## Groups and Group Actions: Lecture 2

In which we meet the dihedral groups, build new groups from old, and explore Cayley tables.

• Definition of the $n$th cyclic group $C_n$.
• Definition of the $n$th dihedral group $D_{2n}$.
• Proposition 4: Let $P_n$ be a regular $n$-gon in the plane.  Write $r$ for the rotation anticlockwise by $2\pi/n$ about the centre of $P_n$, and $s$ for the reflection in an axis of $P_n$.  Then the symmetries of $P_n$ are $e$$r$$r^2$, …, $r^{n-1}$$s$$rs$$r^2s$, …, $r^{n-1}s$. We proved this by labelling the vertices of $P_n$ anticlockwise as $1$, $2$, …, $n$, then picking a symmetry $f$ of $P_n$, and considering where it sends the vertex $1$, splitting into two cases depending on whether the vertices of $f(P_n)$ are numbered anticlockwise or clockwise.
• Definition of the Cartesian product of two sets.
• Definition of the product group $(G\times H, \ast)$ of two groups $(G, \ast_G)$ and $(H, \ast_H)$.
• Proposition 5: The operation $\ast$ just defined is a group operation.  This was a straightforward check of the group axioms, details left as an exercise.
• Definition of the order of a group, and of a finite group.
• Definition of a Cayley table.
• Proposition 6: A Cayley table is a Latin square: each element of the group $G$ appears exactly once in each row and in each column.  For a fixed element $g$ of $G$, we considered row $g$ by defining a map $f_g: G \to G$ via $f_g(g') = gg'$, and checked that this is a bijection.

### Understanding today’s lecture

I encourage you to find the permutations of the vertices of the square corresponding to the eight symmetries we identified.  At the end, a couple of people asked me about the notation I used for the permutations of the vertices of the triangle, and it’s good to be clear about this.  When we rotate by $r$, the vertex in position 1 moves to position 2, the vertex in position 2 to position 3, and the vertex in position 3 to position 1, and that’s what I recorded on the board.  Hope that might help.  A good way to check your understanding would be to work out the permutations for the symmetries of the square, and then to check with Richard Earl’s online notes where he gives the permutations (top of page 11).

Can you fill in the details of the proof of Proposition 5 (that the product group really is a group)?

You could draw up the Cayley table for some groups we’ve seen so far, e.g. $D_8$, or $C_n$ for some sensibly small $n$ like 4 or 5, or $\{0,1\}$ under addition modulo 2 (that was an example from Lecture 1).  What’s the Cayley table for $C_2 \times C_3$?  That would be a really good question to explore, it would give you practice with cyclic groups, with a product group and with a Cayley table, but will also link with ideas we’ll meet later in the course.

Of course Wikipedia has a page about dihedral groups; it has many pretty pictures.  We talked about Cayley tables today; another way to represent a group is via a Cayley graph.  These crop up in various places, for example Fields medallist Terry Tao has written about them on his blog, e.g. here (warning: this post assumes knowledge of more advanced maths than first-year undergraduates usually have, but you might enjoy skim-reading the post to get a flavour without worrying about understanding it!).

We’ve seen lots of definitions so far in the course; you might enjoy reading the musings of Tim Gowers (coincidentally another Fields medallist) on what definitions are.  I also recommend his blog post about alternative definitions, which links nicely with my comment in the lecture today when we showed that $f_g$ (in Proposition 6) is a bijection by writing down an inverse.  Some of the examples he discusses are ones that you’ve already met in Analysis and Linear Algebra, others are things you’ll meet later in the Groups course (so you’ll get lots out of reading a little ahead via his blog post).

### Preparation for Lecture 3

Some of you will want to tackle the first problems sheet before our next lecture, so here are some definitions that you may find helpful.  I’ll give these officially in the lecture too, of course.

• Definition: Let $(G, \ast)$ be a group.  We say that a subset $H \subseteq G$ is a subgroup of $G$ if the restriction of $\ast$ to $H$ makes $H$ into a group, that is,
• $H$ is closed under $\ast$;
• $H$ has an identity;
• $H$ contains inverses.
• Definition: Let $G$ be a group, and take $g \in G$.  We define the order of $g$, $o(g)$, to be the smallest positive integer $k$ such that $g^k = e$.  If no such integer $k$ exists, then we say that $g$ has infinite order.
• Definition: Let $(G, \ast_G)$ and $(H, \ast_H)$ be two groups.  An isomorphism between $G$ and $H$ is a bijective map $\theta : G \to H$ such that $\theta(g_1 \ast_G g_2) = \theta(g_1) \ast_H \theta(g_2)$ for all $g_1, g_2 \in G$.  If such an isomorphism exists, then we say that $G$ and $H$ are isomorphic.

And now some questions for you to consider before the next lecture.

In the definition of a subgroup above, why have I not mentioned associativity?

In the Linear Algebra course, you have studied the structure-preserving maps between vector spaces; these are called linear maps.  In what sense is an isomorphism between groups also an example of a structure-preserving map?  (We shall meet more general structure-preserving maps, which need not always be bijections, later in the course; they are called homomorphisms.)

Last time, I mentioned that composition of functions is a binary operation on the set $\mathrm{Sym}(X)$ of bijections from a set $X$ to itself.  Is $\mathrm{Sym}(X)$ a group under composition of functions?  Is the operation commutative?  If $X$ is a set of size $n$, what is the size of the set $\mathrm{Sym}(X)$?