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  1. I corrected a couple of wrong claims and added the link to a counter-example


    diff, v9, current

    • CommentRowNumber2.
    • CommentAuthorUrs
    • CommentTimeMar 21st 2019

    made the reference-link work, by adding the missing anchor


    diff, v10, current

  2. exibits -> exhibits

    Ian Coley

    diff, v11, current

  3. Added the fact that (weak) localizations are initial and final, together with the fact that the initial vertex map from the category of simplices is a weak localization (and hence initial and final). Added references to Shah and Cisinski about this and noted that this can be used to prove a Bousfield-Kan formula for homotopy (co)limits in infinity categories. Hopefully didn’t screw up the whole page- I’m horrible at html/computers in general…


    diff, v13, current

    • CommentRowNumber5.
    • CommentAuthorUrs
    • CommentTimeJul 3rd 2020
    • (edited Jul 3rd 2020)

    I have added an Example-section “Example: Cofiber products in coslice categories” (here). Currently it reads as follows:

    Consider the inclusion of the walking span-category, into the result of adjoining an initial object tt:

    (1){x b y}AAAA{ t x b y} \mathllap{ (1) \;\;\;\;\;\;\;\;\;\;\;\;\;\;\; } \Big\{ \array{ x &\longleftarrow& b &\longrightarrow& y } \Big\} \;\; \overset{\phantom{AAAA}}{\hookrightarrow} \;\; \left\{ \array{ && t \\ & \swarrow & \downarrow & \searrow \\ x &\longleftarrow& b &\longrightarrow& y } \right\}

    One readily sees that for each object on the right, its comma category over this inclusion has contractible nerve, whence Theorem \ref{Recognition} implies that this inclusion is a final \infty-functor.

    As an application of the finality of (1), observe that for 𝒞\mathcal{C} an (∞,1)-category and T𝒞T \in \mathcal{C} an object, (∞,1)-colimits in the under-(∞,1)-category

    𝒞 T/U𝒞 \mathcal{C}^{T/} \overset{\;\;U\;\;}{\longrightarrow} \mathcal{C}

    are given by the \infty-colimit in 𝒞\mathcal{C} itself of the given cone of the original diagram, with tip XX (by this Prop.): For

    F:𝒞 T/ F \;\colon\; \mathcal{I} \longrightarrow \mathcal{C}^{T/}

    a small diagram, we have

    U(limF)lim(T/U(F)) U \big( \underset{\longrightarrow}{\lim}\, F \big) \;\simeq\; \underset{\longrightarrow}{\lim}\, \big( T/U(F) \big)

    (when either \infty-colimit exists).

    Now for \mathcal{I} the walking span diagram on the left of (1), this means that homotopy cofiber products in 𝒞 T/\mathcal{C}^{T/} are computed as \infty-colimits in 𝒞\mathcal{C} of diagrams of the shape on the right of (1). But since the inclusion in (1) is final, these are just homotopy cofiber products in 𝒞\mathcal{C}.

    Explicitly: Given

    T = T = T ϕ X ϕ B ϕ Y X f B g Y \array{ T &=& T &=& T \\ {}^{\mathllap{ \phi_X }} \big\downarrow && {}^{\mathllap{ \phi_B }} \big\downarrow && {}^{\mathllap{ \phi_Y }} \big\downarrow \\ X & \underset{ f }{\longleftarrow} & B & \underset{ g }{ \longrightarrow } & Y }

    regarded as a span in 𝒞 T\mathcal{C}^T, hence with underlying objects

    U((X,ϕ X))=X,U((B,ϕ B))=B,U((Y,ϕ Y))=Y, U\big( (X,\phi_X) \big) \;=\; X \,, \;\;\;\;\;\; U\big( (B,\phi_B) \big) \;=\; B \,, \;\;\;\;\;\; U\big( (Y,\phi_Y) \big) \;=\; Y \,,

    we have:

    U((X,ϕ X)(B,ϕ B)(Y,ϕ Y))XBY. U \Big( \; (X,\phi_X) \underset{ (B,\phi_B) }{\coprod} (Y,\phi_Y) \; \Big) \;\;\;\simeq\;\;\; X \underset{B}{\coprod} Y \,.

    In particular, if (B,ϕ B)(T,id T)(B,\phi_B) \;\coloneqq\; (T,id_T) is the initial object in 𝒞 T/\mathcal{C}^{T/}, in which case the cofiber product is just the coproduct

    (X,ϕ X)(Y,ϕ Y)=(X,ϕ X)(T,id T)(Y,ϕ Y) (X,\phi_X) \coprod (Y,\phi_Y) \;\;=\;\; (X,\phi_X) \underset{ (T,id_T) }{\coprod} (Y,\phi_Y)

    we find that the coproduct in the co-slice category is the co-fiber product under the given tip object in the underlying category

    U((X,ϕ X)(Y,ϕ Y))XTY. U \Big( \; (X,\phi_X) \coprod (Y,\phi_Y) \; \Big) \;\;\;\simeq\;\;\; X \underset{T}{\coprod} Y \,.

    diff, v14, current

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