ScalingStacks

Lemma 8.8 . [01ZK]

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Lemma 8.8.

For every 0<ϵ≤ϵ⁡(v)0<\epsilon\leq\epsilon({\rm v}), there exists δ=δ⁡(v,ϵ)\delta=\delta({\rm v},\epsilon) with the following properties. Let M4M^{4} satisfy |RicM4|≤3​δ|{\rm Ric}_{M^{4}}|\leq 3\delta and Vol⁡(B1​(p))>v>0{\rm Vol}(B_{1}(p))>{\rm v}>0. Let x∈B1​(p)x\in B_{1}(p) and assume α1∈ℕ\alpha_{1}\in\mathds{N} satisfies Tα1δ​(x)=0T^{\delta}_{\alpha_{1}}(x)=0 with Γα1\Gamma_{\alpha_{1}} the corresponding group. Then if α2∈ℕ\alpha_{2}\in\mathds{N} is such that 𝒱rα2/4δ​(x)≥ln⁡|Γα1|−δ\mathcal{V}^{\delta}_{r_{\alpha_{2}}/4}(x)\geq\ln\big|\Gamma_{\alpha_{1}}\big|-\delta, there exists a subset Arα2/2,2​rα1​(x)⊆U⊆A(1−ϵ)​rα2/2,2​(1+ϵ)​rα1​(x)A_{r_{\alpha_{2}}/2,2r_{\alpha_{1}}}(x)\subseteq U\subseteq A_{(1-\epsilon)r_{\alpha_{2}}/2,2(1+\epsilon)r_{\alpha_{1}}}(x) and a diffeomorphism Φ:Arα2/2,2​rα1​(0)→U\Phi:A_{r_{\alpha_{2}}/2,2r_{\alpha_{1}}}(0)\to U, where 0∈ℝ4/Γα10\in\mathds{R}^{4}/\Gamma_{\alpha_{1}}, such that if gi​j=Φ∗​gg_{ij}=\Phi^{*}g is the pullback metric, we have

‖gi​j−δi​j‖C0​(Arα/2,r2​α)+rα​‖∂kgi​j‖C0​(Arα/2,2​rα)<ϵ\displaystyle||g_{ij}-\delta_{ij}||_{C^{0}(A_{r_{\alpha}/2},r_{2\alpha})}+r_{\alpha}||\partial_{k}g_{ij}||_{C^{0}(A_{r_{\alpha}/2},2r_{\alpha})}<\epsilon (8.30)

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