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    • CommentRowNumber1.
    • CommentAuthorTobias Fritz
    • CommentTimeNov 9th 2012
    • (edited Nov 9th 2012)
    In the definition, the article states "every object in C is a small object (which follows from 2 and 3)". The bracketed remark doesn't seem quite right to me, since neither 2 nor 3 talk about smallness of objects. Presumably this should better be phrased as in A.1.1 of HTT, "assuming 3, this is equivalent to the assertion that every object in S is small".

    Am I right? I don't (yet) feel confident enough with my category theory to change this single-handedly.
    • CommentRowNumber2.
    • CommentAuthorTodd_Trimble
    • CommentTimeNov 9th 2012

    Yes, you’re right. Someone should go through the article again and fix the mistakes (I may do so in the near future).

    • CommentRowNumber3.
    • CommentAuthorTobias Fritz
    • CommentTimeNov 10th 2012
    Thanks, Todd, I fixed this. (I feel a bit stupid for having my name there on the bottom although I've barely done anything...)
    • CommentRowNumber4.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 18th 2019

    Added well-poweredness and well-copoweredness to properties

    diff, v69, current

    • CommentRowNumber5.
    • CommentAuthorJohn Baez
    • CommentTimeFeb 8th 2020

    Changed “presentable” to “locally presentable” in first paragraph, to reduce the chance that people think a second distinct notion is being introduced.

    diff, v71, current

    • CommentRowNumber6.
    • CommentAuthorJohn Baez
    • CommentTimeFeb 8th 2020
    • (edited Feb 8th 2020)

    Why is Emily Riehl’s definition of “locally presentable” category in Categories in Context simpler than the nLab definition? Are they equivalent?

    The nLab says a categorry 𝒞\mathcal{C} is locally presentable iff

    1. 𝒞\mathcal{C} is a locally small category;

    2. 𝒞\mathcal{C} has all small colimits;

    3. there exists a small set SObj(𝒞)S \hookrightarrow Obj(\mathcal{C}) of λ\lambda-small objects that generates 𝒞\mathcal{C} under λ\lambda-filtered colimits for some regular cardinal λ\lambda.

      (meaning that every object of 𝒞\mathcal{C} may be written as a colimit over a diagram with objects in SS);

    4. every object in 𝒞\mathcal{C} is a small object (assuming 3, this is equivalent to the assertion that every object in SS is small).

    Riehl’s definition is that 𝒞\mathcal{C} is locally presentable iff it is locally small, cocomplete, and for some regular cardinal λ\lambda it has a set SS of objects such that:

    1. Every object in 𝒞\mathcal{C} can be written as a colimit of a small diagram whose objects are in SS;

    2. For each object sSs \in S, the functor preserves λ\mathcal{C{\lambda-filtered colimits.

    So, the nnLab definition seems to include two extra conditions. First, that every object in 𝒞\mathcal{C} can be written as a colimit of a λ\lambda-filtered small diagram whose objects are in SS. Second, condition 4, which seems redundant since it seems to be built into condition 3, at least if λ\lambda-small implies small.

    Surely there should be some way to simplify this nLab definition!

    • CommentRowNumber7.
    • CommentAuthorJohn Baez
    • CommentTimeFeb 8th 2020

    The λ\lambda-filtered condition is in Adamek and Rosicky’s book, so either Riehl left it out by accident or somehow she noticed it could be safely dropped - I don’t see how.

    • CommentRowNumber8.
    • CommentAuthorTodd_Trimble
    • CommentTimeFeb 9th 2020

    Quick reaction: I’m not sure what 4. is doing there either, and I agree that Emily’s 1. needs to be fixed (my taste would be to have her 2. coming before 1., i.e. say what the objects in SS are doing before describing other objects in terms of SS).

    • CommentRowNumber9.
    • CommentAuthorJohn Baez
    • CommentTimeNov 3rd 2020

    Added a corollary: locally presentable categories are complete.

    diff, v72, current

    • CommentRowNumber10.
    • CommentAuthorjdc
    • CommentTimeNov 19th 2021

    Item 4 of the definition of locally presentable category (Def 2.1) was there before Kevin Carlson added “λ\lambda-small” to item 3. So item 4 should be changed to a remark. I’ve just done this.

    I also think that “λ\lambda-small” should be changed to “λ\lambda-compact”, to be consistent with the rest of the page and linked pages, unless there is a subtle difference between the two that I’m not aware of. I’ve made this change as well.

    I also removed a parenthetical remark in item 3 that was no longer correct and wasn’t adding anything.

    diff, v77, current

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