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4.2 Comparison Kähler metric II: Regularisation [00BD]

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4.2 Comparison Kähler metric II: Regularisation

We now improve the metric ωF​S,t\omega_{FS,t} on (Xt,1|log⁡|t||​c1​(L))(X_{t},\frac{1}{|\log|t||}c_{1}(L)) to make the volume form approximately CY except on a set with a small percentage of the CY measure.

Let 0<δ≪10<\delta\ll 1. Since ϕ0\phi_{0} solves the real MA equation (8) on the nn-dimensional open faces Int​(ΔJ)\text{Int}(\Delta_{J}), the regularity theory of real MA surveyed in section 2.5 applies. In particular, we can find finitely many small open balls Bl=B⁡(pl,2​r​(δ))B_{l}=B(p_{l},2r(\delta)) properly contained in ∪JInt(ΔJ)\cup_{J}\text{Int}(\Delta_{J}), such that ϕ0\phi_{0} has CkC^{k}-norm uniformly bounded by C⁡(δ)C(\delta) on any BlB_{l}, and the complement of Wδ=∪lB(pl,r(δ))W_{\delta}=\cup_{l}B(p_{l},r(\delta)) has small μ0\mu_{0}-measure less than δ\delta. The open sets WδW_{\delta} form an exhaustion of W0=∪δ>0WδW_{0}=\cup_{\delta>0}W_{\delta}, which is the C∞C^{\infty}-locus of the real MA solution ϕ0\phi_{0}. Recall the normalised CY measure d​μtd\mu_{t} from section 3.1.

Lemma 4.2.

(Regularisation) Let ϵ\epsilon be suffiently small dependent on δ\delta, and construct ωF​S,t\omega_{FS,t} as in Lemma 4.1, for tt sufficiently small dependent on ϵ\epsilon and δ\delta. There is a Lipschitz continuous function ψt\psi_{t} on XtX_{t} with ‖ψt‖L∞≤3​ϵ\left\lVert\psi_{t}\right\rVert_{L^{\infty}}\leq 3\epsilon, such that

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    The function ψt\psi_{t} is smooth away from a closed subset with d​μtd\mu_{t}-measure zero.

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    The (1,1)-current ωψ,t=ωF​S,t+d​dc​ψt≥0\omega_{\psi,t}=\omega_{FS,t}+dd^{c}\psi_{t}\geq 0 is positive on XtX_{t}.

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    The metric estimate (1−C​ϵ)​d​dc​ϕ0∘Log𝒳≤ωψ,t≤(1+C​ϵ)​d​dc​ϕ0∘Log𝒳(1-C\epsilon)dd^{c}\phi_{0}\circ\text{Log}_{\mathcal{X}}\leq\omega_{\psi,t}\leq(1+C\epsilon)dd^{c}\phi_{0}\circ\text{Log}_{\mathcal{X}} holds on the smooth locus of ψ\psi inside Log𝒳−1​(Wδ)\text{Log}_{\mathcal{X}}^{-1}(W_{\delta}).

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    The total variation ∫Xt||log⁡|t||n​ωψ,tn(Ln)−d​μt|<δ.\int_{X_{t}}|\frac{|\log|t||^{n}\omega_{\psi,t}^{n}}{(L^{n})}-d\mu_{t}|<\delta.

Proof.

Choose a smooth nonnegative bump function η\eta on ℝn\mathbb{R}^{n} supported in B⁡(0,2)B(0,2), and equals one on B⁡(0,1)B(0,1), and construct ηi=η⁡(|x−pi|r⁡(δ))\eta_{i}=\eta(\frac{|x-p_{i}|}{r(\delta)}) supported on BiB_{i}. We calculate Log𝒳−1​(Bl)\text{Log}_{\mathcal{X}}^{-1}(B_{l})

