ScalingStacks

0NPF

Theorem 9.10. Let 𝒜{\mathcal{A}} be a small stable ∞\infty-category. Then there is a natural equivalence of spectra

Map⁡(𝒰wlocκ¯​(𝒮∞ω),𝒰wlocκ¯​(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 𝒰addκ¯​(𝒮∞ω)\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}) is compact in ℳaddκ¯\underline{{\mathcal{M}}_{\mathrm{add}}^{\kappa}}. Let SS denote the set of maps in (8.4), S¯\overline{S} the strongly saturated collection of arrows generated by SS [52, 5.5.4.5], and let XX be an SS-local object such that the map 𝒰addκ¯​(𝒜)→X\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}X is an SS-local equivalence (i.e., 𝒰addκ¯​(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⁡(𝒰wlocκ¯​(𝒮∞ω),𝒰wlocκ¯​(𝒜))≃Map⁡(𝒰addκ¯​(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⁡(𝒰addκ¯​(𝒮∞ω),−):ℳaddκ¯⟶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 ℳaddκ¯\underline{{\mathcal{M}}_{\mathrm{add}}^{\kappa}} is a stable ∞\infty-category and 𝒰addκ¯​(𝒮∞ω)\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{S}}_{\infty}^{\omega}) is compact, RR preserves small colimits, so the two-out-of-three property allows us to reduce to checking that RR sends the elements of SS to equivalences.

Consider the following diagram

(9.12) 𝒰addκ¯​(𝒜)\textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}𝒰addκ¯​(ℬ)\textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}𝒰addκ¯​(ℬ)/𝒰addκ¯​(𝒜)\textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})/\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}𝒰addκ¯​(𝒜)\textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{A}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}𝒰addκ¯​(ℬ)\textstyle{\underline{{\mathcal{U}}_{\mathrm{add}}^{\kappa}}({\mathcal{B}})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}𝒰addκ¯​(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. ∎

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

Andrew J. Blumberg, David Gepner, Goncalo Tabuada

Original source: arXiv:1001.2282v4