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    • CommentRowNumber1.
    • CommentAuthorTim_Porter
    • CommentTimeDec 5th 2011
    • (edited Dec 5th 2011)

    At comma object, Eduardo Pareja-Tobes put a query box which does not seem to have been ‘answered’. This was some time ago it seems.

    • CommentRowNumber2.
    • CommentAuthorMike Shulman
    • CommentTimeDec 6th 2011

    Thanks! I’ve replied and suggested to continue here.

    • CommentRowNumber3.
    • CommentAuthorTobyBartels
    • CommentTimeDec 6th 2011

    This same person also wrote for us factorization category and twisted arrow category.

    • CommentRowNumber4.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 27th 2018

    Six years later, I’m copying the conversation here and deleting the query box:

    Eduardo Pareja-Tobes: Not sure about this but, with the strict definition I think you end up having specified isos all around at the level of morphisms; comma categories as normally defined are comma objects in Cat, but not strict ones (of course they’re equivalent to the strict ones). I remember reading something like this in Makkai-Paré Accessible categories book

    Mike Shulman: As far as I can tell, they are strict. Given DD, functors p:DAp':D\to A, q:DBq':D\to B and a natural transformation σ:fpgq\sigma:f p'\Rightarrow g q', these data specify exactly for every dDd\in D, a triple (p(d),q(d),σ d)(p'(d), q'(d), \sigma_d) which is an object of the comma category. Perhaps you are remembering a related remark about pseudo-pullbacks versus iso-comma objects? (If you post your comments at the nForum, for instance on this discussion, other people will be more likely to see it.)

    • CommentRowNumber5.
    • CommentAuthorjesuslop
    • CommentTimeDec 14th 2019

    I’m seeing here that the diagram in the definition section, that is !included, renders badly with excessive clipping. I’m rewriting it in terms of \array and painting the 2-cell α\alpha with double stroke instead of single.

    diff, v34, current

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeNov 27th 2022
    • (edited Nov 27th 2022)

    I have briefly made explicit (here) the notion of “iso-comma objects” and added redirects to this entry (though this might deserve its own entry) so that other entries can refer to this (as the entry 2-pullback has been doing all along).

    While at it, I have made some trivial edits to the wording and formatting of the Definition-section, for streamlining.

    diff, v36, current

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