ScalingStacks

4.3.1. Category

Consider a differential category 𝒲{\mathcal{W}} endowed with two actions of 𝒰{\mathcal{U}} given by (E1,Ο„1)(E_{1},\tau_{1}) and (E2,Ο„2)(E_{2},\tau_{2}) and a closed morphism of functors Οƒ:E2​E1β†’E1​E2\sigma:E_{2}E_{1}\to E_{1}E_{2} such that the following diagrams commute:

(4.3.1) E22​E1\textstyle{E_{2}^{2}E_{1}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E2​σ\scriptstyle{E_{2}\sigma}Ο„2​E1\scriptstyle{\tau_{2}E_{1}}E2​E1​E2\textstyle{E_{2}E_{1}E_{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}σ​E2\scriptstyle{\sigma E_{2}}E1​E22\textstyle{E_{1}E_{2}^{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​τ2\scriptstyle{E_{1}\tau_{2}}E22​E1\textstyle{E_{2}^{2}E_{1}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E2​σ\scriptstyle{E_{2}\sigma}E2​E1​E2\textstyle{E_{2}E_{1}E_{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}σ​E2\scriptstyle{\sigma E_{2}}E1​E22\textstyle{E_{1}E_{2}^{2}}     E2​E12\textstyle{E_{2}E_{1}^{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}σ​E1\scriptstyle{\sigma E_{1}}E2​τ1\scriptstyle{E_{2}\tau_{1}}E1​E2​E1\textstyle{E_{1}E_{2}E_{1}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​σ\scriptstyle{E_{1}\sigma}E12​E2\textstyle{E_{1}^{2}E_{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ο„1​E2\scriptstyle{\tau_{1}E_{2}}E2​E12\textstyle{E_{2}E_{1}^{2}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}σ​E1\scriptstyle{\sigma E_{1}}E1​E2​E1\textstyle{E_{1}E_{2}E_{1}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​σ\scriptstyle{E_{1}\sigma}E12​E2\textstyle{E_{1}^{2}E_{2}}
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Remark 4.3.1. The data of Οƒ\sigma and the relations can be described graphically as follows:

[Uncaptioned image]

We define a differential category 𝒱=Δσ​𝒲{\mathcal{V}}=\Delta_{\sigma}{\mathcal{W}}.

βˆ™\bullet\ The objects of 𝒱{\mathcal{V}} are pairs (m,Ο€)(m,\pi) where mβˆˆπ’²Β―im\in\overline{{\mathcal{W}}}^{i} and Ο€βˆˆZ​Hom𝒲¯i⁑(E2​(m),E1​(m))\pi\in Z\operatorname{Hom}\nolimits_{\overline{{\mathcal{W}}}^{i}}(E_{2}(m),E_{1}(m)) such that the following diagram commutes

(4.3.2) E22​(m)\textstyle{E_{2}^{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E2​π\scriptstyle{E_{2}\pi}Ο„2\scriptstyle{\tau_{2}}E2​E1​(m)\textstyle{E_{2}E_{1}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Οƒ\scriptstyle{\sigma}E1​E2​(m)\textstyle{E_{1}E_{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​π\scriptstyle{E_{1}\pi}E12​(m)\textstyle{E_{1}^{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ο„1\scriptstyle{\tau_{1}}E22​(m)\textstyle{E_{2}^{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E2​π\scriptstyle{E_{2}\pi}E2​E1​(m)\textstyle{E_{2}E_{1}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Οƒ\scriptstyle{\sigma}E1​E2​(m)\textstyle{E_{1}E_{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​π\scriptstyle{E_{1}\pi}E12​(m)\textstyle{E_{1}^{2}(m)}

βˆ™\bullet\ Hom𝒱⁑((m,Ο€),(mβ€²,Ο€β€²))\operatorname{Hom}\nolimits_{\mathcal{V}}((m,\pi),(m^{\prime},\pi^{\prime})) is the differential submodule of Hom𝒲¯i⁑(m,mβ€²)\operatorname{Hom}\nolimits_{\overline{{\mathcal{W}}}^{i}}(m,m^{\prime}) of elements ff such that the following diagram commutes

E2​(m)\textstyle{E_{2}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ο€\scriptstyle{\pi}E2​f\scriptstyle{E_{2}f}E1​(m)\textstyle{E_{1}(m)\ignorespaces\ignorespaces\ignorespaces\ignorespaces}E1​f\scriptstyle{E_{1}f}E2​(mβ€²)\textstyle{E_{2}(m^{\prime})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ο€β€²\scriptstyle{\pi^{\prime}}E1​(mβ€²)\textstyle{E_{1}(m^{\prime})}

The composition of maps is defined by restricting that of 𝒲¯i\overline{{\mathcal{W}}}^{i}. So, we have a faithful forgetful functor Ο‰=ωσ:𝒱→𝒲¯i,(m,Ο€)↦m\omega=\omega_{\sigma}:{\mathcal{V}}\to\overline{{\mathcal{W}}}^{i},\ (m,\pi)\mapsto m. Note that 𝒱{\mathcal{V}} is strongly pretriangulated and idempotent-complete.

