ScalingStacks

6.2. Universal additive invariant

Let ℳadd{\mathcal{M}}_{\mathrm{add}} be the stabilization Stab⁡(ℳaddun)\Stab({\mathcal{M}}_{\mathrm{add}}^{\mathrm{un}}) [53, §1.4] of ℳaddun{\mathcal{M}}_{\mathrm{add}}^{\mathrm{un}}; by construction, this is a stable ∞\infty-category. Denote by 𝒰add{\mathcal{U}}_{\mathrm{add}} the following composite

Cat∞ex⟶𝒰addunℳaddun⟶Stab⁡(ℳaddun).\Cat_{\infty}^{\ex}\stackrel{{\scriptstyle{\mathcal{U}}_{\mathrm{add}}^{\mathrm{un}}}}{{\longrightarrow}}{\mathcal{M}}_{\mathrm{add}}^{\mathrm{un}}\longrightarrow\Stab({\mathcal{M}}_{\mathrm{add}}^{\mathrm{un}})\,.
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Remark 6.8. Note that we have the equivalences

Stab⁡(Pre​((Cat∞perf)ω)∗)\displaystyle\Stab(\mathrm{Pre}((\Cat_{\infty}^{\perf})^{\omega})_{*}) =Stab(Fun(((Cat∞perf)ω)op,𝒯∞∗))\displaystyle=\Stab(\mathrm{Fun}(((\Cat_{\infty}^{\perf})^{\omega})^{\op},{\mathcal{T}}_{\infty*}))
≃Fun(((Cat∞perf)ω)op,Stab(𝒯∞∗)))\displaystyle\simeq\mathrm{Fun}(((\Cat_{\infty}^{\perf})^{\omega})^{\op},\Stab({\mathcal{T}}_{\infty*})))
≃Fun⁡(((Cat∞perf)ω)op,𝒮∞),\displaystyle\simeq\mathrm{Fun}(((\Cat_{\infty}^{\perf})^{\omega})^{\op},{\mathcal{S}}_{\infty}),

the last of which follows from  [53, 1.4.4.11]. Therefore, defining

Pre𝒮∞​((Cat∞perf)ω)=Fun⁡(((Cat∞perf)ω)op,𝒮∞)\mathrm{Pre}_{{\mathcal{S}}_{\infty}}((\Cat_{\infty}^{\perf})^{\omega})=\mathrm{Fun}(((\Cat_{\infty}^{\perf})^{\omega})^{\op},{\mathcal{S}}_{\infty})

and writing

ψ:Cat∞perf⟶Pre𝒮∞​((Cat∞perf)ω)\psi\colon\Cat_{\infty}^{\perf}\longrightarrow\mathrm{Pre}_{{\mathcal{S}}_{\infty}}((\Cat_{\infty}^{\perf})^{\omega})

for the natural functor, we see that ℳadd{\mathcal{M}}_{\mathrm{add}} can alternately be described as the localization of Pre𝒮∞​((Cat∞perf)ω)\mathrm{Pre}_{{\mathcal{S}}_{\infty}}((\Cat_{\infty}^{\perf})^{\omega}) with respect to the set of maps

(6.9) ψ⁡(𝒞)/ψ⁡(𝒜)⟶ψ⁡(ℬ),\psi({\mathcal{C}})/\psi({\mathcal{A}})\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}\psi({\mathcal{B}})\,,

where 𝒜→𝒞→ℬ{\mathcal{A}}\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}{\mathcal{C}}\mathchoice{\longrightarrow}{\rightarrow}{\rightarrow}{\rightarrow}{\mathcal{B}} is a split-exact sequence in ℰ{\mathcal{E}}.

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Theorem 6.10. The functor 𝒰add{\mathcal{U}}_{\mathrm{add}} is the universal additive invariant, i.e., given any stable presentable ∞\infty-category 𝒟{\mathcal{D}}, we have an equivalence of ∞\infty-categories

(𝒰add)∗:FunL​(ℳadd,𝒟)⟶∼Funadd​(Cat∞ex,𝒟).({\mathcal{U}}_{\mathrm{add}})^{\ast}\colon\mathrm{Fun}^{\mathrm{L}}({\mathcal{M}}_{\mathrm{add}},{\mathcal{D}})\stackrel{{\scriptstyle\sim}}{{\longrightarrow}}\mathrm{Fun}_{\mathrm{add}}(\Cat_{\infty}^{\ex},{\mathcal{D}})\,.
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Proof. The result follows from theorem 6.7 and from the universal property of stabilization (i.e., [53, 1.4.5.5]). Note that stabilization preserves colimits and 𝒰addun{\mathcal{U}}_{\mathrm{add}}^{\mathrm{un}} sends split-exact sequences to cofiber sequences, so the split-exact sequence (6.2) is sent to a split cofiber sequence 𝒰add​(𝒞)≃𝒰add​(𝒜)∨𝒰add​(ℬ){\mathcal{U}}_{\mathrm{add}}({\mathcal{C}})\simeq{\mathcal{U}}_{\mathrm{add}}({\mathcal{A}})\vee{\mathcal{U}}_{\mathrm{add}}({\mathcal{B}}) in ℳadd{\mathcal{M}}_{\mathrm{add}}. ∎

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