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    • CommentRowNumber1.
    • CommentAuthorzskoda
    • CommentTimeOct 7th 2010

    New entry internal diagram, generalizing internal functor.

    • CommentRowNumber2.
    • CommentAuthorFinnLawler
    • CommentTimeOct 7th 2010

    Most of this is already at internal profunctor, I think.

    • CommentRowNumber3.
    • CommentAuthorzskoda
    • CommentTimeOct 7th 2010
    • (edited Oct 7th 2010)

    Thanks. In internal diagram I treated an internal diagram in a cartesian category. Entry internal profunctor has a section on internal diagrams in an indexed category, what is a bit more general situation and there described in different terms.

    • CommentRowNumber4.
    • CommentAuthorFinnLawler
    • CommentTimeOct 8th 2010

    Yes, but what internal profunctor describes as an ’internal presheaf’ is the usual sort of internal diagram. Actually I wrote most of that page and I remember that I didn’t know any unambiguous name for ’diagrams’ in an indexed category, so I just used the heading ’Internal diagrams’ (I think that’s what Johnstone calls them), though that may have been a poor choice.

    These things seem to have far too many different names – internal presheaves, diagrams, discrete fibrations… Maybe we should settle on a single term?

    • CommentRowNumber5.
    • CommentAuthorMike Shulman
    • CommentTimeOct 8th 2010

    I don’t think there’s much hope of settling on a single term, given that they’re all used in the literature; we should just make sure that there are plenty of redirects and cross-links.

    Of course, “discrete fibration” is more general in that it applies in any 2-category, not just a 2-category of internal categories. And “diagram” is more general in that it applies to diagrams in any indexed category. So each page can treat the natural level of generality for the concept with the appropriate name.

    • CommentRowNumber6.
    • CommentAuthorFinnLawler
    • CommentTimeOct 8th 2010

    Fair point. Perhaps we should move the section on diagrams in an indexed category from internal profunctor to internal diagram, since it would be more at home there.

    • CommentRowNumber7.
    • CommentAuthorzskoda
    • CommentTimeOct 8th 2010
    • (edited Oct 8th 2010)

    It is OK with me. Somebody could cite then 1984 paper on coherence by MacLane and Pare where they have a part of the paper dedicated to internalization of concepts in an index category with diagram/category in an indexed category an example. If we denote again by Cat(FC)Cat(F\to C) the 2-category of diagrams in a fibered category FCF\to C and Cat(C)Cat(C) the 2-category of internal category in the base, the natural projection among the two is not necessary a 2-fibration, but it satisfies 2 out of 4 universality conditions from Hermida’s paper on 2-fibrations.

    internal presheaves

    When we take the default terminology favoring contravariant internal presheaves then they correspond to discrete opfibrations.

    • CommentRowNumber8.
    • CommentAuthorFinnLawler
    • CommentTimeOct 9th 2010

    I’ve moved that section across now.

    • CommentRowNumber9.
    • CommentAuthorFinnLawler
    • CommentTimeOct 17th 2010

    Some more edits and additions to internal diagram, mostly expanding the definition.

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