Length of A Module
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Length of A Module

In abstract algebra, the length of a module is a generalization of the dimension of a vector space which measures its size.[1] page 153 In particular, as in the case of vector spaces, the only modules of finite length are finitely generated modules. It is defined to be the length of the longest chain of submodules. Modules with finite length share many important properties with finite-dimensional vector spaces.

Other concepts used to 'count' in ring and module theory are depth and height; these are both somewhat more subtle to define. Moreover, their use is more aligned with dimension theory whereas length is used to analyze finite modules. There are also various ideas of dimension that are useful. Finite length commutative rings play an essential role in functorial treatments of formal algebraic geometry and Deformation theory where Artin rings are used extensively.

## Definition

### Length of a module

Let ${\displaystyle M}$ be a (left or right) module over some ring ${\displaystyle R}$. Given a chain of submodules of ${\displaystyle M}$ of the form

${\displaystyle M_{0}\subsetneq M_{1}\subsetneq \cdots \subsetneq M_{n}=M}$

we say that ${\displaystyle n}$ is the length of the chain.[1] The length of ${\displaystyle M}$ is defined to be the largest length of any of its chains. If no such largest length exists, we say that ${\displaystyle M}$ has infinite length.

### Length of a ring

A ring ${\displaystyle R}$ is said to have finite length as a ring if it has finite length as a left ${\displaystyle R}$-module.

## Properties

### Finite length and finite modules

If an ${\displaystyle R}$-module ${\displaystyle M}$ 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

An ${\displaystyle R}$-module ${\displaystyle M}$ 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

Suppose

${\displaystyle 0\rightarrow L\rightarrow M\rightarrow N\rightarrow 0}$

is a short exact sequence of ${\displaystyle R}$-modules. Then M has finite length if and only if L and N have finite length, and we have

${\displaystyle {\text{length}}_{R}(M)={\text{length}}_{R}(L)+{\text{length}}_{R}(N)}$

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

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

${\displaystyle 0=N_{0}\subsetneq N_{1}\subsetneq \cdots \subsetneq N_{n}=M}$

such that

${\displaystyle N_{i+1}/N_{i}{\mbox{ is simple for }}i=0,\dots ,n-1}$

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

### Finite dimensional vector spaces

Any finite dimensional vector space ${\displaystyle V}$ over a field ${\displaystyle k}$ has a finite length. Given a basis ${\displaystyle v_{1},\ldots ,v_{n}}$ there is the chain

${\displaystyle 0\subset {\text{Span}}_{k}(v_{1})\subset {\text{Span}}_{k}(v_{1},v_{2})\subset \cdots \subset {\text{Span}}_{k}(v_{1},\ldots ,v_{n})=V}$

which is of length ${\displaystyle n}$. It is maximal because given any chain,

${\displaystyle V_{0}\subset \cdots \subset V_{m}}$

the dimension of each inclusion will increase by at least ${\displaystyle 1}$. Therefore, its length and dimension coincide.

### Artinian modules

Over a base ring ${\displaystyle R}$, 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

The zero module is the only one with length 0.

#### Simple modules

Modules with length 1 are precisely the simple modules.

#### Artinian modules over Z

The length of the cyclic group ${\displaystyle \mathbb {Z} /n\mathbb {Z} }$ (viewed as a module over the integers Z) is equal to the number of prime factors of ${\displaystyle n}$, with multiple prime factors counted multiple times. This can be found by using the Chinese remainder theorem.

## Use in multiplicity theory

For the need 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

A special case of this general definition of a multiplicity is the order of vanishing of a non-zero algebraic function ${\displaystyle f\in R(X)^{*}}$ on an algebraic variety. Given an algebraic variety ${\displaystyle X}$ and a subvariety ${\displaystyle V}$ of codimension 1[3] the order of vanishing for a polynomial ${\displaystyle f\in R(X)}$ is defined as[4]

${\displaystyle {\text{ord}}_{V}(f)={\text{length}}_{{\mathcal {O}}_{V,X}}\left({\frac {{\mathcal {O}}_{V,X}}{(f)}}\right)}$

where ${\displaystyle {\mathcal {O}}_{V,X}}$ is the local ring defined by the stalk of ${\displaystyle {\mathcal {O}}_{X}}$ along the subvariety ${\displaystyle V}$[3] pages 426-227, or, equivalently, the stalk of ${\displaystyle {\mathcal {O}}_{X}}$ at the generic point of ${\displaystyle V}$[5] page 22. If ${\displaystyle X}$ is an affine variety, and ${\displaystyle V}$ is defined the by vanishing locus ${\displaystyle V(f)}$, then there is the isomorphism

${\displaystyle {\mathcal {O}}_{V,X}\cong R(X)_{(f)}}$

This idea can then be extended to rational functions ${\displaystyle F=f/g}$ on the variety ${\displaystyle X}$ where the order is defined as

${\displaystyle {\text{ord}}_{V}(F):={\text{ord}}_{V}(f)-{\text{ord}}_{V}(g)}$[3]

which is similar to defining the order of zeros and poles in Complex analysis.

