Definition
A subset {aα} of an abelian group is linearly independent (over Z) if the only linear combination of these elements that is equal to zero is trivial: if
where all but finitely many coefficients nα are zero (so that the sum is, in effect, finite), then all coefficients are 0. Any two maximal linearly independent sets in A have the same cardinality, which is called the rank of A.
Rank of an abelian group is analogous to the dimension of a vector space. The main difference with the case of vector space is a presence of torsion. An element of an abelian group A is classified as torsion if its order is finite. The set of all torsion elements is a subgroup, called the torsion subgroup and denoted T(A). A group is called torsion-free if it has no non-trivial torsion elements. The factor-group A/T(A) is the unique maximal torsion-free quotient of A and its rank coincides with the rank of A.
The notion of rank with analogous properties can be defined for modules over any integral domain, the case of abelian groups corresponds to modules over Z.
Read more about this topic: Rank Of An Abelian Group
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