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    • CommentRowNumber1.
    • CommentAuthormaxsnew
    • CommentTimeFeb 26th 2018

    I started comma double category. Since I care about equipments more than double categories in general, and because it actually is an instance of a comma object, I made the article mostly about virtual double categories. I wrote down a couple of conjectures about when the comma has units and composites, but haven’t verified them yet and not sure when I will.

    • CommentRowNumber2.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 26th 2018

    A good start, thanks! I agree that your conjectures are true, because (e.g. in the second case) the F-category of pseudo double categories and strong and lax double functors has comma objects of strong functors over lax ones, since they are a rigged limit.

    One of my long-term projects is to find an abstract context in which to talk about things like comma objects of oplax over lax functors. I feel like it should have something to do with the fact that there’s a double category whose horizontal and vertical arrows are oplax and lax functors respectively, since the squares in the latter are exactly the right shape to be the universal 2-cell of such a comma object, but I haven’t found a good way to formulate the universal property yet.

    • CommentRowNumber3.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 27th 2018

    I added some information about the colax/lax case, and the example of Dialectica constructions.

    • CommentRowNumber4.
    • CommentAuthormaxsnew
    • CommentTimeMay 9th 2018

    Mike, in your description of the Dialectica construction is an internal poset just a monad in span that is jointly monic?

    • CommentRowNumber5.
    • CommentAuthorMike Shulman
    • CommentTimeMay 9th 2018

    Yes, since a monad in Span is an internal category. (Well, to be more precise an internal category with joinly-monic (s,t)(s,t) is an internal preorder, but who’s counting.)