Group Theory, Part 3: Direct and Semidirect Products
October 26th, 2017
Direct Products
Definition: For a finite number of groups Gi, i≤n, the direct product G=G1×G2×⋯×Gn is the direct product of sets, with multiplication defined componentwise.
Evidently, each Gi⊴ in a direct product, as it is exactly the kernel of the projection which erases the ith coordinate. For a particular example, note that:
Z(G_1 \times G_2 \times \dots \times G_n) = Z(G_1)\times \dots \times Z(G_n)
In particular, note that a group is abelian iff Z(G) =G (i.e. every element commutes with every other element). The above tells us that a direct product is abelian iff each of its factors is abelian.
The Fundamental Theorem of Finitely Generated Abelian Groups
Definition: A group G is finitely generated if it is the span of a finite number of elements.
Definition: We call \mathbb{Z} a free abelian group. A free abelian group of rank r is the direct product \mathbb{Z}\times\mathbb{Z} \times \dots \times \mathbb{Z}^r \cong \mathbb{Z}^r.
The Fundamental Theorem of Finitely Generated Abelian Groups
Let G be a finitely generated abelian group. Then:
G \cong \mathbb{Z}^r \times \mathbb{Z}_{n_1} \times \dots \times \mathbb{Z}_{n_s}
Such that n_{i+1} \mid n_i for each i, r \geq 0 and each n_j \geq 2. Furthermore, the expression is unique.
This is really a consequence of the structure theorem for finitely generated modules over a PID, which is covered later on.
Definition: The number r above is called the free rank or Betti number of G. The integers n_i are called the invariant factors of G; the above is called the invariant factor decomposition of G.
Using the above theorem, we can indeed classify all the finitely generated abelian groups of a given order n. The problem is reduced to finding a sequence n_1, \dots n_s of integers such that:
n_1 \dots n_s = n
And furthermore:
n_{i+1} \mid n_i
Note that each n_i divides n_i; so in particular, n_i contains every prime factor of n.
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