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Divisible GroupIn group theory, a divisible group is an abelian group G such that for any positive integer n and any g in G, there exists y in G such that ny = g. One can show that G is divisible if and only if G is an injective object in the category of Z-modules (abelian groups). Examples Structure theorem of divisible groups Let G be a divisible group. One can easily see that the torsion subgroup Tor(G) of G is divisible. Since a divisible group is an injective module, Tor(G) is a direct summand of G. So - .
As a quotient of a divisible group, G/Tor(G) is divisible. Moreover, it is torsion free. Thus, it is a vector space over Q and so there exists a set I such that - .
The structure of the torsion subgroup is harder to determine, but one can show that for all prime numbers p there exists such that -
where is the p-primary component of Tor(G). Thus, if P is the set of prime numbers, - .
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