# Matrices vs. idempotent ultrafilters

Note: as you can see I am not yet in control of how to convert LaTeX to mathml — bear with me, but I thought I should kick myself and start posting…

The other day I was looking for someone to chat about an interesting example of ﬁnite semigroups. So ~~yesterday~~ last week I finally met up with a friend who offered to do just that. The ‘results’ of the morning we spent chatting are perfect blogging material: quite simple, mostly elementary, easily open for discussion and still carry some interest. However, it is much too long, so I’ll split it into a series of posts.

So to start: what’s the example?

**Example** The matrices

generate an 8-element (multiplicative) subsemigroup. Its elements are

So what? Well, what is interesting is that although both

Still, why is it interesting? Well, this example is of interest for people working with ultrafilters on semigroups, in particular on

**Lemma** Every finite (discrete) semigroup is the image of the closed subsemigroup

This can be found as Corollary 6.5 in the book ’Algebra in the Stone–Čech compactification’ by Neil Hindman and Dona Strauss. What can we do with this?

**Corollary** There are idempotent elements in

**Proof**

- Step 1 Consider the (discrete) finite semigroup generated by
and . - Step 2 By the previous lemma, it is a continuous, homomorphic image of
. - Step 3 The preimage of both
and is a closed (by continuity) semigroup (by homomorphy) of . - Step 4 Conversely, the preimage of
cannot contain an idempotent (or else the image of that idempotent, , would be idempotent by homomorphy). - Step 5 In particular, by the Ellis-Numakura Lemma, both preimages contain idempotents
. - Step 6 But
is in the preimage of , hence not idempotent.

One can easily show more, i.e.,

Now of course one can look at finite semigroups abstractly. But the advantage of matrix representations is that it puts some flesh to the bones of abstraction.

Update: since fighting with mathml is tough for the time being, here is something to make up for that. pdf