d​dc​ϕ0∘Log𝒳=14​π​|log⁡|t||2​∑1≤i,j≤n∂2ϕ0∂xi​∂xj​−1​d​log⁡zi∧d​log⁡z¯j≳δ1|log⁡|t||2∑−1dlogzi∧dlogz¯j,\begin{split}dd^{c}\phi_{0}\circ\text{Log}_{\mathcal{X}}=\frac{1}{4\pi|\log|t||^{2}}\sum_{1\leq i,j\leq n}\frac{\partial^{2}\phi_{0}}{\partial x_{i}\partial x_{j}}\sqrt{-1}d\log z_{i}\wedge d\log\bar{z}_{j}\\ \gtrsim_{\delta}\frac{1}{|\log|t||^{2}}\sum\sqrt{-1}d\log z_{i}\wedge d\log\bar{z}_{j},\end{split}
ddcηl∘Log𝒳≥−C|log⁡|t||2∑−1dlogzi∧dlogz¯j,dd^{c}\eta_{l}\circ\text{Log}_{\mathcal{X}}\geq-\frac{C}{|\log|t||^{2}}\sum\sqrt{-1}d\log z_{i}\wedge d\log\bar{z}_{j},

so if 0<ϵ≪10<\epsilon\ll 1 is sufficiently small dependent on δ\delta, we can ensure (ϕ0+2​ϵ​ηl)∘Log𝒳(\phi_{0}+2\epsilon\eta_{l})\circ\text{Log}_{\mathcal{X}} is psh on Log𝒳−1​(Bl)\text{Log}_{\mathcal{X}}^{-1}(B_{l}).

Consider the potential function on Log𝒳−1​(Int​(ΔJ))\text{Log}_{\mathcal{X}}^{-1}(\text{Int}(\Delta_{J}))

ψt​(z)=max⁡(0,maxLog𝒳​(z)∈Bl⁡{(ϕ0+2​ϵ​ηl)∘Log𝒳−ϵ−ϕJ,t}),\psi_{t}(z)=\max(0,\max_{\text{Log}_{\mathcal{X}}(z)\in B_{l}}\{(\phi_{0}+2\epsilon\eta_{l})\circ\text{Log}_{\mathcal{X}}-\epsilon-\phi_{J,t}\}),

with ϕJ,t\phi_{J,t} from Lemma 4.1.

Since |ϕJ,t−ϕ0∘Log𝒳|<ϵ|\phi_{J,t}-\phi_{0}\circ\text{Log}_{\mathcal{X}}|<\epsilon, we see that on ∂Bl\partial B_{l} the maximum is never achieved by (ϕ0+2​ϵ​ηl)∘Log𝒳−ϵ−ϕJ,t(\phi_{0}+2\epsilon\eta_{l})\circ\text{Log}_{\mathcal{X}}-\epsilon-\phi_{J,t}, so the fact that this term is only locally defined causes no problem. By construction every term is d​dc​ϕJ,tdd^{c}\phi_{J,t}-psh, so the maximum ψt\psi_{t} is also d​dc​ϕJ,tdd^{c}\phi_{J,t}-psh. Near the boundary of Int​(ΔJ)\text{Int}(\Delta_{J}) the maximum is achieved by ψt=0\psi_{t}=0, so ψt\psi_{t} globalizes to define a Lipschitz continuous ωF​S,t\omega_{FS,t}-psh function on XtX_{t}, with ‖ψt‖L∞≤3​ϵ\left\lVert\psi_{t}\right\rVert_{L^{\infty}}\leq 3\epsilon.

