The Unapologetic Mathematician

Mathematics for the interested outsider

Diagrammatics for braided monoidal categories with duals

I’m exhausted from spending all morning and much of the afternoon purchasing a new (to me) car. As a result, I’ll just forward you to the excellent notes that Miguel Carrión Alvarez took in John Baez’ seminar on quantum gravity back in fall and winter of 2000-1.

Specifically, pay attention to the diagrammatics. He’s talking mostly about finite-dimensional vector spaces over the field of complex numbers, but most everything applies to a general (braided) monoidal category (with duals). Also, he draws his diagrams from top to bottom, while (as I keep reminding you) I write mine from bottom to top to make it easier to read off the algebraic notation.

We’ve already seen some of the basic pieces as braid, Temperley-Lieb, and tangle diagrams, but here each arc in the diagram carries a label from the objects of a category, and usually an arrow. We can move to a dual object by reversing the arrow or changing the label.

Morphisms can be put in boxes, with the incoming object in the bottom and the outgoing one at the top. The naturality for the dual morphisms basically says we can slide a morphism up over a cup or down under a cap to get its dual. Also, often a morphism will have a number of incoming or outgoing strands, which means that the incoming object is the tensor product of the objects on the incoming strands.

A braiding is written as a crossing (lower-left over upper-right), and the inverse of the braiding is written as the other kind of crossing. Naturality means that we can pull a morphism along a strand through a crossing.

There’s a lot more to the notes than just the diagrammatics, though. If you’re up to it, I highly recommend giving it all a look. If not, just look for the pictures and read the sections around them for the explanations. I’ll be back on Monday with more exposition.

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July 14, 2007 - Posted by | Category theory

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