In mathematics, specifically in abstract algebra, a torsion-free abelian group is an abelian group which has no non-trivial torsion elements; that is, a group in which the group operation is commutative and the identity element is the only element with finite order. That is, multiples of any element other than the identity element generate an infinite number of distinct elements of the group.
An abelian group ⟨ G , ∗ ⟩ is a set G, together with a binary operation * on G, such that the following axioms are satisfied:
AssociativityFor all
a,
b and
c in
G, (
a *
b) *
c =
a * (
b *
c).
Identity elementThere is an element
e in
G, such that
e *
x =
x *
e =
x for all
x in
G. This element
e is an
identity element for * on
G.
Inverse elementFor each
a in
G, there is an element
a′ in
G with the property that
a′ *
a =
a *
a′ =
e. The element
a′ is an
inverse of a with respect to *.
CommutativityFor all
a,
b in
G,
a *
b =
b *
a.
OrderFor this definition, note that in an
abelian group, the binary operation is usually called addition, the symbol for addition is “+” and a repeated sum,
a + a + ⋯ + a of the same element appearing
n times is usually abbreviated “
na”. Let
G be a group and
a ∈
G. If there is a positive integer
n such that
na =
e, the least such positive integer
n is the
order of a. If no such
n exists, then
a is of
infinite order.
TorsionA group
G is a
torsion group if every element in
G is of finite order.
G is
torsion free if no element other than the identity is of finite order.
A torsion-free abelian group has no non-trivial finite subgroups.A finitely generated torsion-free abelian group is free.
