Subject: categories: Martins-Porter paper

From: John Baez <[email-masked]>
Date: Mon, 28 Aug 2006 18:01:21 -0700

Interesting letter from Grothendieck!

Ronnie Brown writes:

>A recent arXiv paper by Jo=E3o Faria Martins and Tim Porter
>math.QA/0608484
>Title: On Yetter's Invariant and an Extension of the Dijkgraaf-Witten
>Invariant to Categorical Groups
>
> seems also relevant to the theme of the Baez-Shulman notes.

Yes; we didn't get into TQFTs at all in our notes, but Martins
and Porter use the classifying space of a discrete 2-group
(=3D categorical group =3D crossed module =3D gr-category) in their
construction, and that's quite relevant.

I wrote a little introduction to the Martins-Porter paper and its
antecedents at the n-category cafe:

http://golem.ph.utexas.edu/category/2006/08/categorifying_the_dijkgraafwit.=
html

Best,
jb