In
mathematics, a
free abelian group is an
abelian group with a
basis. Being an abelian group means that it is a
set with an addition
operation that is
associative,
commutative, and invertible. A basis, also called an
integral basis, is a
subset such that every element of the
group can be uniquely expressed as an
integer combination of finitely many basis elements. For instance the two-dimensional
integer lattice forms a free abelian group, with coordinatewise addition as its operation, and with the two points (1,0) and (0,1) as its basis. Free abelian groups have properties which make them similar to
vector spaces, and may equivalently be called
free
-modules, the
free modules over the integers.
Lattice theory studies free abelian
subgroups of
real vector spaces. In
algebraic topology, free abelian groups are used to define
chain groups, and in
algebraic geometry they are used to define
divisors.
The elements of a free abelian group with basis
![{\displaystyle B}](https://wikimedia.org/api/rest_v1/media/math/render/svg/47136aad860d145f75f3eed3022df827cee94d7a)
may be described in several equivalent ways. These include
formal sums over
, which are expressions of the form
![{\textstyle \sum a_{i}b_{i}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0b5e7d555a59db95749a4362009f177b8cbf7a1c)
where each
![{\displaystyle a_{i}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/0bc77764b2e74e64a63341054fa90f3e07db275f)
is a nonzero integer, each
![{\displaystyle b_{i}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/40a8c2db2990a53c683e75961826167c5adac7c3)
is a distinct basis element, and the sum has finitely many terms. Alternatively, the elements of a free abelian group may be thought of as signed
multisets containing finitely many elements
of
, with the multiplicity of an element in the multiset equal to its coefficient in the formal sum.
Another way to represent an element of a free abelian group is as a
function from
![{\displaystyle B}](https://wikimedia.org/api/rest_v1/media/math/render/svg/47136aad860d145f75f3eed3022df827cee94d7a)
to the integers with finitely many nonzero values; for this functional representation, the group operation is the
pointwise addition of functions. (
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