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nLab > Latest Changes: commutativity of limits and colimits

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  1.  
    • CommentRowNumber1.
    • CommentAuthorMarc Hoyois
    • CommentTimeJan 12th 2014

    Added to commutativity of limits and colimits the case of coproducts commuting with connected limits in a topos, and the generalization to higher topoi. This particular instance of commutativity is not mentioned very often, probably because it’s not very impressive in Set, but its generalization to higher topoi (for which I couldn’t find a reference) is more interesting. For instance, cofiltered limits commute with taking quotients by an ∞-group in an ∞-topos.

    • CommentRowNumber2.
    • CommentAuthorZhen Lin
    • CommentTimeJan 13th 2014

    I can believe the claim for finite connected limits – that property is really just inherited from SetSet. But why is true in the general case?

    • CommentRowNumber3.
    • CommentAuthorMarc Hoyois
    • CommentTimeJan 13th 2014

    I meant Grothendieck topos, was that the issue?

    Here’s a proof of the general case. If AA is an ∞-groupoid, the colimit functor H A→H\mathbf{H}^A\to\mathbf{H} can be identified with the forgetful functor H /Disc(A)→H\mathbf{H}_{/Disc(A)}\to\mathbf{H}, by the ∞-Giraud axioms. If CC is an ∞-category such that lim CDisc(A)≃Disc(A)\lim_C Disc(A)\simeq Disc(A) (e.g. if A is n-truncated and C is n-connected), then that forgetful functor creates CC-limits.

    • CommentRowNumber4.
    • CommentAuthorZhen Lin
    • CommentTimeJan 13th 2014

    Not being expert in ∞\infty-categories, I will take your word for it. In the ordinary case, you are saying that the forgetful functor ℰ /ΔX→ℰ\mathcal{E}_{/ \Delta X} \to \mathcal{E} creates connected limits (which is well-known) and identifying ℰ /ΔX\mathcal{E}_{/ \Delta X} with ℰ X\mathcal{E}^X. This is true so long as ℰ\mathcal{E} is an infinitary extensive category.

    • CommentRowNumber5.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 17th 2017

    At https://ncatlab.org/nlab/show/commutativity+of+limits+and+colimits#ColimitsStableByBaseChange surely one could also list regular extensive categories? Just checking I’m not being stupid (could be the case…)

    • CommentRowNumber6.
    • CommentAuthorMike Shulman
    • CommentTimeDec 18th 2017

    Yes… but why is that section there at all? It’s not an example of commutativity of limits and colimits. And we already have universal colimit.

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeDec 18th 2017

    It’s not an example of commutativity of limits and colimits.

    Are we talking about

    colimd∈D(F(d)× ZY)⟶≃(colimd∈DF(d))× ZY \underset{d \in D}{colim} (F(d) \times_Z Y) \stackrel{\simeq}{\longrightarrow} (\underset{d \in D}{colim} F(d)) \times_Z Y

    ?

    • CommentRowNumber8.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 18th 2017

    I thought so.

    • CommentRowNumber9.
    • CommentAuthorMike Shulman
    • CommentTimeDec 18th 2017

    Yes, that’s what we’re talking about.

    • CommentRowNumber10.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 18th 2017

    So how is it not an example?

    • CommentRowNumber11.
    • CommentAuthorMike Shulman
    • CommentTimeDec 18th 2017

    How is it an example? Commutativity of limits and colimits means colim D∘lim E=lim E∘colim Dcolim^D \circ lim^E = lim^E \circ colim^D as functors on a diagram category C D×EC^{D\times E}.

    • CommentRowNumber12.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 18th 2017
    • (edited Dec 18th 2017)

    Ah, I see your point. Not immediately obvious, if at all true.