#### Example on a projective variety

For example, consider a projective surface ${\displaystyle Z(h)\subset \mathbb {P} ^{3}}$ defined by a polynomial ${\displaystyle h\in k[x_{0},x_{1},x_{2},x_{3}]}$, then the order of vanishing of a rational function

${\displaystyle F={\frac {f}{g}}}$

is given by

${\displaystyle {\text{ord}}_{Z(h)}(F)={\text{ord}}_{Z(h)}(f)-{\text{ord}}_{Z(h)}(g)}$

where

${\displaystyle {\text{ord}}_{Z(h)}(f)={\text{length}}_{{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}\left({\frac {{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}{(f)}}\right)}$

For example, if ${\displaystyle h=x_{0}^{3}+x_{1}^{3}+x_{2}^{3}+x_{2}^{3}}$ and ${\displaystyle f=x^{2}+y^{2}}$ and ${\displaystyle g=h^{2}(x_{0}+x_{1}-x_{3})}$ then

${\displaystyle {\text{ord}}_{Z(h)}(f)={\text{length}}_{{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}\left({\frac {{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}{(x^{2}+y^{2})}}\right)=0}$

since ${\displaystyle x^{2}+y^{2}}$ is a Unit (ring theory) in the local ring ${\displaystyle {\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}$. In the other case, ${\displaystyle x_{0}+x_{1}-x_{3}}$ is a unit, so the quotient module is isomorphic to

${\displaystyle {\frac {{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}{(h^{2})}}}$

so it has length ${\displaystyle 2}$. This can be found using the maximal proper sequence

${\displaystyle (0)\subset {\frac {{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}{(h)}}\subset {\frac {{\mathcal {O}}_{Z(h),\mathbb {P} ^{3}}}{(h^{2})}}}$

#### Zero and poles of an analytic function

The order of vanishing is a generalization of the order of zeros and poles for meromorphic functions in Complex analysis. For example, the function

${\displaystyle {\frac {(z-1)^{3}(z-2)}{(z-1)(z-4i)}}}$

has zeros of order 2 and 1 at ${\displaystyle 1,2\in \mathbb {C} }$ and a pole of order ${\displaystyle 1}$ at ${\displaystyle 4i\in \mathbb {C} }$. This kind of information can be encoded using the length of modules. For example, setting ${\displaystyle R(X)=\mathbb {C} [z]}$ and ${\displaystyle V=V(z-1)}$, there is the associated local ring ${\displaystyle {\mathcal {O}}_{V,X}}$ is ${\displaystyle \mathbb {C} [z]_{(z-1)}}$ and the quotient module

${\displaystyle {\frac {\mathbb {C} [z]_{(z-1)}}{((z-4i)(z-1)^{2})}}}$

Note that ${\displaystyle z-4i}$ is a unit, so this is isomorphic to the quotient module

${\displaystyle {\frac {\mathbb {C} [z]_{(z-1)}}{((z-1)^{2})}}}$

Its length is ${\displaystyle 2}$ since there is the maximal chain

${\displaystyle (0)\subset {\frac {\mathbb {C} [z]_{(z-1)}}{((z-1))}}\subset {\displaystyle {\frac {\mathbb {C} [z]_{(z-1)}}{((z-1)^{2})}}}}$

of submodules.[6] More generally, using the Weierstrass factorization theorem a meromorphic function factors as

${\displaystyle F={\frac {f}{g}}}$

which is a (possibly infinite) product of linear polynomials in both the numerator and denominator.

## References

1. ^ a b c "A Term of Commutative Algebra". www.centerofmathematics.com. pp. 153-158. Archived from the original on 2013-03-02. Retrieved . Alt URL
2. ^ "Lemma 10.51.2 (02LZ)--The Stacks project". stacks.math.columbia.edu. Retrieved .
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.CS1 maint: multiple names: authors list (link)
4. ^ "Section 31.26 (0BE0): Weil divisors--The Stacks project". stacks.math.columbia.edu. Retrieved .
5. ^ Hartshorne, Robin (1977). Algebraic Geometry. Graduate Texts in Mathematics. 52. New York, NY: Springer New York. doi:10.1007/978-1-4757-3849-0. ISBN 978-1-4419-2807-8.
6. ^ "Section 10.120 (02MB): Orders of vanishing--The Stacks project". stacks.math.columbia.edu. Retrieved .