# The Unapologetic Mathematician

## Group actions

Okay, now we’ve got all the setup for one big use of group theory.

Most mathematical structures come with some notion of symmetries. We can rotate a regular $n$-sided polygon $\frac{1}{n}$ of a turn, or we can flip it over. We can rearrange the elements of a set. We can apply an automorphism of a group. The common thread in all these is a collection of reversible transformations. Performing one transformation and then the other is certainly also a transformation, so we can use this as a composition. The symmetries of a given structure form a group!

What if we paint one side of the above $n$-gon black and the other white, so flipping it over definitely changes it. Then we can only rotate. The rotations are the collection of symmetries that preserve the extra structure of which side is which, and they form a subgroup of the group of all symmetries of the $n$-gon. The symmetries of a structure preserving some extra structure form a subgroup!

As far as we’re concerned right now, mathematical structures are all built on sets. So the fundamental notion of symmetry is rearranging the elements of a set. Given a set $S$, the set of all bijections from $S$ to itself ${\rm Bij}(S)$ is a group. We’ve actually seen a lot of these before. If $S$ is a finite set with $n$ elements, ${\rm Bij}(S)$ is the symmetric group on $n$ letters.

Now, a group action on a set is simply this: a homomorphism from $G$ to ${\rm Bij}(S)$. That is, for each element $g$ in $G$ there is a bijection $p_g$ of $S$ and $p_{gh}(x)=p_g(p_h(x))$.

It’s important to note what seems like a switch here. It’s really due to the notation, but can easily seem confusing. What’s written on the right is “do the permutation corresponding to $h$, then the one corresponding to $g$“. So we have to think of the multiplication in $G$ as “first do $h$, then do $g$“.

In what follows I’ll often write $p_g(x)$ as $gx$. The homomorphism property then reads $(gh)x=g(hx)$

I’ll throw out a few definitions now, and follow up with examples in later posts.

We can slice up $S$ into subsets so that if $x$ and $x'$ are in the same subset, $x'=gx$ for some $g$, and not if they’re not in the same subset. In fact, this is rather like how we sliced up $G$ itself into cosets of $H$. We call these slices the “orbits” of the $G$ action.

As an important special case of the principle that fixing additional structure induces subgroups, consider the “extra structure” of one identified point. We’re given an element $x$ of $S$, and want to consider those transformations in $G$ which send $x$ to itself. Verify that this forms a subgroup of $G$. We call it the “isotropy group” or the “stabilizer” of $x$, and write it $G_x$.

I’ll leave you to ponder this: if $x$ and $x'$ are in the same $G$-orbit, show that $G_x$ and $G_{x'}$ are isomorphic.

February 20, 2007

## Today was a good day

I didn’t even have to use my AK.

Dartmouth went well. The drive was completely clear — about two and a half hours in each direction. I got some time with Dr. Chernov, after which I have to go check on some technical points about sorts of graded Lie algebras. Also I got to see his student, Allison Henrich, and find out what she’s doing with Legendrian virtual knots. The talk itself was well-received, and the dinner afterwards was very enjoyable.

About the only sour note was the fact that Dartmouth has completed its postdoc search, and I’m not it. It was a bit of a stretch to fit into what they were looking for anyhow, so I’m not horribly surprised. At least now I know. And knowing is half the battle.

So I’m back in New Haven, and the other instructors from my calculus class are raising points now about the test that’s been printed already and is to be given tomorrow. No rest for the wicked.