    • CommentRowNumber13.
    • CommentAuthorRichard Williamson
    • CommentTimeDec 18th 2017
    • (edited Dec 18th 2017)

    Surely, if EE is •→•←•\bullet \rightarrow \bullet \leftarrow \bullet, and J:D×E→CJ : D \times E \rightarrow C sends (d,•→•←•)(d, \bullet \rightarrow \bullet \leftarrow \bullet) to F(d)→Z←YF(d) \rightarrow Z \leftarrow Y, and does the obvious levelwise thing on arrows in DD (i.e. constant on ZZ and YY), then the equation colim D∘lim E(J)=lim E∘colim D(J)colim^{D} \circ lim^{E}(J) = lim^{E} \circ \colim^{D}(J) expresses exactly that colimd∈D(F(d)× ZY)⟶≃(colimd∈DF(d))× ZY \underset{d \in D}{colim} (F(d) \times_Z Y) \stackrel{\simeq}{\longrightarrow} (\underset{d \in D}{colim} F(d)) \times_Z Y?

    • CommentRowNumber14.
    • CommentAuthorMike Shulman
    • CommentTimeDec 18th 2017

    @Richard: only if the colimit of a constant DD-diagram is equal to the object that it’s constant at. For 1-colimits this is true if DD is connected; for ∞\infty-colimits it’s only true if DD has a contractible nerve.

    • CommentRowNumber15.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 19th 2017

    I’m only interested in the case of 1-categories, which is not treated at universal colimit.

    • CommentRowNumber16.
    • CommentAuthorMike Shulman
    • CommentTimeDec 19th 2017

    Well, it should be. Just copy and paste the definition given and remove all the ∞\inftys.

    • CommentRowNumber17.
    • CommentAuthorUrs
    • CommentTimeDec 19th 2017
    • (edited Dec 19th 2017)

    While it’s not generally an example of the functorial commutaticity

    lim⟶D∘lim⟵C≃lim⟵C∘lim⟶D:𝒞 C×D→𝒞 \underset{\underset{D}{\longrightarrow}}{\lim} \circ \underset{\underset{C}{\longleftarrow}}{\lim} \simeq \underset{\underset{C}{\longleftarrow}}{\lim} \circ \underset{\underset{D}{\longrightarrow}}{\lim} \;\colon\; \mathcal{C}^{C \times D} \to \mathcal{C}

    the formula

    colimd∈D(F(d)× ZY)⟶≃(colimd∈DF(d))× ZY \underset{d \in D}{colim} (F(d) \times_Z Y) \stackrel{\simeq}{\longrightarrow} (\underset{d \in D}{colim} F(d)) \times_Z Y

    is still syntactically an exchange of the order of taking limits and colimits and as such is something that a reader may appreciate finding discussed on this page.

    So I vote for leaving the material there. But I added the warning: here

    • CommentRowNumber18.
    • CommentAuthorMike Shulman
    • CommentTimeDec 19th 2017

    I agree that a reader may appreciate finding something about it, but I think what they should find is a link to the place where it is properly discussed.

    • CommentRowNumber19.
    • CommentAuthorUrs
    • CommentTimeDec 19th 2017

    Sounds hardly objectionable. Maybe I am missing what is under debate.

    • CommentRowNumber20.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 19th 2017

    I just wanted to expand the list of examples, wherever the list is given. I was trying to find alternative hypotheses for a result of Todd’s (at this point, Lemma 1.5 here) and double checking before adding to the page on colimits/limits.

  21. Re #14: Thanks, Mike!

    To me, though, it seems more like the definition at commutativity of limits and colimits should not necessarily be taken as the default one in the non-connected case. Take a coproduct, for instance. Using the fold arrows out of the coproduct, one recovers the canonical arrow we are discussing, and it seems to me that the latter one is actually the one that more ’correctly’ expresses a useful notion of commutativity of limits and colimits. As Urs put it, syntactically it seems equally justifiable as well.

    Great that the warning has been added to the page, that is the most important thing I think.

    • CommentRowNumber22.
    • CommentAuthorMike Shulman
    • CommentTimeDec 19th 2017

    If anyone can formulate a general notion of “commutativity of limits and colimits” that includes pullback-stability of colimits and also all the other examples on the page, I’d love to hear it. Otherwise, I’d like to move the former to a different page. Having precise definitions and categorizing things appropriately helps to alleviate just this sort of confusion.

