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To qualify as an abelian group, the set and operation, (,), must satisfy four requirements known as the abelian group axioms (some authors include in the axioms some properties that belong to the definition of an operation: namely that the operation is defined for any ordered pair of elements of A, that the result is well-defined, and that the ...
An object in Ab is injective if and only if it is a divisible group; it is projective if and only if it is a free abelian group. The category has a projective generator (Z) and an injective cogenerator (Q/Z). Given two abelian groups A and B, their tensor product A⊗B is defined; it is again an abelian group.
Finally, the name of the torsion subgroup of an infinite group shows the legacy of topology in group theory. A torus. Its abelian group structure is induced from the map C → C/(Z + τZ), where τ is a parameter living in the upper half plane.
Every elementary abelian p-group is a vector space over the prime field with p elements, and conversely every such vector space is an elementary abelian group. By the classification of finitely generated abelian groups, or by the fact that every vector space has a basis, every finite elementary abelian group must be of the form (Z/pZ) n for n a ...
This definition is equivalent [5] to the following "piecemeal" definition: A category is preadditive if it is enriched over the monoidal category Ab of abelian groups. This means that all hom-sets are abelian groups and the composition of morphisms is bilinear. A preadditive category is additive if every finite set of objects has a biproduct.
The universal cover of any connected Lie group is a simply connected Lie group, and conversely any connected Lie group is a quotient of a simply connected Lie group by a discrete normal subgroup of the center. Any Lie group G can be decomposed into discrete, simple, and abelian groups in a canonical way as follows. Write
D 1 and D 2 are the only abelian dihedral groups. Otherwise, D n is non-abelian. D n is a subgroup of the symmetric group S n for n ≥ 3. Since 2n > n! for n = 1 or n = 2, for these values, D n is too large to be a subgroup. The inner automorphism group of D 2 is trivial, whereas for other even values of n, this is D n / Z 2.
An Abelian 2-group is a groupoid (that is, a category in which every morphism is an isomorphism) with a bifunctor +: and natural transformations: + +: (+) + + (+) which satisfy a host of axioms ensuring these transformations behave similarly to commutativity and associativity () for an Abelian group.