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    • CommentRowNumber1.
    • CommentAuthorTodd_Trimble
    • CommentTimeFeb 26th 2012

    I made some much-needed corrections at simplicial complex, directed mostly at errors which had been introduced by yours truly. I also created quasi-topological space (the notion due to Spanier).

    I haven’t thought this through, but regarding the process of turning a simplicial complex into a simplicial set, the usual sequence of words seems to involve putting a non-canonical ordering on the set of vertices and then getting ordered simplices from that. But is there anything “wrong” with taking the composite

    SimpCompSet Fin + opSet i opSet Δ opSimpComp \hookrightarrow Set^{Fin_{+}^{op}} \stackrel{Set^{i^{op}}}{\to} Set^{\Delta^{op}}

    where the inclusion is the realization of simplicial complexes as concrete presheaves on nonempty finite sets, and the second arrow is pulling back along the forgetful functor ii from nonempty totally ordered finite sets to nonempty finite sets? This looks much more canonical.

    • CommentRowNumber2.
    • CommentAuthorYaron
    • CommentTimeFeb 26th 2012
    • (edited Feb 26th 2012)

    Todd, this may be complete nonsense, but something looks a little strange at the second condition of the “Canonical construction” section. Shouldn’t an α\alpha be thought of as convex combination of the vertices of one simplex? In that case, shouldn’t the second condition be: for each α\alpha in |K||K|, vα(v)=1\sum_v \alpha(v)=1? (I’d probably feel pretty silly when I hear the answer…)

    • CommentRowNumber3.
    • CommentAuthorTodd_Trimble
    • CommentTimeFeb 26th 2012

    Good catch, Yaron – those terms α,v\alpha, v got mixed up. I’m having trouble getting into the Lab right now, but once I do I’ll fix it up.

    • CommentRowNumber4.
    • CommentAuthorjim_stasheff
    • CommentTimeFeb 27th 2012
    Not all polyhedra are simplicial complexes, though one could consider simplicial subdivisions.
    Also suggest what you define be (abstract) simplicial complex
    • CommentRowNumber5.
    • CommentAuthorTodd_Trimble
    • CommentTimeFeb 27th 2012

    Jim, the term ’polyhedron’ as used on that page was taken from Spanier’s book, where it reads (page 107): a polyhedron is a topological space which admits a triangulation by a simplicial complex. I can add a little disambiguation though.

    The notion of abstract simplicial complex was defined in the Idea and Definition section, although the word ’abstract’ was left out. Later in the article, it is effectively redefined to mean a concrete presheaf on nonempty finite sets, which seems for categorical purposes to be a very helpful formulation.

    • CommentRowNumber6.
    • CommentAuthorMike Shulman
    • CommentTimeFeb 27th 2012

    I think the middle category in your canonical composite is called the category of symmetric simplicial sets. It also seems to me that that composite is going to give you the simplicial set that has (n+1)!(n+1)! copies of each nn-simplex?

    • CommentRowNumber7.
    • CommentAuthorTodd_Trimble
    • CommentTimeFeb 27th 2012

    I think the middle category in your canonical composite is called the category of symmetric simplicial sets.

    Yes, that is so.

    It also seems to me that that composite is going to give you the simplicial set that has (n+1)!(n+1)! copies of each nn-simplex?

    Ah, that makes sense now. Thanks!

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