# SF&PA – the Temperley-Lieb algebra

Hi all,

First, I’d like to thank the organizers for inviting me to post on their blog, and apologize for the low tech pictures in what follows.

As Noah mentioned, my name is Emily, I study subfactors and planar algebras, and that’s the back of my head at the top of this page (still). While Noah is taking you through the delights of subfactors sans analysis, I’ll say a few words about planar algebras to set the stage for their later appearance in subfactorland. For now, let’s leave definitions to a future post, and say a little bit about my favorite planar algebra: the Temperley-Lieb algebra.

To get a Temperley-Lieb picture, arrange $n$ points at the bottom of your page, and $n$ points at the top, and connect the points up among themselves in a non-crossing way:

We only consider such pictures up to isotopy — then the number of such pictures is exactly the $n^{th}$ Catalan number (since you can, for instance, read matching parenthesizations as directions for connecting up the $2n$ points). Now, form a vector space $TL_n$ whose basis is Temperley-Lieb pictures on $2n$ points. For instance,

We turn this vector space into an algebra by defining multiplication: The product of two boxes is the picture you get by stacking them:

But what about that loop in the middle? It’s not part of the data of a Temperley-Lieb picture, so we have to throw it out — but let’s remember it was there by multiplying the resulting picture by $\delta$ (If there had been $k$ circles, we’d have multiplied the picture by $\delta^k$).

If you enjoy multiplying Temperley-Lieb pictures, try this fun exercise: show that Temperley-Lieb is multiplicatively generated by elements $e_i$, which consist of $n-2$ through strings and a cup and a cap starting at the $i^{th}$ string:

and satisfy the relations $e_i^2 = \delta e_i$, $e_i e_j = e_j e_i$ if $|i-j|>1$ and $e_i e_{i \pm 1} e_i = e_i$ (hmm, don’t those last two relations sort of remind you of the braid group?)

One of the reasons we subfactoralists (subfactorers?) like Temperley-Lieb is that it has a lot of structure to it. For instance, we can define an involution $^*$ on $TL_n$ by horizontal reflection: So, for example:

and we can also define a trace by connecting the top points to the bottom points — the result is some number of loops in a $TL_0$ diagram, ie a power of $\delta$:

We call this a trace because it doesn’t care about the order of multiplication (just slide the bottom picture along the strings until it ends up on top).

This combination of a trace and an involution is pretty powerful, as it lets us define a bilinear form $\left< x, y \right> := \text{tr}(y^* x)$ on $TL_n$. Here’s a hard one for you: For which values of $\delta$ is this form positive definite?

Maybe that’s a good place to stop for now. Coming soon: why is Temperley-Lieb a planar algebra, instead of a just plain algebra?

## 5 thoughts on “SF&PA – the Temperley-Lieb algebra”

1. I would opt for “subfactotum/subfactota.”

2. Anonymous says:

Thanks this is a nice post.

3. muz says:

The old-skool drawings look pretty cool. Can we have some more?

4. hi if posible for you please sent for me leter or any writing about representation of temperley-lieb algebra that study n*n matricess and satisfied the constraints of this algebra

thank you very much