# Dihedral Group on a square

1. Jan 30, 2012

### Ted123

1. The problem statement, all variables and given/known data

Let $G=D_4$ (the group of symmetries (reflections/rotations) of a square) and let $$X=\{ \text{colourings of the edges of a square using the colours red or blue} \}$$ so a typical element of X is:

What is the size of $X$?

Let $G$ act on X in the obvious way. You are given that $G$ has 6 orbits on X. Find a representative for each $G$-orbit, and its size.

3. The attempt at a solution

Obviously there are going to be various ways of colouring the edges of a square, but how can I be sure that I have them all or is there a quicker way to find the size?

How do I find a representative for each G-orbit?

2. Jan 30, 2012

### Joffan

Size of X: Each side has a choice of two colours, and there are 4 sides.

Orbits of G - Some properties of X will not be affected by the symmetry operations. This will naturally lead to separate orbits.

3. Jan 30, 2012

### Ted123

The definition of orbit is $$\text{orb}_G(x)= \{ gx : g\in G \}$$
Saying that G has 6 orbits on X: does this mean there are 6 $x$'s?

So by saying 'find a representative' does it mean find 6 different $x$'s?

4. Jan 30, 2012

### Joffan

It means (as I read it) there are six categories of $x$ that will be mapped only onto their own category by the symmetries in G.

For example: The $x$ that consists of all blue edges will be mapped to itself by any G. It is an orbit of size one.

5. Jan 30, 2012

### Ted123

Is the size of X 16?

6. Jan 30, 2012

### Joffan

That's what I get. It's a conveniently small-enough number that you can actually draw them all out, too, if you need to.

Last edited: Jan 30, 2012