ScalingStacks

[05YI]

Lemma 4.2.5. Let 𝒞\mathcal{C} and 𝒟\mathcal{D} be ∞\infty-categories that admit finite limits and let F:𝒞⇆𝒟:GF\colon\mathcal{C}\leftrightarrows\mathcal{D}\penalty\mskip 6.0mu plus 1.0mu\mathpunct{}\nonscript\mkern-3.0mu{:}\mskip 2.0muG be an adjunction with F⊣GF\dashv G,

  1. (1)

    For every d≥−2d\geq-2 and a dd-truncated morphism gg in 𝒟\mathcal{D}, the morphism G⁡(g)G\left(g\right) is a dd-truncated morphism in 𝒞\mathcal{C}.

  2. (2)

    For every n≥−2n\geq-2 and an nn-connected morphism ff in 𝒞\mathcal{C}, the morphism F⁡(f)F\left(f\right) is an nn-connected morphism in 𝒟\mathcal{D}.

[05YJ]

Proof. As a right adjoint, GG is left exact and therefore preserves dd-truncated morphisms by T.5.5.6.16. Since GG preserves nn-truncated morphisms and the space of lifts in the square

F⁡(A)\textstyle{F\left(A\right)\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}X\textstyle{X\ignorespaces\ignorespaces\ignorespaces\ignorespaces}F⁡(B)\textstyle{F\left(B\right)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Y\textstyle{Y}

is homotopy equivalent to the space of lifts in the adjoint square

A\textstyle{A\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}G⁡(X)\textstyle{G\left(X\right)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}B\textstyle{B\ignorespaces\ignorespaces\ignorespaces\ignorespaces}G⁡(Y)\textstyle{G\left(Y\right)}

given by 4.1.4, we see that if ff is left orthogonal to all nn-truncated morphisms then so is F⁡(f)F\left(f\right). ∎

Original mathematics by the credited authors. Source collection and HTML conversion remain in progress.

Tomer Schlank, Lior Yanovski

Original source: arXiv:1808.06006v3

Original source page 29

Original source · 1808.06006v3