[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 : G F\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 ⊣ G F\dashv G ,
(1)
For every d ≥ − 2 d\geq-2 and a d d -truncated morphism g g in 𝒟 \mathcal{D} ,
the morphism G ( g ) G\left(g\right) is a d d -truncated morphism in 𝒞 \mathcal{C} .
(2)
For every n ≥ − 2 n\geq-2 and an n n -connected morphism f f in 𝒞 \mathcal{C} ,
the morphism F ( f ) F\left(f\right) is an n n -connected morphism in 𝒟 \mathcal{D} .
[05YJ]
Proof. As a right adjoint, G G is left exact and therefore preserves d d -truncated
morphisms by T.5.5.6.16 . Since G G preserves n n -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 f f is left orthogonal to all n n -truncated morphisms
then so is F ( f ) F\left(f\right) .
∎