occasional meanderings in physics' brave new world

Name:
Location: New Zealand

Marni D. Sheppeard

## Monday, June 29, 2009

### M Theory Lesson 282

Recall that the vertices of associahedra are described by all rooted binary trees of a certain height, such that degeneracies in the level of the nodes is permitted. For example, for trees with three vertices (including a root) there must be four leaves, and we obtain the five vertices of a pentagon. The edges of the pentagon are labelled by trees with only two nodes, which are the contractions of the trees on the boundary vertices. And the face of the pentagon itself is labelled by the single vertex four leaved tree. The associahedron for two leaves is a point and the associahedron for three leaves is a single edge. For each real dimension, there is an associahedron.

What about ternary trees? First observe that the real dimension must increase by two at each step, because ternary vertices increase the number of leaves by two at each branching. The first two ternary polytopes are described by the following trees. The second case has three points on a surface, with no marked edges, just like a Riemann sphere. The next case naturally lives in dimension four, so we only draw the seven leaved trees marking the \$12\$ points:

PhilG said...

It's funny to think that they started out looking at generalised hypergeometric functions. The hyperdeterminant was the inverse of the first term in special cases(or something like that) so they thought they had better understand it first. They ended up writing the book on just that.

One day somebody should go back and finish understanding the rest of the terms in the functions.

For the benefit of anyone else who wants to buy it, you could have ordered from Amazon.co.uk and got the cheaper paperback version.

July 02, 2009 10:44 PM
Kea said...

I did order from Amazon, and I only paid \$61 for a nice hard bound copy.

July 02, 2009 10:53 PM
PhilG said...

OOps I replied to the wrong post, glad to hear you got such a good deal.

Ordering stuff from the US to the UK can sometimes be a pain because the customs may randomly open it up and charge you VAT, duty and a big fee. I think books are fairly safe though.

July 02, 2009 11:44 PM