Length of a module

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In algebra, the length of a module over a ring is a generalization of the dimension of a vector space which measures its size.[1] page 153 It is defined to be the length of the longest chain of submodules. For vector spaces (modules over a field), the length equals the dimension. If is an algebra over a field , the length of a module is at most its dimension as a -vector space.

In commutative algebra and algebraic geometry, a module over a Noetherian commutative ring can have finite length only when the module has Krull dimension zero. Modules of finite length are finitely generated modules, but most finitely generated modules have infinite length. Modules of finite length are called Artinian modules and are fundamental to the theory of Artinian rings.

The degree of an algebraic variety inside an affine or projective space is the length of the coordinate ring of the zero-dimensional intersection of the variety with a generic linear subspace of complementary dimension. More generally, the intersection multiplicity of several varieties is defined as the length of the coordinate ring of the zero-dimensional intersection.

Definition

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Length of a module

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Let   be a (left or right) module over some ring  . Given a chain of submodules of   of the form

 

one says that   is the length of the chain.[1] The length of   is the largest length of any of its chains. If no such largest length exists, we say that   has infinite length. Clearly, if the length of a chain equals the length of the module, one has   and  

Length of a ring

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The length of a ring   is the length of the longest chain of ideals; that is, the length of   considered as a module over itself by left multiplication. By contrast, the Krull dimension of   is the length of the longest chain of prime ideals.

Properties

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Finite length and finite modules

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If an  -module   has finite length, then it is finitely generated.[2] If R is a field, then the converse is also true.

Relation to Artinian and Noetherian modules

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An  -module   has finite length if and only if it is both a Noetherian module and an Artinian module[1] (cf. Hopkins' theorem). Since all Artinian rings are Noetherian, this implies that a ring has finite length if and only if it is Artinian.

Behavior with respect to short exact sequences

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Suppose is a short exact sequence of  -modules. Then M has finite length if and only if L and N have finite length, and we have   In particular, it implies the following two properties

  • The direct sum of two modules of finite length has finite length
  • The submodule of a module with finite length has finite length, and its length is less than or equal to its parent module.

Jordan–Hölder theorem

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A composition series of the module M is a chain of the form

 

such that

 

A module M has finite length if and only if it has a (finite) composition series, and the length of every such composition series is equal to the length of M.

Examples

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Finite dimensional vector spaces

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Any finite dimensional vector space   over a field   has a finite length. Given a basis   there is the chain which is of length  . It is maximal because given any chain, the dimension of each inclusion will increase by at least  . Therefore, its length and dimension coincide.

Artinian modules

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Over a base ring  , Artinian modules form a class of examples of finite modules. In fact, these examples serve as the basic tools for defining the order of vanishing in intersection theory.[3]

Zero module

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The zero module is the only one with length 0.

Simple modules

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Modules with length 1 are precisely the simple modules.

Artinian modules over Z

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The length of the cyclic group   (viewed as a module over the integers Z) is equal to the number of prime factors of  , with multiple prime factors counted multiple times. This follows from the fact that the submodules of   are in one to one correspondence with the positive divisors of  , this correspondence resulting itself from the fact that   is a principal ideal ring.

Use in multiplicity theory

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For the needs of intersection theory, Jean-Pierre Serre introduced a general notion of the multiplicity of a point, as the length of an Artinian local ring related to this point.

The first application was a complete definition of the intersection multiplicity, and, in particular, a statement of Bézout's theorem that asserts that the sum of the multiplicities of the intersection points of n algebraic hypersurfaces in a n-dimensional projective space is either infinite or is exactly the product of the degrees of the hypersurfaces.

This definition of multiplicity is quite general, and contains as special cases most of previous notions of algebraic multiplicity.

Order of vanishing of zeros and poles

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A special case of this general definition of a multiplicity is the order of vanishing of a non-zero algebraic function   on an algebraic variety. Given an algebraic variety   and a subvariety   of codimension 1[3] the order of vanishing for a polynomial   is defined as[4] where   is the local ring defined by the stalk of   along the subvariety  [3] pages 426-227, or, equivalently, the stalk of   at the generic point of  [5] page 22. If   is an affine variety, and   is defined the by vanishing locus  , then there is the isomorphism This idea can then be extended to rational functions   on the variety   where the order is defined as[3]  which is similar to defining the order of zeros and poles in complex analysis.

Example on a projective variety

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For example, consider a projective surface   defined by a polynomial  , then the order of vanishing of a rational function is given by where For example, if   and   and   then since   is a unit in the local ring  . In the other case,   is a unit, so the quotient module is isomorphic to so it has length  . This can be found using the maximal proper sequence 

Zero and poles of an analytic function

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The order of vanishing is a generalization of the order of zeros and poles for meromorphic functions in complex analysis. For example, the function has zeros of order 2 and 1 at   and a pole of order   at  . This kind of information can be encoded using the length of modules. For example, setting   and  , there is the associated local ring   is   and the quotient module  Note that   is a unit, so this is isomorphic to the quotient module Its length is   since there is the maximal chain of submodules.[6] More generally, using the Weierstrass factorization theorem a meromorphic function factors as which is a (possibly infinite) product of linear polynomials in both the numerator and denominator.

See also

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References

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  1. ^ a b c "A Term of Commutative Algebra". www.centerofmathematics.com. pp. 153–158. Archived from the original on 2013-03-02. Retrieved 2020-05-22. Alt URL
  2. ^ "Lemma 10.51.2 (02LZ)—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-05-22.
  3. ^ a b c d Fulton, William, 1939- (1998). Intersection theory (2nd ed.). Berlin: Springer. pp. 8–10. ISBN 3-540-62046-X. OCLC 38048404.{{cite book}}: CS1 maint: multiple names: authors list (link) CS1 maint: numeric names: authors list (link)
  4. ^ "Section 31.26 (0BE0): Weil divisors—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-05-22.
  5. ^ Hartshorne, Robin (1977). Algebraic Geometry. Graduate Texts in Mathematics. Vol. 52. New York, NY: Springer New York. doi:10.1007/978-1-4757-3849-0. ISBN 978-1-4419-2807-8. S2CID 197660097.
  6. ^ "Section 10.120 (02MB): Orders of vanishing—The Stacks project". stacks.math.columbia.edu. Retrieved 2020-05-22.
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