ScalingStacks

Example 5.2 . [00AR]

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Example 5.2.

When n=mn=m, namely in the maximal degeneration case, for simplicity we consider an (n−1)(n-1)-dimensional face ΔJ\Delta_{J} of S​k​(X)Sk(X), such that there are only two nn-dimensional faces of Δ𝒳\Delta_{\mathcal{X}} containing ΔJ\Delta_{J}, and they both lie on S​k​(X)Sk(X). Then the degree condition 𝒟J​(x,‖⋅‖C​Y)⋅EJ=0\mathcal{D}_{J}(x,\left\lVert\cdot\right\rVert_{CY})\cdot E_{J}=0 imposes a matching condition on the gradient of ϕ0\phi_{0} across the (n−1)(n-1)-dim face.

We consider a very concrete local example where ℙ1≃EJ=∩1nEi\mathbb{P}^{1}\simeq E_{J}=\cap_{1}^{n}E_{i},

deg𝒪(Ei)|EJ=−di≤0,i=1,2,…n,∑1ndi=2,\deg\mathcal{O}(E_{i})|_{E_{J}}=-d_{i}\leq 0,\quad i=1,2,\ldots n,\quad\sum_{1}^{n}d_{i}=2,

and the two divisors E0,E∞E_{0},E_{\infty} intersect EJE_{J} transversely at 0,∞∈ℙ10,\infty\in\mathbb{P}^{1}, giving rise to the two nn-dimensional faces. All divisors are reduced. The complex geometric local model for 𝒳\mathcal{X} comprises of two charts ℂz0,…​znn+1\mathbb{C}^{n+1}_{z_{0},\ldots z_{n}} and ℂw0,…,wnn+1\mathbb{C}^{n+1}_{w_{0},\ldots,w_{n}}. On the first chart Ei={zi=0}E_{i}=\{z_{i}=0\} for i=0,1,…​ni=0,1,\ldots n, and z0z_{0} is the affine coordinate on ℙ1∖{∞}\mathbb{P}^{1}\setminus\{\infty\}. On the second chart Ei={wi=0}E_{i}=\{w_{i}=0\} for i=1,…​ni=1,\ldots n, E∞={w0=0}E_{\infty}=\{w_{0}=0\}, and w0w_{0} is the affine coordinate on ℙ1∖{0}\mathbb{P}^{1}\setminus\{0\}. The transition on the overlap is

w0=z0−1,wi=ziz0di,i=1,2,…n.w_{0}=z_{0}^{-1},\quad w_{i}=z_{i}z_{0}^{d_{i}},\quad i=1,2,\ldots n.

The holomorphic volume form Ω∼d​z0∧…​d​zn=−d​w0∧…​d​wn\Omega\sim dz_{0}\wedge\ldots dz_{n}=-dw_{0}\wedge\ldots dw_{n}, and the coordinate t=z0​…​zn=w0​…​wnt=z_{0}\ldots z_{n}=w_{0}\ldots w_{n}. Locally on XtX_{t}

Ωt∼d​log​z1∧…​d​log​zn=−d​log​w1∧…​d​log​wn.\Omega_{t}\sim d\log z_{1}\wedge\ldots d\log z_{n}=-d\log w_{1}\wedge\ldots d\log w_{n}.

On the two nn-dimensional faces the coordinates are xi=log⁡|zi|log⁡|t|x_{i}=\frac{\log|z_{i}|}{\log|t|} and xi′=log⁡|wi|log⁡|t|x_{i}^{\prime}=\frac{\log|w_{i}|}{\log|t|} respectively, satisfying the linear constraints ∑0nxi=0\sum_{0}^{n}x_{i}=0 and ∑0nxi′=0\sum_{0}^{n}x_{i}^{\prime}=0, so we can eliminate xn,xn′x_{n},x_{n}^{\prime}. By adjusting ℒ\mathcal{L} we can make it zero in the local model. The potential ϕ0\phi_{0} satisfies the real MA equation on the two nn-dimensional faces:

det(∂2ϕ0∂xi​∂xj)0≤i,j≤n−1=const,det(∂2ϕ0∂xi′​∂xj′)0≤i,j≤n−1=const,\det\left(\frac{\partial^{2}\phi_{0}}{\partial x_{i}\partial x_{j}}\right)_{0\leq i,j\leq n-1}=\text{const},\quad\det\left(\frac{\partial^{2}\phi_{0}}{\partial x_{i}^{\prime}\partial x_{j}^{\prime}}\right)_{0\leq i,j\leq n-1}=\text{const},

and the problem is to match them on ΔJ\Delta_{J}. The complex geometry suggests that the domain of the coordinates xi,xi′x_{i},x_{i}^{\prime} can be extended outside the original nn-simplices, by the identificaton

x0′=−x0,xi′=xi+dix0,i=1,2,…n−1,x_{0}^{\prime}=-x_{0},\quad x_{i}^{\prime}=x_{i}+d_{i}x_{0},\quad i=1,2,\ldots n-1,

and the real MA equation is satisfied also on ΔJ\Delta_{J}. In terms of gradients at x∈ΔJx\in\Delta_{J},

∂ϕ0∂xi′=∂ϕ0∂xi,i=1,…n−1,∂ϕ0∂x0′=−∂ϕ0∂x0+∑1n−1di∂ϕ0∂xi.\frac{\partial\phi_{0}}{\partial x_{i}^{\prime}}=\frac{\partial\phi_{0}}{\partial x_{i}},\quad i=1,\ldots n-1,\quad\frac{\partial\phi_{0}}{\partial x_{0}^{\prime}}=-\frac{\partial\phi_{0}}{\partial x_{0}}+\sum_{1}^{n-1}d_{i}\frac{\partial\phi_{0}}{\partial x_{i}}.

The first (n−1)(n-1) conditions merely mean the tangent derivatives along ΔJ\Delta_{J} agree. The normal derivative matching condition precisely says

𝒟J​(x,‖⋅‖)⋅EJ=∑1n−1di​∂ϕ0∂xi−∂ϕ0∂x0−∂ϕ0∂x0′=0.\mathcal{D}_{J}(x,\left\lVert\cdot\right\rVert)\cdot E_{J}=\sum_{1}^{n-1}d_{i}\frac{\partial\phi_{0}}{\partial x_{i}}-\frac{\partial\phi_{0}}{\partial x_{0}}-\frac{\partial\phi_{0}}{\partial x_{0}^{\prime}}=0.

From a different perspective, the zero degree condition explains why ℙ1\mathbb{P}^{1} can be invisible to the metric, so ϕ0\phi_{0} is allowed to remain smooth across Int​(ΔJ)\text{Int}(\Delta_{J}).

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