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    • CommentRowNumber1.
    • CommentAuthorDmitri Pavlov
    • CommentTimeMay 1st 2019

    Page created, but author did not leave any comments.

    v1, current

    • CommentRowNumber2.
    • CommentAuthorMike Shulman
    • CommentTimeMay 5th 2019

    Added a link to quasi-topological space.

    diff, v2, current

    • CommentRowNumber3.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    Added a comparison made to condensed sets as not forming a topos.

    diff, v3, current

    • CommentRowNumber4.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    Added that the topos of pyknotic sets is not cohesive.

    diff, v3, current

    • CommentRowNumber5.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    Added a few more bits and pieces.

    diff, v3, current

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeJun 11th 2019

    Have not been following any details, but just glancing at the entry right now, something needs qualification here: Presently it says that condensed sets do not form a topos, but following the link gives that condensed sets are sheaves on some site. Something amiss here. I guess some size issues?

    • CommentRowNumber7.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    Oh yes, I remember thinking that was odd.

    As emphasised by Scholze, however, the distinction between pyknotic and condensed does have some consequences beyond philosophical matters. For example, the indiscrete topological space {0,1}\{0,1\}, viewed as a sheaf on the site of compacta, is pyknotic but not condensed (relative to any universe). By allowing the presence of such pathological objects into the category of pyknotic sets, we guarantee that it is a topos, which is not true for the category of condensed sets. (p. 4)

    But Scholtze speaks of the topos of condensed sets. It sounds like size is at stake. What’s going on exactly?

    • CommentRowNumber8.
    • CommentAuthorMike Shulman
    • CommentTimeJun 11th 2019

    It sounds like it’s just size technicalities.

    • CommentRowNumber9.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019
    • (edited Jun 11th 2019)

    Seems likely. The paragraph before the one I cited in #7, says of the difference between approaches that ” it is a matter of set theory”.

    But still, what should be written on our pages when one person says condensed sets form a topos and another says they don’t?

    • CommentRowNumber10.
    • CommentAuthorMike Shulman
    • CommentTimeJun 11th 2019

    In the other discussion I just linked to, it sounded to me as though condensed sets were an elementary topos but not a Grothendieck one. But I have not actually read any of the literature myself.

    Could we include on the page some explanation of the origin of the bizarre word “pyknotic”, whatever it might be?

    • CommentRowNumber11.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    Added etymology.

    diff, v5, current

    • CommentRowNumber12.
    • CommentAuthorDavid_Corfield
    • CommentTimeJun 11th 2019

    For any uncountable strong limit cardinal κ\kappa, the category of κ\kappa-condensed sets is the category of sheaves on the site of profinite sets of cardinality less than κ\kappa, with finite jointly surjective families of maps as covers.

    The category of condensed sets is the (large) colimit of the category of κ\kappa-condensed sets along the filtered poset of all uncountable strong limit cardinals κ\kappa.

    • CommentRowNumber13.
    • CommentAuthorMike Shulman
    • CommentTimeJun 11th 2019

    Thanks. I wonder whether it is related to the infinitary-pretopos of small sheaves on the large site of all profinite sets.

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