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    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeMay 5th 2010

    added the statement of the Fubini theorem for ends to a new section Properties.

    (I wish this page would eventually give a good introduction to ends. I remember the long time when I banged my head against Kelly’s book and just didn’t get it. Then suddenly it all became obvious. It’s some weird effect with this enriched category theory that some of it is obvious once you understand it, but looks deeply mystifying to the newcomer. Kelly’s book for instance is a magnificently elegant resource for everyone who already understands the material, but hardly serves as an exposition of the ideas involved. I am hoping that eventually the nLab entries on enriched category theory can fill this gap. Currently they do not really. But I don’t have time for it either.)

    • CommentRowNumber2.
    • CommentAuthorTodd_Trimble
    • CommentTimeMay 5th 2010
    I'll look to see what can be done, but can you give an idea of how you would improve it?

    If one understands what an extranatural transformation is, then I claim ends *are* obvious. And I've heard several very positive comments about the page on extranatural transformations (Dan Piponi, and John Baez no less!). So we may not be in such bad shape, really.

    I never had quite this difficulty with enriched category theory, or if I did I don't remember it. So maybe I'm not the best one to do edits.
    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeMay 5th 2010

    There are some things where one can read and understand each single definition, and still not get the hang of how the subject ticks.

    We’d need somebody familiar with category theory but unfamiliar with enriched category theory to tell us about his feelings when reading our entries on enriched category theory.

    (The page on extranatural transformations is indeed nice, by the way. )

    • CommentRowNumber4.
    • CommentAuthorMike Shulman
    • CommentTimeMay 5th 2010

    Ends and coends made a lot more sense to me when I started thinking of them as tensor products and homs of (pro)functors. The definition as a universal extranatural transformation never really clicked for me until after that. Specifically, I have found that it sometimes helps people not to think about the definition in terms of an arbitrary functor of two variables, but rather just as a tensor product or hom between two functors of one variable.

    One other thing that it might be helpful to add to the page would be an leisurely explanation of why the end of a homset chom(Fc,Gc)\int_c hom(F c,G c) is the set of natural transformations from FF to GG. This would then help motivate why enriched ends are used to define enriched objects of natural transformations.

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeMay 5th 2010
    • (edited May 5th 2010)

    Interesting. I had a similar experience:

    I had been looking at Kelly’s book with the impression that I see the columns of green letters, but not the Matrix encoded by them, if you know what I mean.

    Then at some point I drew a big diagram expressing the descent omega-category of an omega-category valued presheaf on some cover, which seemed to me to be a simplification of the expression in Street’s article. I understood that, but was wondering about that big diagram. Then Dominic Verity in a blog comment dropped the remark that this diagram expresses an end. After he said that, I could suddenly see all of the Matrix behind the green letters.

    And now with hindsight, it is so obvious that I feel a bit upset with myself that I had that trouble at the beginning. It was this experience that made me think that if a book like Kelly’s had existed, but one which had taken the time to chat a bit about basic examples and waste a little time here and there on becoming friends with the concepts, I would have avoided wasting a certain amount of my life time on not getting the obvious.

    But then I look now at the nLab material on enriched category theory that I coauthored, and I realize that I am just repeating history: the concept obvious to me I express in a few symbols in an nLab entry. Since it’s all obvious, it’s hard not to. Right? That’s the big problem with math in general. All ideas in math are obvious and evident. The trouble is communicating them via a bunch of black marks on paper.

    • CommentRowNumber6.
    • CommentAuthorMike Shulman
    • CommentTimeMay 5th 2010

    Yeah. (-:

    • CommentRowNumber7.
    • CommentAuthorEric
    • CommentTimeMay 6th 2010

    I still don’t get the obvious, but have hope :)

    • CommentRowNumber8.
    • CommentAuthorDavid_Corfield
    • CommentTimeJan 13th 2014

    I added a reference to the nn-Café discussion on Ends.

    • CommentRowNumber9.
    • CommentAuthorUrs
    • CommentTimeJun 8th 2018
    • (edited Jun 8th 2018)

    Is there a quick way to directly show, for 𝒞\mathcal{C} an enriched category, that 𝒞(c,)\mathcal{C}(c,-) preserves enriched ends, without passing through the general concept of weighted limits?

    • CommentRowNumber10.
    • CommentAuthorMike Shulman
    • CommentTimeJun 8th 2018

    Well, take whatever definition of “enriched end” you are using, prove that it’s equivalent to the definition in terms of weighted limits, transfer the proof that C(c,)C(c,-) preserves weighted limits across that equivalence, and beta-reduce. (-:

    • CommentRowNumber11.
    • CommentAuthorUrs
    • CommentTimeJun 9th 2018

    My question was if there is a direct way, shortcutting this route, without invoking the general concept of weighted limits.

    • CommentRowNumber12.
    • CommentAuthorTodd_Trimble
    • CommentTimeJun 9th 2018

    I would render Mike’s suggestion like this: for F:J opJCF: J^{op} \otimes J \to C, the end of FF if it exists is the representing object jF(j,j)\int_j F(j, j) for the functor C opVC^{op} \to V that takes cc to V J opJ(J(,),C(c,F(,)))V^{J^{op} \otimes J}(J(-, -), C(c, F(-, -))). So we have VV-natural isomorphisms

    C(c, jF(j,j)) V J opJ(J(,),C(c,F(,))) j,jJ(j,j)C(c,F(j,j)) endformulaforfunctorcategoryhom j jJ(j,j)C(c,F(j,j)) Fubiniformula jC(c,F(j,j)) Yonedalemma\array{ C(c, \int_j F(j, j)) & \cong & V^{J^{op} \otimes J}(J(-, -), C(c, F(-, -))) & \\ & \cong & \int_{j, j'} J(j, j') \multimap C(c, F(j, j')) & end\; formula\; for\; functor\; category\; hom \\ & \cong & \int_j \int_{j'} J(j, j') \multimap C(c, F(j, j')) & Fubini\; formula \\ & \cong & \int_j C(c, F(j, j)) & Yoneda\; lemma }
    • CommentRowNumber13.
    • CommentAuthorMike Shulman
    • CommentTimeJun 10th 2018

    My suggestion was a way to derive a direct way, by starting out with the general concept of weighted limits and then eliminating it, as Todd has done.

    • CommentRowNumber14.
    • CommentAuthorUrs
    • CommentTimeJun 10th 2018

    Todd, thanks, that is the derivation via the end realized as a weighted limit. I was hoping for something that would not need to introduce this concept. But I guess in the end it will be easiest to just introduce it after all.

    • CommentRowNumber15.
    • CommentAuthorTodd_Trimble
    • CommentTimeJun 10th 2018

    You don’t have to introduce the concept of “weighted limit” if you don’t want – the exact text of #12 never mentions it.

    Alternatively, consider that in SetSet-enriched category theory, limits in categories are ultimately reducible to limits in SetSet, being jointly preserved and reflected by representables. Similarly, once the notion of an end in a base of enrichment VV has been established, say following Kelly’s book, then the only sensible notion of end jF(j,j)\int_j F(j, j) in a general VV-category CC is simply dictated by the requirement that C(, jF(j,j))C(-, \int_j F(j, j)) takes cCc \in C to the appropriate end in VV, i.e., by the requirement that the representables C(c,)C(c, -) jointly preserve and reflect that end. In other words, arrange matters so that representables preserve ends by the very definition of end. The only work is a consistency check that this applies to C=VC = V, but this is trivial.

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