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3.7.1. Relative Gibbons-Hawking potential [043L]

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3.7.1. Relative Gibbons-Hawking potential

We plan to exhibit a T2T^{2}-bundle preserving diffeomorphism Ψ1\Psi_{1} between the Taub-NUT type ℂ3\mathbb{C}^{3} and the positive vertex space Mν+M^{+}_{\nu} over the common base {0<|μ→|a≤13​A1/2}\{0<|\vec{\mu}|_{a}\leq\frac{1}{3}A^{1/2}\}, with good estimates on the deviations between both Kähler structures. Since 13​A1/2<12​A1/2\frac{1}{3}A^{1/2}<\frac{1}{2}A^{1/2}, the η\eta-periodic copies of such punctured discs do not overlap. The topology of the T2T^{2}-bundle structures on both spaces agree by construction. The remaining degrees of freedom in defining Ψ1\Psi_{1} amounts to a gauge choice, which is the same as a prescription of ϑiℂ3−Ψ1∗​ϑi\vartheta_{i}^{\mathbb{C}^{3}}-\Psi_{1}^{*}\vartheta_{i}.

As a general guideline, the corresponding quantities on ℂ3\mathbb{C}^{3} and Mν+M^{+}_{\nu} have the same singularity, so their difference are smooth quantities. We use superscripts for quantities on ℂ3\mathbb{C}^{3} to disambiguate from quantities on Mν+M^{+}_{\nu}.

Lemma 3.25.

Over the region {|μ→|a≤13A1/2}\{|\vec{\mu}|_{a}\leq\frac{1}{3}A^{1/2}\},

|Ψ1−1∗αi−α~i|≤CA−3/4|μ→|a,|∇gak(Ψ1−1∗αi−α~i)|ga≤CA−1/4−k/2,i=1,2,3.|\Psi_{1}^{-1*}\alpha_{i}-\tilde{\alpha}_{i}|\leq CA^{-3/4}|\vec{\mu}|_{a},\quad|\nabla_{g_{a}}^{k}(\Psi_{1}^{-1*}\alpha_{i}-\tilde{\alpha}_{i})|_{g_{a}}\leq CA^{-1/4-k/2},\quad i=1,2,3.
Proof.

The absolute estimate follows from Lemma 3.2. The higher order estimate follows from Δa\Delta_{a}-harmonicity. ∎

Lemma 3.26.

Over the disc {|μ→|a≤13A1/2}\{|\vec{\mu}|_{a}\leq\frac{1}{3}A^{1/2}\}

|β~i−Ψ1−1∗βi|≤CA−1/2|μ→|a,|∇gak(β~i−Ψ1−1∗βi)|ga≤CA−k/2.|\tilde{\beta}_{i}-\Psi_{1}^{-1*}\beta_{i}|\leq CA^{-1/2}|\vec{\mu}|_{a},\quad|\nabla^{k}_{g_{a}}(\tilde{\beta}_{i}-\Psi_{1}^{-1*}\beta_{i})|_{g_{a}}\leq CA^{-k/2}.
Proof.

The absolute estimate is contained in Lemma 3.7. The higher order estimates follow from the differential relations between β~i\tilde{\beta}_{i} and α~i\tilde{\alpha}_{i}, vis-a-vis βi{\beta}_{i} and αi{\alpha}_{i} (cf. Lemma 2.7). ∎

Corollary 3.27.

There is a real-valued relative Gibbons-Hawking potential φ2\varphi_{2} on the disc {|μ→|a≤13A1/2}\{|\vec{\mu}|_{a}\leq\frac{1}{3}A^{1/2}\}, such that its second derivatives are given by

{∂2φ2∂μi​∂μj=V(1)i​j−V~(1)i​j−∂2φ1∂μi​∂μj,i,j=1,2,∂2φ2∂η​∂η¯=−14​(W(1)−W~(1))−∂2φ1∂η​∂η¯,∂2φ2∂η​∂μi=12(βi−β~i)−∂2φ1∂η​∂μi,i=1,2.\begin{cases}\frac{\partial^{2}\varphi_{2}}{\partial\mu_{i}\partial\mu_{j}}=V_{(1)}^{ij}-\tilde{V}_{(1)}^{ij}-\frac{\partial^{2}\varphi_{1}}{\partial\mu_{i}\partial\mu_{j}},\quad i,j=1,2,\\ \frac{\partial^{2}\varphi_{2}}{\partial\eta\partial\bar{\eta}}=-\frac{1}{4}(W_{(1)}-\tilde{W}_{(1)})-\frac{\partial^{2}\varphi_{1}}{\partial\eta\partial\bar{\eta}},\\ \frac{\partial^{2}\varphi_{2}}{\partial\eta\partial\mu_{i}}=\frac{1}{2}(\beta_{i}-\tilde{\beta}_{i})-\frac{\partial^{2}\varphi_{1}}{\partial\eta\partial\mu_{i}},\quad i=1,2.\end{cases}

We can demand the estimates in {|μ→|a≤13A1/2}\{|\vec{\mu}|_{a}\leq\frac{1}{3}A^{1/2}\}:

|∇gakφ2|ga≤C​ν​A1/4−k/2,k≥0.|\nabla^{k}_{g_{a}}\varphi_{2}|_{g_{a}}\leq C\nu A^{1/4-k/2},\quad k\geq 0.
Proof.

The existence of φ2\varphi_{2} with presecribed second order derivatives is a consequence of integrability, notably Lemma 2.7 and its counterpart for Mν+M^{+}_{\nu}. If we impose that φ\varphi and its first order derivatives vanish at the origin, then the estimates follow immediately from the Lemmas above and Proposition 3.23. ∎

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