Abstract Algebra: Theory and Applications

Subsection Classification of Finite Groups

It does not take any sophisticated tools to understand groups of order \(2p\text{,}\) where \(p\) is an odd prime. There are two possibilities — a cyclic group of order \(2p\) and the dihedral group of order \(2p\) that is the set of symmetries of a regular \(p\)-gon. The proof requires some close, tight reasoning, but the required theorems are generally just concern orders of elements, Lagrange's Theorem and cosets. See Exercise 9.4.55. This takes care of orders \(n=6,\,10,\,14\text{.}\)

For \(n=9\text{,}\) the upcoming Corollary 14.16 will tell us that any group of order \(p^2\) (where \(p\) is a prime) is abelian. So we know from this section that the only two possibilities are \({\mathbb Z}_9\) and \({\mathbb Z}_3\times{\mathbb Z}_3\text{.}\) Similarly, the upcoming Theorem 15.10 will tell us that every group of order \(n=15\) is abelian. Now this leaves just one possibility for this order: \({\mathbb Z}_3\times{\mathbb Z}_5\cong{\mathbb Z}_{15}\text{.}\)

We have just two orders left to analyze: \(n=8\) and \(n=12\text{.}\) The possibilities are groups we already know, with one exception. However, the analysis that these are the only possibilities is more complicated, and will not be pursued now, nor in the next few chapters. Notice that \(n=16\) is more complicated still, with \(14\) different possibilities (which explains why we stopped here).

For \(n=8\) there are \(3\) abelian groups, and the two non-abelian groups are the dihedral group (symmetries of a square) and the quaternions.

For \(n=12\) there are \(2\) abelian groups, and \(3\) non-abelian groups. We know two of the non-abelian groups as a dihedral group, and the alternating group on \(4\) symbols (which is also the symmetries of a tetrahedron). The third non-abelian group is an example of a “dicyclic” group, which is an infinite family of groups, each with order divisible by \(4\text{.}\) The order \(12\) dicyclic group can also be constructed as a “semi-direct product” of two cyclic groups — this is a construction worth knowing as you pursue further study of group theory. The order \(8\) dicyclic group is also the quaternions and more generally, the dicyclic groups of order \(2^k\text{,}\) \(k>2\) are known as “generalized quaternion groups.”

The following examples will show you how to construct some of these groups, while also exercising a few of the commands and allowing us to be more certain the following table is accurate.

Subsection Groups of Small Order as Permutation Groups

We list here constructions, as permutation groups in Sage, for all of the groups of order less than \(16\text{.}\)