In the theory of abelian groups, the torsion subgroup A_{T} of an abelian group A is the subgroup of A consisting of all elements that have finite order. An abelian group A is called a torsion (or periodic) group if every element of A has finite order and is called torsionfree if every element of A except the identity is of infinite order.
The proof that A_{T} is closed under addition relies on the commutativity of addition (see examples section).
If A is abelian, then the torsion subgroup T is a fully characteristic subgroup of A and the factor group A/T is torsionfree. There is a covariant functor from the category of abelian groups to the category of torsion groups that sends every group to its torsion subgroup and every homomorphism to its restriction to the torsion subgroup. There is another covariant functor from the category of abelian groups to the category of torsionfree groups that sends every group to its quotient by its torsion subgroup, and sends every homomorphism to the obvious induced homomorphism (which is easily seen to be welldefined).
If A is finitely generated and abelian, then it can be written as the direct sum of its torsion subgroup T and a torsionfree subgroup (but this is not true for all infinitely generated abelian groups). In any decomposition of A as a direct sum of a torsion subgroup S and a torsionfree subgroup, S must equal T (but the torsionfree subgroup is not uniquely determined). This is a key step in the classification of finitely generated abelian groups.
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ppower torsion subgroups
For any abelian group A and any prime number p the set A_{Tp} of elements of A that have order a power of p is a subgroup called the ppower torsion subgroup or, more loosely, the ptorsion subgroup:
The torsion subgroup A_{T} is isomorphic to the direct sum of its ppower torsion subgroups over all prime numbers p:
When A is a finite abelian group, A_{Tp} coincides with the unique Sylow psubgroup of A.
Each ppower torsion subgroup of A is a fully characteristic subgroup. More strongly, any homomorphism between abelian groups sends each ppower torsion subgroup into the corresponding ppower torsion subgroup.
For each prime number p, this provides a functor from the category of abelian groups to the category of ppower torsion groups that sends every group to its ppower torsion subgroup, and restricts every homomorphism to the ptorsion subgroups. The product over the set of all prime numbers of the restriction of these functors to the category of torsion groups, is a faithful functor from the category of torsion groups to the product over all prime numbers of the categories of ptorsion groups. In a sense, this means that studying ptorsion groups in isolation tells us everything about torsion groups in general.
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