  23. As long as CC has an initial object, maybe we can just formally add an initial object ∅\emptyset to DD and send (∅,•→•←•)(\emptyset, \bullet \rightarrow \bullet \leftarrow \bullet) to ∅→Z←Y\emptyset \rightarrow Z \leftarrow Y? Haven’t checked carefully…

    • CommentRowNumber24.
    • CommentAuthorMarc Hoyois
    • CommentTimeDec 20th 2017

    I think universal colimits is an example of distributivity of limits over colimits, for the functor (a→b←c)→Cat(a\to b\leftarrow c) \to Cat sending aa to the given indexing category and b,cb,c to the point.

    • CommentRowNumber25.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 20th 2017

    FWIW I added the example I originally asked about.

  26. Re #24, I think is indeed the construction I was getting at in #21. Seems more relevant than plain commutativity in the non-connected case. But maybe #23 provides an alternative way to see it as an actual instance of plain commutativity if one insists on that (one can always add an initial object to CC as well, of course, and then just use the forgetful functor back to CC at the end). If I have not made a mistake, then #23 should also work in the ∞\infty-case.

    • CommentRowNumber27.
    • CommentAuthorMike Shulman
    • CommentTimeDec 26th 2017

    Yes, I think #24 works. Good job Marc!

    I think #23 works as long as the pullback of ∅→Z←Y\emptyset \to Z\leftarrow Y in CC is ∅\emptyset, i.e. as long as we already know that initial objects are stable under pullback (which is equivalent to the initial object being strict). But if we don’t yet know whether the initial object of CC is strict, and we want to formulate the condition for its being pullback-stable, then #23 doesn’t work to write that as a commutativity condition. I don’t think it helps to add a new initial object to CC either, since then the colimit of the modified diagram would be the new initial object rather than the “real” one in CC that we want to be talking about.

    Given that pullback-stability of colimits is an instance of distributivity, but is also sometimes an instance of commutativity, and the technicalities of regarding it as the latter are a little subtle, I still think we should move all substantial discussion and examples of pullback-stability to universal colimit. We can include explanations at commutativity of limits and colimits and distributivity of limits over colimits of how (and when) they specialize to pullback-stability, and links to these discussions from universal colimit. Objections?

    • CommentRowNumber28.
    • CommentAuthorMike Shulman
    • CommentTimeDec 26th 2017

    I went ahead and added #24 as an example to distributivity of limits over colimits.

    • CommentRowNumber29.
    • CommentAuthorDavidRoberts
    • CommentTimeDec 26th 2017

    Re #27 good idea, thanks.

    • CommentRowNumber30.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 25th 2018

    I implemented my suggestion at #27, and also moved the definition into a “Definition” section and added a section on preservation by functor categories and localizations (explaining in particular why filtered colimits commute with finite limits in all toposes).

    • CommentRowNumber31.
    • CommentAuthorUrs
    • CommentTimeJun 14th 2018

    tocuched the section Sifted colimits commute with finite products. Added hyperlinks and pointer to the reference given.

    diff, v24, current

    • CommentRowNumber32.
    • CommentAuthorvarkor
    • CommentTimeMay 30th 2022

    Added some references.

    diff, v27, current

    • CommentRowNumber33.
    • CommentAuthorvarkor
    • CommentTimeMay 30th 2022

    One thing I’m not sure about is that Eckmann and Hilton state:

    The transformation w, together with the question when w is an equivalence, have already been considered by Roos [8], under the heading of the distributivity of lim with respect to lim

    but on distributivity of limits over colimits, Roos is cited as defining distributivity rather than commutativity. So either Eckmann and Hilton are misinformed, or the nLab page is incorrect.

    • CommentRowNumber34.
    • CommentAuthorvarkor
    • CommentTimeMay 30th 2022

    It strikes me that commutation of limits and colimits can be seen as a kind of limit–colimit coincidence, which is essentially an absolute colimit phenomenon (see Mike’s answer to this MO question, for instance). Does this suggest that the enriched perspective of absolute weighted (co)limits could shed some light on commutation of limits and colimits?

    • CommentRowNumber35.
    • CommentAuthorUrs
    • CommentTimeMay 1st 2023

    added pointer to

    diff, v30, current

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