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
jim stasheff wrote:
link didn't work even if I tacked the html onto the =
Sorry, the equals sign should not be there. Here's the link to the Martins-Porter paper on categorifying the Dijkgraaf-Witten model: http://golem.ph.utexas.edu/category/2006/08/categorifying_the_dijkgraafwit.h... and here, just for fun, is a letter from Grothendieck to Ronnie Brown: http://golem.ph.utexas.edu/category/2006/08/letter_from_grothendieck.html Best, jb
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John Baez