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nLab > Latest Changes: Segal condition

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  1.  
    • CommentRowNumber1.
    • CommentAuthorUrs
    • CommentTimeApr 17th 2012
    • (edited Oct 30th 2012)

    finally a stub for Segal condition. Just for completeness (and to have a sensible place to put the references about Segal conditions in terms of sheaf conditions).

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeOct 30th 2012
    • (edited Oct 30th 2012)

    added discussion to Segal condition of how it is not quite a sheaf condition on Δ\Delta but is related to a sheaf condition on some Δ 0\Delta_0.

    • CommentRowNumber3.
    • CommentAuthorUrs
    • CommentTimeOct 31st 2012
    • (edited Oct 31st 2012)

    I have further expanded at Segal condition. In particular I wrote a section In terms of sheaf conditions (which maybe got a bit more detailed than is enjoyable, but anyway).

    Let’s look at the upshot, I am wondering if this is supposed to be telling us something:

    write

    {1⇐0}→iΔ 0→jΔ \left\{1 \Leftarrow 0\right\} \; \stackrel{i}{\to} \; \Delta_0 \; \stackrel{j}{\to} \; \Delta

    for the canonical inclusion functors, where Δ 0\Delta_0 is the full subcategory of graphs on the Δ[n]\Delta[n] regarded as graphs (so morphisms Δ[k 1]→Δ[k 2]\Delta[k_1] \to \Delta[k_2] in Δ 0\Delta_0 send elementary edges to elementary edges: they just pick a k 1k_1-long subequence in k 2k_2. In particular they are all injections, both on vertices and on edges).

    Then the statement is that

    Cat ↪N PSh(Δ) ↓ U ↓ j * Graph≃Sh(Δ 0) ↪i * PSh(Δ 0) \array{ Cat &\stackrel{N}{\hookrightarrow}& PSh(\Delta) \\ \downarrow^{\mathrlap{U}} && \downarrow^{\mathrlap{j^*}} \\ Graph \simeq Sh(\Delta_0) &\stackrel{i_*}{\hookrightarrow}& PSh(\Delta_0) }

    is a pullback diagram (in the 1-category of categories).

    But let’s think about this further: unless I am mixed up we have:

    • i *i_* is the direct image of a geometric morphism;

    • j *j^* is the direct image of a geometric morphism

    • all three toposes Graph,PSh(Δ 0),PSh(Δ)Graph, PSh(\Delta_0), PSh(\Delta) are cohesive.

    So this characterizes CatCat as a limit of a pullback diagram in the Image of CohesiveTopos↪Topos→FCatCohesiveTopos \hookrightarrow Topos \stackrel{F}{\to} Cat. Of course this FF does not preserve pullbacks, and of course CatCat is not a topos. But it’s still curious that this way it is the pullback in CatCat of a diagram of cohesive toposes. (Unless I am making a mistake somewhere.) I am wondering if this is telling us something about how to speak about (pre-)category objects in cohesive homotopy type theory.

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeNov 1st 2012

    Added brief paragraphs

    Somewhat telegraphic for the moment. Am running out of steam now.

    • CommentRowNumber5.
    • CommentAuthorStephan A Spahn
    • CommentTimeNov 1st 2012
    • (edited Nov 1st 2012)

    We have no entry precategory.I understand this term as referring to a weighted (with the name of the arrow) directed graph where it is allowed that the same name is assigned to different arrows. Is this also intended in your naming?

    edit: ok, precategory redirects to paracategory.

    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeNov 1st 2012

    We have no entry precategory.I understand this term as referring to a weighted (with the name of the arrow)

    Where Segal condition refers and points to “pre-categories” it points to pre-category object in an infinity-category (see there).

    This refers to a simplicial object X •X_\bullet in an ∞\infty-category which does satisfy the Segal condition, but which does not necessarily satisfy the additional “completess” condition (saying that Core(X •)Core(X_\bullet) is a homotopy-constant simplicial object or equivalently that id:X 0→Equiv(X 1)id \colon X_0 \to Equiv(X_1) is an equivalence).

