0NPF
Theorem 9.10 . Let 𝒜 {\mathcal{A}} be a small stable ∞ \infty -category. Then there is a natural
equivalence of spectra
Map ( 𝒰 wloc κ ¯ ( 𝒮 ∞ ω ) , 𝒰 wloc κ ¯ ( 𝒜 ) ) ≃ K ( 𝒜 ) . \mathrm{Map}(\underline{{\mathcal{U}}_{\mathrm{wloc}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}),\,\underline{{\mathcal{U}}_{\mathrm{wloc}}^{\kappa}}({\mathcal{A}}))\simeq K({\mathcal{A}})\,.
0NPG
Proof. By construction, the object 𝒰 add κ ¯ ( 𝒮 ∞ ω ) \underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}) is compact in
ℳ add κ ¯ \underline{{\mathcal{M}}_{\mathrm{add}}^{\kappa}} . Let S S denote the set of maps in (8.4 ),
S ¯ \overline{S} the strongly saturated collection of arrows generated
by S S [52 , 5.5.4.5] , and let X X be an S S -local
object such that the map 𝒰 add κ ¯ ( 𝒜 ) → X \underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}X is an S S -local
equivalence (i.e., 𝒰 add κ ¯ ( 𝒜 ) → X \underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}X is in S ¯ \overline{S} ).
Then by definition,
Map ( 𝒰 wloc κ ¯ ( 𝒮 ∞ ω ) , 𝒰 wloc κ ¯ ( 𝒜 ) ) ≃ Map ( 𝒰 add κ ¯ ( 𝒮 ∞ ω ) , X ) , \mathrm{Map}(\underline{{\mathcal{U}}_{\mathrm{wloc}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}),\underline{{\mathcal{U}}_{\mathrm{wloc}}^{\kappa}}({\mathcal{A}}))\simeq\mathrm{Map}(\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}),X),
so it suffices to show that the functor
(9.11)
R := Map ( 𝒰 add κ ¯ ( 𝒮 ∞ ω ) , − ) : ℳ add κ ¯ ⟶ 𝒮 ∞ R:=\mathrm{Map}(\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}),-):\underline{{\mathcal{M}}_{\mathrm{add}}^{\kappa}}\longrightarrow{\mathcal{S}}_{\infty}
sends the maps in S ¯ \overline{S} to equivalences of
spectra. Since ℳ add κ ¯ \underline{{\mathcal{M}}_{\mathrm{add}}^{\kappa}} is a stable ∞ \infty -category and
𝒰 add κ ¯ ( 𝒮 ∞ ω ) \underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}) is compact, R R preserves small colimits,
so the two-out-of-three property allows us to reduce to checking that
R R sends the elements of S S to equivalences.
Consider the following diagram
(9.12)
𝒰 add κ ¯ ( 𝒜 ) \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces} 𝒰 add κ ¯ ( ℬ ) \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces} 𝒰 add κ ¯ ( ℬ ) / 𝒰 add κ ¯ ( 𝒜 ) \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})/\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} 𝒰 add κ ¯ ( 𝒜 ) \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} 𝒰 add κ ¯ ( ℬ ) \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} 𝒰 add κ ¯ ( ℬ / 𝒜 ) . \textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}}/{\mathcal{A}})\,.}
By applying the functor (9.11 ) to the above diagram
(9.12 ) we obtain by theorem 9.9 a diagram in
𝒮 \mathcal{S}
(9.13)
K ( 𝒜 ) \textstyle{K({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces} K ( ℬ ) \textstyle{K({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces} K ( ℬ ) / K ( 𝒜 ) \textstyle{K({\mathcal{B}})/K({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} K ( 𝒜 ) \textstyle{K({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} K ( ℬ ) \textstyle{K({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces} K ( ℬ / 𝒜 ) , \textstyle{K({\mathcal{B}}/{\mathcal{A}})\,,}
where the upper row is a homotopy cofiber sequence. Now, an
argument analogous to the one used in the proof of
proposition 7.19 (where we make use of Waldhausen’s
fibration theorem) allow us to conclude that the lower row in the
above diagram (9.13 ) is also a homotopy
cofiber sequence. This completes the argument.
∎