Morever, on Log𝒳−1​(Wδ)\text{Log}_{\mathcal{X}}^{-1}(W_{\delta}) the maximum of ψt\psi_{t} is strictly greater than zero. Perturbing the bump function in the construction if necessary, we may assume the locus on XtX_{t} where the maximum is achieved by at least two terms is a subset of codimension one, and it is automatically closed. So a.e on Log𝒳−1​(Wδ)\text{Log}_{\mathcal{X}}^{-1}(W_{\delta}), the metric ωψ,t=ωF​S,t+d​dc​ψt\omega_{\psi,t}=\omega_{FS,t}+dd^{c}\psi_{t} is smooth and equals d​dc​(ϕ0+2​ϵ​ηl)∘Log𝒳dd^{c}(\phi_{0}+2\epsilon\eta_{l})\circ\text{Log}_{\mathcal{X}} for some ll. We calculate using the regularity estimates on d​dc​ϕ0dd^{c}\phi_{0} that

ωψ,tn=(d​dc​(ϕ0+2​ϵ​ηl)∘Log𝒳)n=(1+O⁡(ϵ))​(d​dc​ϕ0∘Log𝒳)n.\omega_{\psi,t}^{n}=(dd^{c}(\phi_{0}+2\epsilon\eta_{l})\circ\text{Log}_{\mathcal{X}})^{n}=(1+O(\epsilon))(dd^{c}\phi_{0}\circ\text{Log}_{\mathcal{X}})^{n}.

Using the real MA equation (8),

(d​dc​ϕ0∘Log𝒳)n=n!​det(D2​ϕ0)​∏i14​π​|log⁡|t||2​−1​d​log⁡zi∧d​log⁡z¯i,(dd^{c}\phi_{0}\circ\text{Log}_{\mathcal{X}})^{n}=n!\det(D^{2}\phi_{0})\prod_{i}\frac{1}{4\pi|\log|t||^{2}}\sqrt{-1}d\log z_{i}\wedge d\log\bar{z}_{i},

where (Ln)n!​d​μ0=det(D2​ϕ0)​d​x1​…​d​xn\frac{(L^{n})}{n!}d\mu_{0}=\det(D^{2}\phi_{0})dx_{1}\ldots dx_{n} determines the constant det(D2​ϕ0)\det(D^{2}\phi_{0}). Comparing with section 3.1, the normalized CY measure d​μtd\mu_{t} satisfies

(Ln)n!​d​μt=(1+O⁡(1|log⁡|t||))​det(D2​ϕ0)​∏i14​π​|log⁡|t||​−1​d​log⁡zi∧d​log⁡z¯i.\frac{(L^{n})}{n!}d\mu_{t}=(1+O(\frac{1}{|\log|t||}))\det(D^{2}\phi_{0})\prod_{i}\frac{1}{4\pi|\log|t||}\sqrt{-1}d\log z_{i}\wedge d\log\bar{z}_{i}.

For sufficiently small tt depending on ϵ\epsilon and δ\delta, we combine the above to deduce

ωψ,tn=(1+O⁡(ϵ))​(Ln)|log⁡|t||n​d​μt.\omega_{\psi,t}^{n}=(1+O(\epsilon))\frac{(L^{n})}{|\log|t||^{n}}d\mu_{t}.

The total complex MA measure is ∫Xtωψ,tn=(Ln)|log⁡|t||n\int_{X_{t}}\omega_{\psi,t}^{n}=\frac{(L^{n})}{|\log|t||^{n}}, and the contribution from the smooth region in Log𝒳−1​(Wδ)\text{Log}_{\mathcal{X}}^{-1}(W_{\delta}) is greater than (1−δ)​(Ln)|log⁡|t||n(1-\delta)\frac{(L^{n})}{|\log|t||^{n}} for very small ϵ\epsilon dependent on δ\delta. Thus the measure contribution from the complement must be less than δ​(Ln)|log⁡|t||n\delta\frac{(L^{n})}{|\log|t||^{n}}, namely δ\delta-percent of the total measure. Thus the total variation of the signed measure d​μt−|log⁡|t||n(Ln)​ωψ,tnd\mu_{t}-\frac{|\log|t||^{n}}{(L^{n})}\omega_{\psi,t}^{n} is smaller than δ\delta for small enough ϵ\epsilon. ∎

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