Martins-Porter paper

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