a polynomial ring. By induction on the number of variables, it is enough to consider the case  . Since R' is flat over R,

 
By Noether's normalization lemma, the second term on the right side is:
 
Next, suppose   is generated by a single element; thus,  . If I = 0, then we are already done. Suppose not. Then   is algebraic over R and so  . Since R is a subring of R',   and so   since   is algebraic over  . Let   denote the pre-image in   of  . Then, as  , by the polynomial case,
 
Here, note that the inequality is the equality if R' is catenary. Finally, working with a chain of prime ideals, it is straightforward to reduce the general case to the above case. Q.E.D.

Homological methodsEdit

Regular ringsEdit

Let R be a noetherian ring. The projective dimension of a finite R-module M is the shortest length of any projective resolution of M (possibly infinite) and is denoted by  . We set  ; it is called the global dimension of R.

Assume R is local with residue field k.

Lemma —   (possibly infinite).

Proof: We claim: for any finite R-module M,

 
By dimension shifting (cf. the proof of Theorem of Serre below), it is enough to prove this for  . But then, by the local criterion for flatness,   Now,
 
completing the proof. Q.E.D.

Remark: The proof also shows that   if M is not free and   is the kernel of some surjection from a free module to M.

Lemma — Let  , f a non-zerodivisor of R. If f is a non-zerodivisor on M, then

 

Proof: If  , then M is R-free and thus   is  -free. Next suppose  . Then we have:   as in the remark above. Thus, by induction, it is enough to consider the case  . Then there is a projective resolution:  , which gives:

 
But   Hence,   is at most 1. Q.E.D.

Theorem of Serre — R regular  

Proof:[3] If R is regular, we can write  ,   a regular system of parameters. An exact sequence  , some f in the maximal ideal, of finite modules,  , gives us:

 
But f here is zero since it kills k. Thus,   and consequently  . Using this, we get:
 
The proof of the converse is by induction on  . We begin with the inductive step. Set  ,   among a system of parameters. To show R is regular, it is enough to show   is regular. But, since  , by inductive hypothesis and the preceding lemma with  ,
 

The basic step remains. Suppose  . We claim   if it is finite. (This would imply that R is a semisimple local ring; i.e., a field.) If that is not the case, then there is some finite module   with   and thus in fact we can find M with  . By Nakayama's lemma, there is a surjection   from a free module F to M whose kernel K is contained in  . Since  , the maximal ideal   is an associated prime of R; i.e.,   for some nonzero s in R. Since  ,  . Since K is not zero and is free, this implies  , which is absurd. Q.E.D.

Corollary — A regular local ring is a unique factorization domain.

Proof: Let R be a regular local ring. Then  , which is an integrally closed domain. It is a standard algebra exercise to show this implies that R is an integrally closed domain. Now, we need to show every divisorial ideal is principal; i.e., the divisor class group of R vanishes. But, according to Bourbaki, Algèbre commutative, chapitre 7, §. 4. Corollary 2 to Proposition 16, a divisorial ideal is principal if it admits a finite free resolution, which is indeed the case by the theorem. Q.E.D.

Theorem — Let R be a ring. Then

 

DepthEdit

Let R be a ring and M a module over it. A sequence of elements   in   is called an M-regular sequence if   is not a zero-divisor on   and   is not a zero divisor on   for each  . A priori, it is not obvious whether any permutation of a regular sequence is still regular (see the section below for some positive answer.)

Let R be a local Noetherian ring with maximal ideal   and put  . Then, by definition, the depth of a finite R-module M is the supremum of the lengths of all M-regular sequences in  . For example, we have   consists of zerodivisors on M   is associated with M. By induction, we find

 
for any associated primes   of M. In particular,  . If the equality holds for M = R, R is called a Cohen–Macaulay ring.

Example: A regular Noetherian local ring is Cohen–Macaulay (since a regular system of parameters is an R-regular sequence.)

In general, a Noetherian ring is called a Cohen–Macaulay ring if the localizations at all maximal ideals are Cohen–Macaulay. We note that a Cohen–Macaulay ring is universally catenary. This implies for example that a polynomial ring   is universally catenary since it is regular and thus Cohen–Macaulay.

Proposition (Rees) — Let M be a finite R-module. Then  .

More generally, for any finite R-module N whose support is exactly  ,

 

Proof: We first prove by induction on n the following statement: for every R-module M and every M-regular sequence   in  ,

 

 

 

 

 

()

The basic step n = 0 is trivial. Next, by inductive hypothesis,  . But the latter is zero since the annihilator of N contains some power of  . Thus, from the exact sequence   and the fact that   kills N, using the inductive hypothesis again, we get

 
proving (). Now, if  , then we can find an M-regular sequence of length more than n and so by () we see  . It remains to show   if  . By () we can assume n = 0. Then   is associated with M; thus is in the support of M. On the other hand,   It follows by linear algebra that there is a nonzero homomorphism from N to M modulo  ; hence, one from N to M by Nakayama's lemma. Q.E.D.

The Auslander–Buchsbaum formula relates depth and projective dimension.

Theorem — Let M be a finite module over a noetherian local ring R. If  , then

 

Proof: We argue by induction on  , the basic case (i.e., M free) being trivial. By Nakayama's lemma, we have the exact sequence   where F is free and the image of f is contained in  . Since   what we need to show is  . Since f kills k, the exact sequence yields: for any i,

 
Note the left-most term is zero if  . If  , then since   by inductive hypothesis, we see   If  , then   and it must be   Q.E.D.

