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The '''invariant factors''' of a [[module (mathematics)|module]] over a [[principal ideal domain]] (PID) occur in one form of the [[structure theorem for finitely generated modules over a principal ideal domain]].
 
If <math>R</math> is a [[Principal ideal domain|PID]] and <math>M</math> a [[Finitely-generated module|finitely generated]] <math>R</math>-module, then
 
:<math>M\cong R^r\oplus R/(a_1)\oplus R/(a_2)\oplus\cdots\oplus R/(a_m)</math>
 
for some integer <math>r\geq0</math> and a (possibly empty) list of nonzero elements <math>a_1,\ldots,a_m\in R</math> for which <math>a_1 \mid a_2 \mid \cdots \mid a_m</math>. The nonnegative integer <math>r</math> is called the ''free rank'' or ''Betti number'' of the module <math>M</math>, while <math>a_1,\ldots,a_m</math> are the ''invariant factors'' of <math>M</math> and are unique up to [[associatedness]].
 
The invariant factors of a [[Matrix (mathematics)|matrix]] over a PID occur in the [[Smith normal form]] and provide a means of computing the structure of a module from a set of generators and relations.
 
==See also==
* [[Elementary divisors]]
 
==References==
* {{cite book | author=B. Hartley | authorlink=Brian Hartley | coauthors=T.O. Hawkes | title=Rings, modules and linear algebra | publisher=Chapman and Hall | year=1970 | isbn=0-412-09810-5 }}  Chap.8, p.128.
* Chapter III.7, p.153 of {{Lang Algebra|edition=3}}
 
[[Category:Module theory]]
 
 
{{Abstract-algebra-stub}}

Revision as of 10:01, 7 December 2012

The invariant factors of a module over a principal ideal domain (PID) occur in one form of the structure theorem for finitely generated modules over a principal ideal domain.

If R is a PID and M a finitely generated R-module, then

MRrR/(a1)R/(a2)R/(am)

for some integer r0 and a (possibly empty) list of nonzero elements a1,,amR for which a1a2am. The nonnegative integer r is called the free rank or Betti number of the module M, while a1,,am are the invariant factors of M and are unique up to associatedness.

The invariant factors of a matrix over a PID occur in the Smith normal form and provide a means of computing the structure of a module from a set of generators and relations.

See also

References


Template:Abstract-algebra-stub