ScalingStacks

8.1.2. Approximation

Assume ξ−1​(M)\xi^{-1}(M) has no maximum. Fix an increasing sequence m0,m1,…m_{0},m_{1},\ldots of points of (0,1)(0,1) with ξ⁡(mi)∈M\xi(m_{i})\in M for all ii and with limimi>t\lim_{i}m_{i}>t for all t∈ξ−1​(M)t\in\xi^{-1}(M).

Fix n≥0n\geq 0 and define the braid βr:{mr,…,mr+n−1}→{1,…,n}\beta_{r}:\{m_{r},\ldots,m_{r+n-1}\}\to\{1,\ldots,n\} of 𝐑>0{\mathbf{R}}_{>0} by (βr)mr+i=[mr+i→i+1](\beta_{r})_{m_{r+i}}=[m_{r+i}\to i+1].

Let SS and TT be two finite subsets of MM. Consider rr such that mr>ξ−1​(t)m_{r}>\xi^{-1}(t) for all t∈T∩ξ⁡(𝐑>0)t\in T\cap\xi({\mathbf{R}}_{>0}). There is an isomorphism

Hom𝒮∙​(Z)⁡(S,T⊔ξ⁡({mr,…,mr+n−1}))→∼L∙​(T,S,en),α↦(idT⊠ξ⁡(βr))⋅α.\operatorname{Hom}\nolimits_{{\mathcal{S}}^{\bullet}(Z)}(S,T\sqcup\xi(\{m_{r},\ldots,m_{r+n-1}\}))\mathrel{\mathop{\kern 0.0pt\to}\limits^{\sim}}L^{\bullet}(T,S,e^{n}),\ \alpha\mapsto(\operatorname{id}\nolimits_{T}\boxtimes\xi(\beta_{r}))\cdot\alpha.

It follows that there are isomorphisms functorial in SS and TT

(8.1.1) colimr→∞⁡Hom𝒮M∙​(Z)​(S,T⊔ξ⁡({mr,…,mr+n−1}))→∼L∙​(T,S,en).\operatorname{colim}\nolimits_{r\to\infty}\operatorname{Hom}\nolimits_{{\mathcal{S}}^{\bullet}_{M}(Z)}(S,T\sqcup\xi(\{m_{r},\ldots,m_{r+n-1}\}))\mathrel{\mathop{\kern 0.0pt\to}\limits^{\sim}}L^{\bullet}(T,S,e^{n}).

Here, the colimit is taken over the invertible maps ξ⁡(θr)\xi(\theta_{r}), where θr:{mr,…,mr+n−1}→{mr+1,…,mr+n}\theta_{r}:\{m_{r},\ldots,m_{r+n-1}\}\to\{m_{r+1},\ldots,m_{r+n}\} is the braid in 𝐑>0{\mathbf{R}}_{>0} given by (θr)ms=[ms→ms+1](\theta_{r})_{m_{s}}=[m_{s}\to m_{s+1}].

We deduce that T↦(S↦L∙​(T,S,en))T\mapsto(S\mapsto L^{\bullet}(T,S,e^{n})) is isomorphic to the functor

𝒮M∙​(Z)→𝒮M∙​(Z)​−diff,T↦colimr→∞⁡T⊔ξ⁡({mr,…,mr+n−1}).{\mathcal{S}}^{\bullet}_{M}(Z)\to{\mathcal{S}}^{\bullet}_{M}(Z)\operatorname{\!-diff}\nolimits,\ T\mapsto\operatorname{colim}\nolimits_{r\to\infty}T\sqcup\xi(\{m_{r},\ldots,m_{r+n-1}\}).

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

Andrew Manion, Raphael Rouquier

Original source: arXiv:2009.09627v2