ScalingStacks

Proof. [01ZQ]

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Proof.

Let δ>0\delta>0 be chosen with δ′​(v,δ)\delta^{\prime}({\rm v},\delta) to be chosen later. Note that by Lemma 8.5, for each x∈B1​(p)x\in B_{1}(p) there exists αx≤α¯​(v,δ′)\alpha_{x}\leq\bar{\alpha}(v,\delta^{\prime}) such that Tαxδ′​(x)=0T^{\delta^{\prime}}_{\alpha_{x}}(x)=0. Consider the covering {Brαx​(x)}\{B_{r_{\alpha_{x}}}(x)\} of B1​(p)B_{1}(p), and choose an efficient subcovering {Brj​(xj′)}1N\{B_{r_{j}}(x^{\prime}_{j})\}_{1}^{N}, where rj=rαxj′r_{j}=r_{\alpha_{x^{\prime}_{j}}} and the balls in {Brj/4​(xj′)}\{B_{r_{j}/4}(x^{\prime}_{j})\} are disjoint. The usual doubling arguments imply that N≤N⁡(v,δ′)N\leq N(v,\delta^{\prime}).

By Theorem 8.3, if we are given ϵ>0\epsilon>0, then we can choose δ′​(v,ϵ,δ)\delta^{\prime}({\rm v},\epsilon,\delta) such that for each x∈Bϵ​rj​(xj′)x\in B_{\epsilon r_{j}}(x^{\prime}_{j}) we have Tαjδ​(x)=0T^{\delta}_{\alpha_{j}}(x)=0, while for each x∈Aϵ​rj,2​rj​(xj)x\in A_{\epsilon r_{j},2r_{j}}(x_{j}) we have rh​(x)>r¯​(v,ϵ)​rj≥r0​(v,ϵ,δ′)r_{h}(x)>\bar{r}(v,\epsilon)r_{j}\geq r_{0}(v,\epsilon,\delta^{\prime}). Let Γj\Gamma_{j} be the group associated to Brj​(xj′)B_{r_{j}}(x^{\prime}_{j}), and for each x∈Bϵ​rj​(xj′)x\in B_{\epsilon r_{j}}(x^{\prime}_{j}) let βj​(x)\beta_{j}(x) be the largest integer such that Vrβj/4δ​(x)≥ln⁡|Γj|−δV^{\delta}_{r_{\beta_{j}}/4}(x)\geq\ln|\Gamma_{j}|-\delta. Let βj=max⁡βj​(x)\beta_{j}=\max\beta_{j}(x) with xjx_{j} the corresponding point. Note that for ϵ⁡(v,δ)\epsilon(v,\delta) sufficiently small, we have B2​rβj​(xj)⊆Bϵ​rj​(xj′)B_{2r_{\beta_{j}}}(x_{j})\subseteq B_{\epsilon r_{j}}(x^{\prime}_{j}), and in particular, for every x∈B2​rβj​(xj)x\in B_{2r_{\beta_{j}}}(x_{j})

Vrβj/8δ​(x)<ln⁡|Γj|−δ.\displaystyle V^{\delta}_{r_{\beta_{j}}/8}(x)<\ln|\Gamma_{j}|-\delta\,. (8.50)

Consider the collection of balls {Brj​(xj)}\{B_{r_{j}}(x_{j})\}. Clearly, by construction, conditions (1) and (3) are satisfied. If x∈B1​(p)∖{Brj​(xj)}x\in B_{1}(p)\setminus\{B_{r_{j}}(x_{j})\} then since {B2​rj​(xj)}\{B_{2r_{j}}(x_{j})\} cover B1​(p)B_{1}(p) we have that for some xjx_{j} that x∈Arj,2​rj​(xj)x\in A_{r_{j},2r_{j}}(x_{j}), which implies rh​(x)≥r0​(v,δ)r_{h}(x)\geq r_{0}(v,\delta), as claimed. ∎

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