As a matter of notation, for any R-module M, we let

 
One sees without difficulty that   is a left-exact functor and then let   be its j-th right derived functor, called the local cohomology of R. Since  , via abstract nonsense,
 
This observation proves the first part of the theorem below.

Theorem (Grothendieck) — Let M be a finite R-module. Then

  1.  .
  2.   and   if  
  3. If R is complete and d its Krull dimension and if E is the injective hull of k, then
     
    is representable (the representing object is sometimes called the canonical module especially if R is Cohen–Macaulay.)

Proof: 1. is already noted (except to show the nonvanishing at the degree equal to the depth of M; use induction to see this) and 3. is a general fact by abstract nonsense. 2. is a consequence of an explicit computation of a local cohomology by means of Koszul complexes (see below).  

Koszul complexEdit

Let R be a ring and x an element in it. We form the chain complex K(x) given by   for i = 0, 1 and   for any other i with the differential

 
For any R-module M, we then get the complex   with the differential   and let   be its homology. Note:
 
 

More generally, given a finite sequence   of elements in a ring R, we form the tensor product of complexes:

 
and let   its homology. As before,
 
 

We now have the homological characterization of a regular sequence.

Theorem — Suppose R is Noetherian, M is a finite module over R and   are in the Jacobson radical of R. Then the following are equivalent

  1.   is an M-regular sequence.
  2.  .
  3.  .

Corollary — The sequence   is M-regular if and only if any of its permutations is so.

Corollary — If   is an M-regular sequence, then   is also an M-regular sequence for each positive integer j.

A Koszul complex is a powerful computational tool. For instance, it follows from the theorem and the corollary

 
(Here, one uses the self-duality of a Koszul complex; see Proposition 17.15. of Eisenbud, Commutative Algebra with a View Toward Algebraic Geometry.)

Another instance would be

Theorem — Assume R is local. Then let

 
the dimension of the Zariski tangent space (often called the embedding dimension of R). Then
 

Remark: The theorem can be used to give a second quick proof of Serre's theorem, that R is regular if and only if it has finite global dimension. Indeed, by the above theorem,   and thus  . On the other hand, as  , the Auslander–Buchsbaum formula gives  . Hence,  .

We next use a Koszul homology to define and study complete intersection rings. Let R be a Noetherian local ring. By definition, the first deviation of R is the vector space dimension

 
where   is a system of parameters. By definition, R is a complete intersection ring if   is the dimension of the tangent space. (See Hartshorne for a geometric meaning.)

Theorem — R is a complete intersection ring if and only if its Koszul algebra is an exterior algebra.

Injective dimension and Tor dimensionsEdit

Let R be a ring. The injective dimension of an R-module M denoted by   is defined just like a projective dimension: it is the minimal length of an injective resolution of M. Let   be the category of R-modules.

Theorem — For any ring R,

 

Proof: Suppose  . Let M be an R-module and consider a resolution

 
where   are injective modules. For any ideal I,
 
which is zero since   is computed via a projective resolution of  . Thus, by Baer's criterion, N is injective. We conclude that  . Essentially by reversing the arrows, one can also prove the implication in the other way. Q.E.D.

The theorem suggests that we consider a sort of a dual of a global dimension:

 
It was originally called the weak global dimension of R but today it is more commonly called the Tor dimension of R.

Remark: for any ring R,  .

Proposition — A ring has weak global dimension zero if and only if it is von Neumann regular.

Multiplicity theoryEdit

Dimensions of non-commutative ringsEdit

Let A be a graded algebra over a field k. If V is a finite-dimensional generating subspace of A, then we let   and then put

 
It is called the Gelfand–Kirillov dimension of A. It is easy to show   is independent of a choice of V.

Example: If A is finite-dimensional, then gk(A) = 0. If A is an affine ring, then gk(A) = Krull dimension of A.

Bernstein's inequality — See [1]

See alsoEdit

NotesEdit

  1. ^ Eisenbud 1995, Theorem 10.10
  2. ^ Matsumura 1987, Theorem 15.5.
  3. ^ Weibel 1995, Theorem 4.4.16

ReferencesEdit

  • Bruns, Winfried; Herzog, Jürgen (1993), Cohen-Macaulay rings, Cambridge Studies in Advanced Mathematics, vol. 39, Cambridge University Press, ISBN 978-0-521-41068-7, MR 1251956
  • Part II of Eisenbud, David (1995), Commutative algebra. With a view toward algebraic geometry, Graduate Texts in Mathematics, vol. 150, New York: Springer-Verlag, ISBN 0-387-94268-8, MR 1322960.
  • Chapter 10 of Atiyah, Michael Francis; Macdonald, I.G. (1969), Introduction to Commutative Algebra, Westview Press, ISBN 978-0-201-40751-8.
  • Kaplansky, Irving, Commutative rings, Allyn and Bacon, 1970.
  • Matsumura, H. (1987). Commutative Ring Theory. Cambridge Studies in Advanced Mathematics. Vol. 8. Translated by M. Reid. Cambridge University Press. doi:10.1017/CBO9781139171762. ISBN 978-0-521-36764-6.
  • Serre, Jean-Pierre (1975), Algèbre locale. Multiplicités, Cours au Collège de France, 1957–1958, rédigé par Pierre Gabriel. Troisième édition, 1975. Lecture Notes in Mathematics (in French), vol. 11, Berlin, New York: Springer-Verlag
  • Weibel, Charles A. (1995). An Introduction to Homological Algebra. Cambridge University Press.