    Calling this a “pre-category object” is not standard, but it is what we ended up with after some discussion here. There is no really standard term for this at the moment. Elsewhere it is called just a “category object” and then with the other conidition imposed it would be a “complete Segal object”. But the idea here was that “complete Segal object” is an unfortunately undescriptive term for such an important concept.

    But, yeah, eventually we might want to settle on something else. We had had plenty of discussion on this elsewhere, not entirely conclusive maybe.

    • CommentRowNumber7.
    • CommentAuthorMike Shulman
    • CommentTimeNov 3rd 2012

    No, not conclusive, although since as Stephan points out the word “precategory” is used for some other very different things, perhaps we should change it to something which is at least unambiguous.

    • CommentRowNumber8.
    • CommentAuthorStephan A Spahn
    • CommentTimeNov 9th 2012
    • (edited Nov 9th 2012)

    The notation in Definition 2 at Segal condition seems to be not in line with that on simplex category - what in the first article is denoted by Δ[n]\Delta[n] would have been denoted by [n][n] in the latter article.

    • CommentRowNumber9.
    • CommentAuthorTodd_Trimble
    • CommentTimeNov 9th 2012

    Stephan, I’m not sure of a good polite way to say this, but: that’s bound to happen with different people working on different articles. Does it bother you that there are different notations? The meaning seems clear enough in either case, and striving for absolute consistency of notation across the nLab seems close to impossible (and in many cases, including this one IMO, not worth worrying about).

    • CommentRowNumber10.
    • CommentAuthorUrs
    • CommentTimeNov 13th 2012
    • (edited Nov 13th 2012)

    I have added a further Remark on how the pullback diagram that characterizes CatCat as in Prop. 6 involves a bunch of adjoints

    Cat ↪N⊥←τ PSh(Δ)≃sSet U↓⊢↑ F j *↓⊢↑ j ! Graph≃Sh(Δ 0) ↪i *⊥←i * PSh(Δ 0) \array{ Cat &\stackrel{\overset{\tau}{\leftarrow}}{\underoverset{N}{\bottom}{\hookrightarrow}}& PSh(\Delta) \simeq sSet \\ {}^{\mathllap{U}}\downarrow \vdash \uparrow^{\mathrlap{F}} && {}^{\mathllap{j^*}}\downarrow \vdash \uparrow^{\mathrlap{j_!}} \\ Graph \simeq Sh(\Delta_0) &\stackrel{\overset{i^*}{\leftarrow}}{\underoverset{i_*}{\bottom}{\hookrightarrow}}& PSh(\Delta_0) }

    and how in terms of this one characterizes categories equivalently as those algebras of the j *j !j^* j_!-monad which satisfy a Segal condition in that they are in the image of i *i_*.

    • CommentRowNumber11.
    • CommentAuthorUrs
    • CommentTimeNov 25th 2012
    • (edited Nov 25th 2012)

    [ never mind ]

    • CommentRowNumber12.
    • CommentAuthorUrs
    • CommentTimeJun 18th 2021

    added pointer to

    • Alexander Grothendieck, Prop. 4.1 of: Techniques de construction et théorèmes d’existence en géométrie algébrique III : préschémas quotients, Séminaire Bourbaki : années 1960/61, exposés 205-222, Séminaire Bourbaki, no. 6 (1961), Exposé no. 212, (numdam:SB_1960-1961__6__99_0, pdf)

    which is the actual source of the “Segal conditions”

    (thanks to Tim Porter for digging out this reference in another thread here)

    diff, v20, current

  13. Link to a page on algebraic patterns in the sense of Barkan-Chu-Haugseng-Steinebrunner

    Natalie Stewart

    diff, v25, current

  14. Fixed false/misleading statement, see https://arxiv.org/abs/2404.12924

    Brendan Murphy

    diff, v26, current

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