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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 FinFin of finite sets such that models of a Lawvere theory are the presheaves that restrict to sheaves on FinFin. I wondered if this (replacing the theory with some Kl(T)Kl(T) and the ‘arities’ FinFin 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 TT-algebras to free TT-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 TC_T and F TF_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.

    • CommentRowNumber24.
    • CommentAuthorTim_Porter
    • CommentTimeJul 27th 2021

    Fixed two faulty links

    diff, v34, current

    • CommentRowNumber25.
    • CommentAuthorThomas Holder
    • CommentTimeJul 26th 2022

    Added RelRel as an example where not all monads have Eilenberg-Moore objects.

    diff, v35, current

    • CommentRowNumber26.
    • CommentAuthorjonsterling
    • CommentTimeAug 2nd 2022

    I’m a little confused by the discussion of ’universal TT-module’ — I assume it’s just the metaphor that I’m having trouble grasping. A (left) TT-module in the category CC is defined toward the beginning; then later we are suggested to view the forgetful functor C TCC^T\to C as the “universal (left) TT-module”, but we have not yet been instructed on how to think of this as a module of any kind, much less a universal one.

    Can someone help me understand what is meant here?

    • CommentRowNumber27.
    • CommentAuthorvarkor
    • CommentTimeAug 2nd 2022
    • (edited Aug 2nd 2022)

    Any adjunction LRL \dashv R, which induces a monad TT, induces a left TT-module whose underlying 1-cell is RR, and whose action is induced by the counit of the adjunction. The universal left TT-module is the left TT-module induced by F TU TF_T \dashv U_T. It is universal in the sense that it is induced by the representing object for the 2-presheaf sending each object XT\text{-Alg} : \mathcal KX to the category of XX-indexed left TT-modules. One place to read about this is §3 of Kelly–Street’s Review of the elements of 2-categories.

    • CommentRowNumber28.
    • CommentAuthorjonsterling
    • CommentTimeAug 2nd 2022

    @varkor Thanks, this reference has been helpful for me; in particular the discussion on page 169.

    • CommentRowNumber29.
    • CommentAuthorvarkor
    • CommentTimeOct 4th 2022

    Added redirects for name variants with en-dashes.

    diff, v37, current

    • CommentRowNumber30.
    • CommentAuthorarthuraa
    • CommentTimeOct 4th 2022
    • (edited Oct 4th 2022)

    It is claimed that the forgetful functor U:C TCU : C^T \to C is a terminal object in the full subcategory of Cat/C\mathrm{Cat} / C of right-adjoint functors whose induced monad is TT. I am having trouble seeing why this is the case. I do know that, given some right adjoint R:DCR : D \to C whose corresponding monad is TT, we can construct a functor F:DC TF : D \to C^T such that UF=RUF = R, where the algebras are induced by the counit of the adjunction. But I don’t see why this functor is unique. For instance, take C TC^T to be the category of bi-pointed sets (i.e., TX=X+2T X = X + 2). There is a functor G:C TC TG : C^T \to C^T that swaps the two points of a bi-pointed set. This functor satisfies UG=UU G = U, but it is not the same as the canonical functor FF constructed above, which in this case would be the identity. Am I missing something?

    • CommentRowNumber31.
    • CommentAuthorUrs
    • CommentTimeNov 6th 2022

    I have completed the publication data for

    and added pointer to:

    diff, v38, current

    • CommentRowNumber32.
    • CommentAuthorTim Campion
    • CommentTimeNov 22nd 2022

    I am pretty sure there was an erroneous dualization here in the treatment of comonads. The construction of the co-EM category is still a “limit-type” construction, so I am pretty sure that its universal property involves mapping in from other categories, rather than mapping out as was indicated here. I have taken the liberty of changing what the page says to reflect this, but it is possible I am mistaken.

    diff, v40, current

    • CommentRowNumber33.
    • CommentAuthorvarkor
    • CommentTimeNov 30th 2022

    Added reference to Structures pseudo-algébriques (1ère partie).

    diff, v41, current

    • CommentRowNumber34.
    • CommentAuthorBryceClarke
    • CommentTimeJan 25th 2023

    Added the original reference for the Eilenberg-Moore category: Samuel Eilenberg, John Moore, Adjoint functors and triples, Illinois J. Math. 9 (3), pp. 381 - 398, September 1965. (doi:10.1215/ijm/1256068141)

    diff, v42, current

    • CommentRowNumber35.
    • CommentAuthoranuyts
    • CommentTimeMar 2nd 2023

    Cite voutas.

    diff, v43, current

  1. Added a diagram

    Bartosz Milewski

    diff, v50, current

  2. Translated diagram to tikzcd

    Bartosz Milewski

    diff, v50, current