In abstract algebra, the length of a module is a generalization of the dimension of a vector space which measures its size. 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.
Length of a module
Let be a (left or right) module over some ring . Given a chain of submodules of of the form
we say that is the length of the chain. The length of is defined to be the largest length of any of its chains. If no such largest length exists, we say that has infinite length.
Length of a ring
A ring is said to have finite length as a ring if it has finite length as a left -module.
Finite length and finite modules
If an -module has finite length, then it is finitely generated. If R is a field, then the converse is also true.
Relation to Artinian and Noetherian modules
An -module has finite length if and only if it is both a Noetherian module and an Artinian module (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
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.
A composition series of the module M is a chain of the form
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.
Finite dimensional vector spaces
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.
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.
The zero module is the only one with length 0.
Modules with length 1 are precisely the simple modules.
Artinian modules over Z
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 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 on an algebraic variety. Given an algebraic variety and a subvariety of codimension 1 the order of vanishing for a polynomial is defined as
where is the local ring defined by the stalk of along the subvariety  pages 426-227, or, equivalently, the stalk of at the generic point of  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
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 defined by a polynomial , then the order of vanishing of a rational function
is given by
For example, if and and then
since is a Unit (ring theory) 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
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. 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.