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• CommentRowNumber1.
• CommentAuthorFinnLawler
• CommentTimeNov 4th 2010

I’ve added to Eilenberg-Moore category an explicit definition of EM objects in a 2-category and some other universal properties of EM categories, including Linton’s construction of the EM category as a subcategory of the presheaves on the Kleisli category.

Question: can anyone tell me what Street–Walters mean when they say that this construction (and their generalised one, in a 2-category with a Yoneda structure) exhibits the EM category as the ‘category of sheaves for a certain generalised topology on’ the Kleisli category?

• CommentRowNumber2.
• CommentAuthorTim_Porter
• CommentTimeNov 4th 2010
• (edited Nov 4th 2010)

I created a page for Paul-André Melliès. I noted that he has a neat paper: that I had not seen.

• CommentRowNumber3.
• CommentAuthorFinnLawler
• CommentTimeNov 4th 2010

It’s a nice paper, all right. I’ve put that link to the PDF on the Eilenberg-Moore category page.

• CommentRowNumber4.
• CommentAuthorMike Shulman
• CommentTimeNov 4th 2010

can anyone tell me what Street–Walters mean when they say that this construction … exhibits the EM category as the ‘category of sheaves for a certain generalised topology on’ the Kleisli category?

My only guess is that they mean that the EM category is a subcategory of presheaves on the Kleisli category, just as the category of sheaves on a site is a subcategory of the category of presheaves. I don’t see any closer relationship than that, but perhaps they had something else in mind.

• CommentRowNumber5.
• CommentAuthorFinnLawler
• CommentTimeNov 4th 2010

Hmm. I’m still trying to understand Melliès’s paper (among many others), but he refers to Berger’s A cellular nerve for higher categories — remark 1.7 there describes a Grothendieck topology on the category $Fin$ of finite sets such that models of a Lawvere theory are the presheaves that restrict to sheaves on $Fin$. I wondered if this (replacing the theory with some $Kl(T)$ and the ‘arities’ $Fin$ with the base category) was what Street & Walters were alluding to. Then again, I’m not even sure if that makes sense…

• CommentRowNumber6.
• CommentAuthorzskoda
• CommentTimeNov 4th 2010

they mean that the EM category is a subcategory of presheaves on the Kleisli category

Reflective or not ?

• CommentRowNumber7.
• CommentAuthorMike Shulman
• CommentTimeNov 4th 2010

I can’t imagine that it would be reflective, since the EM and Kleisli categories are the same “size” whereas the presheaf category is one size bigger. And no, I wouldn’t either be inclined to call the objects of a non-reflective subcategory of a presheaf category any kind of “sheaves;” I was just saying that’s the only relationship I can think of.

• CommentRowNumber8.
• CommentAuthorTim_Porter
• CommentTimeNov 6th 2010

@Finn Why not ask Paul-André himself. We were talking on Skype about the area last night and I mentioned that you had some problem understanding the point you mentioned (I gave him a link to the discussion.) Send him an e-mail. Or more generally ask the Cat list for help. They are usually very helpful.

• CommentRowNumber9.
• CommentAuthorFinnLawler
• CommentTimeNov 6th 2010

@Tim: Good idea, thanks! I might just do that (in between the other hundred and one things I have to do. Oh, for the life of a Ph.D. student…)

• CommentRowNumber10.
• CommentAuthorTim_Porter
• CommentTimeNov 6th 2010

@Finn I can think of worse places to be a PhD student than Baile atha Cliath. I have fond memories of TCD back in the 1970s. I was in UCC in those days and would go up to Dublin for a meeting usually in December (essentially Irish Math Soc., but it did not yet exist.) I stayed several times on a floor in Trinity. Breakfast at Bewley’s … I remember a full Irish breakfast, but that is perhaps a memory only. Then off to the talks.

• CommentRowNumber11.
• CommentAuthorFinnLawler
• CommentTimeNov 6th 2010

Yes, I suppose it could be worse. That was just me putting on the ’poor mouth’!

• CommentRowNumber12.
• CommentAuthorTim_Porter
• CommentTimeNov 6th 2010

An Béal Bocht by Myles na gCopaleen??