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Remark 4.3.2. The structure of objects and maps in 𝒱{\mathcal{V}} can be described graphically as follows:

[Uncaptioned image]
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Remark 4.3.3. Assume E1E_{1} admits a left adjoint F1F_{1}. The data of the map Οƒ:E2​E1β†’E1​E2\sigma:E_{2}E_{1}\to E_{1}E_{2} corresponds by adjunction to the data of a map

Ξ»:F1​E2β†’βˆ™Ξ·1F1​E2​E1​F1β†’F1​σ​F1F1​E1​E2​F1β†’Ξ΅1βˆ™E2​F1.\lambda:F_{1}E_{2}\xrightarrow{\bullet\eta_{1}}F_{1}E_{2}E_{1}F_{1}\xrightarrow{F_{1}\sigma F_{1}}F_{1}E_{1}E_{2}F_{1}\xrightarrow{\varepsilon_{1}\bullet}E_{2}F_{1}.

The commutativity of the diagrams (4.3.1) is equivalent to the commutativity of the diagrams (4.2.1). Assume the diagrams commute. We obtain a lax bi-22-representation (Ei,j)(E_{i,j}) on 𝒲{\mathcal{W}} (cf Β§4.2.1).

Let (m,Ο‚)βˆˆΞ”E​𝒲(m,\varsigma)\in\Delta_{E}{\mathcal{W}}. We have an adjunction isomorphism

Ο•:Hom⁑(E2​(m),E1​(m))β†’βˆΌHom⁑(F1​E2​(m),m).\phi:\operatorname{Hom}\nolimits(E_{2}(m),E_{1}(m))\mathrel{\mathop{\kern 0.0pt\to}\limits^{\sim}}\operatorname{Hom}\nolimits(F_{1}E_{2}(m),m).

Let Ο€=Ο•βˆ’1​(Ο‚)∈Z​Hom⁑(E2​(m),E1​(m))\pi=\phi^{-1}(\varsigma)\in Z\operatorname{Hom}\nolimits(E_{2}(m),E_{1}(m)). The object (m,Ο€)(m,\pi) is in Δσ​𝒲\Delta_{\sigma}{\mathcal{W}} and (m,Ο‚)↦(m,Ο€)(m,\varsigma)\mapsto(m,\pi) defines a fully faithful functor of differential categories Ξ”E​𝒲→Δσ​𝒲\Delta_{E}{\mathcal{W}}\to\Delta_{\sigma}{\mathcal{W}}.

Assume now Ξ»\lambda is invertible. The canonical map fi:(E0,1​E1,0)iβ†’Ei,if_{i}:(E_{0,1}E_{1,0})^{i}\to E_{i,i} is invertible. Let Ο‚i=bi∘fiβˆ’1\varsigma_{i}=b_{i}\circ f_{i}^{-1}. Consider r∈{1,…,iβˆ’1}r\in\{1,\ldots,i-1\}. We have

Ο‚i∘(TrβŠ—1)\displaystyle\varsigma_{i}\circ(T_{r}\otimes 1) =brβˆ’1∘(E01​E10)rβˆ’1​(Ο‚2∘(T1βŠ—1)∘f2)∘(E01​E10)r+1​biβˆ’rβˆ’1∘fiβˆ’1\displaystyle=b_{r-1}\circ(E_{01}E_{10})^{r-1}(\varsigma_{2}\circ(T_{1}\otimes 1)\circ f_{2})\circ(E_{01}E_{10})^{r+1}b_{i-r-1}\circ f_{i}^{-1}
=brβˆ’1∘(E01​E10)rβˆ’1​(Ο‚2∘(1βŠ—T1)∘f2)∘(E01​E10)r+1​biβˆ’rβˆ’1∘fiβˆ’1\displaystyle=b_{r-1}\circ(E_{01}E_{10})^{r-1}(\varsigma_{2}\circ(1\otimes T_{1})\circ f_{2})\circ(E_{01}E_{10})^{r+1}b_{i-r-1}\circ f_{i}^{-1}
=Ο‚i∘(1βŠ—Tr)\displaystyle=\varsigma_{i}\circ(1\otimes T_{r})

As a consequence, the functor above is an isomorphism of differential categories Ξ”Eβ€‹π’²β†’βˆΌΞ”Οƒβ€‹π’²\Delta_{E}{\mathcal{W}}\mathrel{\mathop{\kern 0.0pt\to}\limits^{\sim}}\Delta_{\sigma}{\mathcal{W}}.

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

Andrew Manion, Raphael Rouquier

Original source: arXiv:2009.09627v2