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    • CommentRowNumber1.
    • CommentAuthorBartek
    • CommentTimeApr 15th 2017
    • (edited Apr 15th 2017)

    I was wondering if anyone could add some more examples to the pages on powers and copowers?

    The only examples featured on both pages are the obvious ones:

    1. a closed monoidal category is (co)powered over itself

    2. (co)complete locally small categories are (co)powered over Sets.

    For instance, are there any good examples of abelian categories, such as abelian sheaves, which are (co)powered over abelian groups? Or categories which are (co)powered over chain complexes?

    • CommentRowNumber2.
    • CommentAuthorTodd_Trimble
    • CommentTimeApr 15th 2017

    This was asked also at MO; I guess you are the author. I think the reason the question wasn’t received better is that there’s an “embarrassment of riches”: most of the reasonable examples encountered in the wild, such as abelian sheaves, have powers and copowers. I added a comment on that at MO.

    Here’s an example where an enriched category is SetSet-complete but not complete in the enriched sense: consider monoids MM as 1-object categories BMB M, so that monoids are enriched in CatCat (the CatCat-hom being given by Cat(BM,BN)Cat(B M, B N)). Then CatCat-powers don’t exist in MonMon. For example, if 2\mathbf{2} is the arrow category and MM has more than one element, then the power M 2M^\mathbf{2} doesn’t exist in MonMon (in CatCat it’s the category whose objects are elements of MM and where morphisms mnm \to n are elements aa such that am=naa m = n a).

    • CommentRowNumber3.
    • CommentAuthorBartek
    • CommentTimeApr 19th 2017
    • (edited Apr 19th 2017)

    Thanks! Your answer on MO was very helpful. When I have some time I’ll add some of your examples to the nLab page.

    • CommentRowNumber4.
    • CommentAuthorTodd_Trimble
    • CommentTimeApr 19th 2017

    Thanks, Bartek. I had added something ever so brief to copower, but there’s room for other additions. :-)