ScalingStacks

8.2.7. Functoriality

Consider f:Z→Z′f:Z\to Z^{\prime} a morphism of curves and assume f∘ξ1+f\circ\xi_{1}^{+} is outgoing for Z′Z^{\prime} and f∘ξ2−f\circ\xi_{2}^{-} is incoming for Z′Z^{\prime}.

The morphism ff extends uniquely to a morphism of curves f:Zξ→Zf∘ξ′f:Z_{\xi}\to Z^{\prime}_{f\circ\xi}.

The functor f:𝒮f,M∙​(Z)→𝒮f⁡(M)∙​(Z′)f:{\mathcal{S}}^{\bullet}_{f,M}(Z)\to{\mathcal{S}}^{\bullet}_{f(M)}(Z^{\prime}) can be equipped with a structure of morphism of 22-representations Lξ1+∙→Lf∘ξ1+∙L_{\xi_{1}^{+}}^{\bullet}\to L_{f\circ\xi_{1}^{+}}^{\bullet} and of morphism of 22-representations Rξ2−∙→Rf∘ξ2−∙R_{\xi_{2}^{-}}^{\bullet}\to R_{f\circ\xi_{2}^{-}}^{\bullet} (Lemma 8.1.7 and §8.1.5), and it induces a differential pointed functor (cf §4.3.4)

Δ​f:ΔRξ2−,Lξ1+,λ​𝒮f,M∙​(Z)→ΔRf∘ξ2−,Lf∘ξ1+,λ′​𝒮f⁡(M)∙​(Z′),\Delta f:\Delta_{R_{\xi_{2}^{-}},L_{\xi_{1}^{+}},\lambda}{\mathcal{S}}^{\bullet}_{f,M}(Z)\to\Delta_{R_{f\circ\xi_{2}^{-}},L_{f\circ\xi_{1}^{+}},\lambda^{\prime}}{\mathcal{S}}^{\bullet}_{f(M)}(Z^{\prime}),

where λ′\lambda^{\prime} is the analog of the map λ\lambda for Z′Z^{\prime}.

We obtain a commutative diagram of differential pointed functors

(8.2.1) ΔRξ2−,Lξ1+,λ​𝒮f,M∙​(Z)\textstyle{\Delta_{R_{\xi_{2}^{-}},L_{\xi_{1}^{+}},\lambda}{\mathcal{S}}^{\bullet}_{f,M}(Z)\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ξ\scriptstyle{\Xi}Δ​f\scriptstyle{\Delta f}𝒮f,M∙​(Zξ)\textstyle{{\mathcal{S}}^{\bullet}_{f,M}(Z_{\xi})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}f\scriptstyle{f}ΔRf∘ξ2−,Lf∘ξ1+,λ′​𝒮f⁡(M)∙​(Z′)\textstyle{\Delta_{R_{f\circ\xi_{2}^{-}},L_{f\circ\xi_{1}^{+}},\lambda^{\prime}}{\mathcal{S}}^{\bullet}_{f(M)}(Z^{\prime})\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ξ\scriptstyle{\Xi}𝒮f⁡(M)∙​(Zf∘ξ′)\textstyle{{\mathcal{S}}^{\bullet}_{f(M)}(Z^{\prime}_{f\circ\xi})}

Assume f|Zf_{|Z} is strict. It follows that ff is strict. The functor f#:add⁡(𝒮f⁡(M)​(Z′))→add⁡(𝒮M​(Z))f^{\#}:\operatorname{add}\nolimits({\mathcal{S}}_{f(M)}(Z^{\prime}))\to\operatorname{add}\nolimits({\mathcal{S}}_{M}(Z)) can be equipped with a structure of morphism of 22-representations Lf∘ξ1+→Lξ1+L_{f\circ\xi_{1}^{+}}\to L_{\xi_{1}^{+}} and of morphism of 22-representations Rf∘ξ2−→Rξ2−R_{f\circ\xi_{2}^{-}}\to R_{\xi_{2}^{-}} (Lemma 8.1.4 and §8.1.5), and it induces a differential functor (cf §4.3.4)

Δ​f#:ΔRf∘ξ2−,Lf∘ξ1+,λ′​add⁡(𝒮f⁡(M)​(Z′))→ΔRξ2−,Lξ1+,λ​add⁡(𝒮M​(Z)).\Delta f^{\#}:\Delta_{R_{f\circ\xi_{2}^{-}},L_{f\circ\xi_{1}^{+}},\lambda^{\prime}}\operatorname{add}\nolimits({\mathcal{S}}_{f(M)}(Z^{\prime}))\to\Delta_{R_{\xi_{2}^{-}},L_{\xi_{1}^{+}},\lambda}\operatorname{add}\nolimits({\mathcal{S}}_{M}(Z)).

We obtain a commutative diagram of differential functors commuting with coproducts

(8.2.2) ΔRξ2−,Lξ1+,λ​add⁡(𝒮M​(Z))\textstyle{\Delta_{R_{\xi_{2}^{-}},L_{\xi_{1}^{+}},\lambda}\operatorname{add}\nolimits({\mathcal{S}}_{M}(Z))\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ξ\scriptstyle{\Xi}add⁡(𝒮M​(Zξ))\textstyle{\operatorname{add}\nolimits({\mathcal{S}}_{M}(Z_{\xi}))}ΔRf∘ξ2−,Lf∘ξ1+,λ′​add⁡(𝒮f⁡(M)​(Z′))\textstyle{\Delta_{R_{f\circ\xi_{2}^{-}},L_{f\circ\xi_{1}^{+}},\lambda^{\prime}}\operatorname{add}\nolimits({\mathcal{S}}_{f(M)}(Z^{\prime}))\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}Ξ\scriptstyle{\Xi}Δ​f#\scriptstyle{\Delta f^{\#}}OPENadd⁡(𝒮f⁡(M)​(Zf∘ξ′)))\textstyle{\operatorname{add}\nolimits({\mathcal{S}}_{f(M)}(Z^{\prime}_{f\circ\xi})))\ignorespaces\ignorespaces\ignorespaces\ignorespaces}f#\scriptstyle{f^{\#}}

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

Andrew Manion, Raphael Rouquier

Original source: arXiv:2009.09627v2