I have never read that. I have the Third Policeman. That would be a good book to mention for its quantum theory!

• CommentRowNumber13.
• CommentAuthorFinnLawler
• CommentTimeNov 6th 2010

The ’poor mouth’ as in a particularly Irish kind of maudlin self-pity.

I’ve never read An Béal Bocht either (my Irish was never up to it), but The Third Policeman is one of the funniest books I’ve ever read. Whenever I see a sheep I think

What is a sheep only millions of little bits of sheepness all whirling around and doing intricate convolutions inside the sheep? What is it but that?

• CommentRowNumber14.
• CommentAuthorTim_Porter
• CommentTimeNov 6th 2010

But Myles was tongue in cheek. (There is a translation of An Béal Bocht.)

• CommentRowNumber15.
• CommentAuthorFinnLawler
• CommentTimeNov 6th 2010

As I understand it, An Béal Bocht is a (pretty vicious) satire on Gaeltacht misery memoirs by people like Peig Sayers and Tomás Ó Criomhthain, and the slum-tourist industry they brought about, hence the title — ’putting on the poor mouth’ means engaging in self-pitying lamentation, usually with an ulterior motive.

• CommentRowNumber16.
• CommentAuthorTim_Porter
• CommentTimeNov 6th 2010

I think he could be very vicious at times.

• CommentRowNumber17.
• CommentAuthorzskoda
• CommentTimeNov 7th 2010

And no, I wouldn’t either be inclined to call the objects of a non-reflective subcategory of a presheaf category any kind of “sheaves;”

Well, there are cases where this is justified, namely the sheaves on noncommutative spaces and, similarly, sheaves on a Q-category. The sheaf condition is over there not with respect to covers which are cones over discrete set of objects but rather over cones over more general diagrams. This is alike the situation in enriched category theory where conical diagrams are replaced by weighted limits, and they are still called limits. In noncommutative geometry, the sheaf condition is always more general than one coming from Grothendieck topology. There are lots of examples of noncommutative sheaves and bundles which are rich enough and behave too well not to justify the name of sheaf.

• CommentRowNumber18.
• CommentAuthorzskoda
• CommentTimeNov 7th 2010

Sheaves are about 1-categorical local gluing conditions. The fact that for the sheaves of sets on commutative spaces, and on sites in particular one has a general nonsense characterization of sheafification is not in my opinion more important than the original motivation of gluing from local patches. Now every sensible notion of local has appropriate version of gluing, hence appropriate kind of sheaf theory.

• CommentRowNumber19.
• CommentAuthorMike Shulman
• CommentTimeNov 7th 2010

@Zoran: fair enough. I should have said “I wouldn’t be inclined to call the objects of a category ’sheaves’ just because they are a full subcategory of some presheaf category, without some additional reason to believe that they behave in a sheaf-like manner.”

• CommentRowNumber20.
• CommentAuthorzskoda
• CommentTimeNov 8th 2010

That is also fair enough :=)

• CommentRowNumber21.
• CommentAuthorUrs
• CommentTimeOct 20th 2015
• (edited Oct 20th 2015)

At Eilenberg-Moore category I have tried to make the paragraph on the relation of $T$-algebras to free $T$-algebras more explicit and more comprehensive, now a small new subsection titled As a colimit completion of the Kleisli category.

First of all I added the statement of the universal Beck coequalizer, for completeness, and then I edited the formatting and the citations for the characterization via presheaves on the Kleisli category a bit.

Similarly I have touched the Definition section, trying to edit a bit for readability.

• CommentRowNumber22.
• CommentAuthorZhen Lin
• CommentTimeOct 20th 2015

Proposition 1 is a bit ambiguous. It is an absolute coequaliser in the base but not in the category of algebras. Also, if you choose the definitions of $C_T$ and $F_T$ right, the pullback diagram in Proposition 2 is also a bicategorical pullback diagram.

• CommentRowNumber23.
• CommentAuthorUrs
• CommentTimeOct 20th 2015

True, I have fixed the wording.