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    • CommentRowNumber1.
    • CommentAuthorMike Shulman
    • CommentTimeApr 4th 2012

    I added a bit to category of simplices, including the fact that the category of nondegenerate simplices is final and thus colimits can be computed using only that, and that the nerve of the category of simplices itself is colimit-preserving.

    • CommentRowNumber2.
    • CommentAuthorEmily Riehl
    • CommentTimeApr 5th 2012

    This relates to a question I was thinking about today. How do you prove that the “last vertex map” N(ΔX)XN(\Delta \downarrow X) \to X is a weak equivalence for any simplicial set? Because the colimits over the category of simplices are homotopy colimits, by your remarks on cocontinuity, it would suffice to prove this in the case X=Δ nX=\Delta^n. But then both N(ΔΔ n)N(\Delta \downarrow \Delta^n) and Δ n\Delta^n are nerves of categories, and I suppose it’s reasonably clear that the functor ΔΔ n[n]\Delta \downarrow \Delta^n \to [n] is final. Does this make any sense?

    • CommentRowNumber3.
    • CommentAuthorMike Shulman
    • CommentTimeApr 6th 2012

    Hmm, yes, that makes sense. Actually, in Higher Topos Theory asserts that N(ΔX)XN(\Delta\downarrow X)\to X is a final map for any simplicial set XX.

    • CommentRowNumber4.
    • CommentAuthorUrs
    • CommentTimeFeb 1st 2013
    • (edited Feb 1st 2013)

    Somebody kindly pointed out by email to me that there was a false statement in the entry category of simplicies (that the non-degenerate nn-simplices of XX are equivalently the monos Δ nX\Delta^n \to X).

    I have fixed that and in the course of this I have tried to slightly polish the entry a bit more. Added formal proposition-environments, stated the relation to barycentric subdivision and added a textbook reference.

    More could be done here. But I am out of time now.

    • CommentRowNumber5.
    • CommentAuthorrognes
    • CommentTimeFeb 4th 2013
    The statement that the nerve of the category of non-degenerate simplices is a model for the barycentric subdivision is still wrong. At least if by barycentric subdivision you mean Kan (normal) subdivision, as the link to barycentric subdivision suggests. Think about X = \Delta^2/\partial\Delta^2, for instance. It helps if the simplicial set is what Waldhausen calls non-singular, i.e., each non-degenerate simplex is embedded. Then the category of non-degenerate simplices is a partially ordered set, and its nerve is the same as Kan's subdivision.
    • CommentRowNumber6.
    • CommentAuthorUrs
    • CommentTimeFeb 4th 2013

    Maybe we should rather add more comments to the entry on barycentric subdivision then. For instance Lurie in HTT, variant uses the term as I did.

    • CommentRowNumber7.
    • CommentAuthorUrs
    • CommentTimeFeb 4th 2013

    Okay, I have edited a bit further.

    (But I do need to look into something else now. Feel free to hit the edit-button if there is more you’d want to see clarified.)

    • CommentRowNumber8.
    • CommentAuthorTim_Porter
    • CommentTimeApr 9th 2019

    added a ‘de’ to non-generate! plus some hyphens in nondegenerate.

    diff, v15, current

    • CommentRowNumber9.
    • CommentAuthorHurkyl
    • CommentTimeOct 29th 2020

    The article here says that the inclusion of the full subcategory of nondegenerate simplices has a left adjoint when every nondegenerate simplex of XX has nondegerate faces. But HTT variant claims this is true for every XX.

    Is HTT correct here? That the assumption can be omitted?

    I’m inclined to think HTT is correct, but I’ve been confused on this point in the past… but I think my confusion might have involved this very nLab page, so my instincts are all messed up here.

    • CommentRowNumber10.
    • CommentAuthorJohn Dougherty
    • CommentTimeMar 22nd 2021

    The condition on XX is necessary. If the inclusion has a left adjoint LL then the unit η σ:σLσ\eta_{\sigma} : \sigma \to L\sigma exhibits the unique factorization of σ\sigma as a surjection η σ\eta_{\sigma} followed by a nondegenerate simplex LσL\sigma. If σ\sigma is nondegenerate then the adjunction factors the injection δ i:d iσσ\delta^{i} : d_{i}\sigma \to \sigma through the surjection η d iσ\eta_{d_{i}\sigma}, making η d iσ\eta_{d_{i}\sigma} the identity and so d iσd_{i}\sigma nondegenerate.

    • CommentRowNumber11.
    • CommentAuthorMike Shulman
    • CommentTimeMar 10th 2022

    Back on this inclusion of the subcategory of nondegenerate simplices. We’ve observed that if every nondegenerate simplex has nondegenerate faces, then the inclusion is a right adjoint, and John says the converse also holds. This implies in particular that the inclusion is a final functor, so that colimits over the category of simplices can be computed on the nondegenerate simplices. But being final is a weaker statement than being a right adjoint, so can the inclusion of nondegenerate simplices be final even if not every nondegenerate simplex has nondegenerate faces? Are there simplicial sets for which this inclusion fails to be final?

    • CommentRowNumber12.
    • CommentAuthorMike Shulman
    • CommentTimeMar 15th 2022

    By the way, I had a look at the current version of HTT, and I think it includes this condition: the restriction to nondegenerate simplices only happens in step (5), after we’ve already passed to a category of simplices several times in a way that ensures that at this point the faces of nondegenerate simplices are all nondegenerate. Perhaps that was an update in the past 1.5 years.

    • CommentRowNumber13.
    • CommentAuthorMike Shulman
    • CommentTimeMar 15th 2022

    I guess it isn’t too hard to come up with examples where the inclusion of nondegenerate simplices isn’t final. E.g. take a square Δ 1×Δ 1\Delta^1\times \Delta^1 and collapse the diagonal to a point. Then for that degenerate 1-simplex σ\sigma, the category σ(ΔX) nondeg\sigma \downarrow (\Delta\downarrow X)_{nondeg} is, I think, not connected.

    • CommentRowNumber14.
    • CommentAuthorHurkyl
    • CommentTimeMar 15th 2022

    Sounds right. Embedding the square in Δ 3\Delta^3 so that I can name the vertices 0123 and so that XX is the quotient by the 03 simplex….

    The only factorizations Δ 1Δ nX\Delta^1 \hookrightarrow \Delta^n \to X of σ\sigma with Δ nX\Delta^n \to X nondegenerate are through the 013 and 023 simplices. There are no morphisms out of these factorizations, since they have to be monic on the Δ n\Delta^n term, and these are the top simplices. There are no morphisms into these factorizations, because all of the other ones have a nonmonic Δ 1Δ n\Delta^1 \to \Delta^n part,

    • CommentRowNumber15.
    • CommentAuthorUrs
    • CommentTimeAug 19th 2022