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2.3 Simplifications for proportional line bundles [0226]

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2.3 Simplifications for proportional line bundles

A special case of the generalized Calabi ansatz is when LL is trivial, and Li=di​L0L_{i}=d_{i}L_{0} for some positive line bundle L0L_{0} and positive integers di>0d_{i}>0. In this case, we can take hL=1h_{L}=1, and hLih_{L_{i}} is the suitable tensor power of hL0h_{L_{0}}, where hL0h_{L_{0}} can be chosen to correspond to the Calabi-Yau metric in the class c1​(L0)c_{1}(L_{0}). The ansatz metric is simply

d​dc​u=∑∂2u∂xi​∂xj​d​log⁡ri∧dc​log⁡rj+∑∂u∂xi​d​dc​ϕi=∑∂2u∂xi​∂xj​d​log⁡ri∧dc​log⁡rj+(∑∂u∂xi​di)​d​dc​ϕ0.\begin{split}dd^{c}u=\sum\frac{\partial^{2}u}{\partial x_{i}\partial x_{j}}d\log r_{i}\wedge d^{c}\log r_{j}+\sum\frac{\partial u}{\partial x_{i}}dd^{c}\phi_{i}\\ =\sum\frac{\partial^{2}u}{\partial x_{i}\partial x_{j}}d\log r_{i}\wedge d^{c}\log r_{j}+(\sum\frac{\partial u}{\partial x_{i}}d_{i})dd^{c}\phi_{0}.\end{split}

Observe that for rank reasons

(∑∂2u∂xi​∂xj​d​log⁡ri∧dc​log⁡rj)m+1=0,(d​dc​ϕ0)n−m+1=0.(\sum\frac{\partial^{2}u}{\partial x_{i}\partial x_{j}}d\log r_{i}\wedge d^{c}\log r_{j})^{m+1}=0,\quad(dd^{c}\phi_{0})^{n-m+1}=0.

We compute the volume form using binomial expansion

(d​dc​u)n=n!(n−m)!​(∑∂u∂xi​di)n−m​det(D2​u)​(∏1md​log⁡ri∧dc​log⁡ri)∧(d​dc​ϕ0)n−m.(dd^{c}u)^{n}=\frac{n!}{(n-m)!}(\sum\frac{\partial u}{\partial x_{i}}d_{i})^{n-m}\det(D^{2}u)(\prod_{1}^{m}d\log r_{i}\wedge d^{c}\log r_{i})\wedge(dd^{c}\phi_{0})^{n-m}.

Since (d​dc​ϕ0)n−m(dd^{c}\phi_{0})^{n-m} already exhaust all base terms, we can replace log⁡ri\log r_{i} by log⁡|ξi|\log|\xi_{i}|, and obtain

(d​dc​u)n=n!(n−m)!​(∑∂u∂xi​di)n−m​det(D2​u)​(∏1m−14​π​d​log⁡ξi∧d​log⁡ξi¯)∧(d​dc​ϕ0)n−m.(dd^{c}u)^{n}=\frac{n!}{(n-m)!}(\sum\frac{\partial u}{\partial x_{i}}d_{i})^{n-m}\det(D^{2}u)(\prod_{1}^{m}\frac{\sqrt{-1}}{4\pi}d\log\xi_{i}\wedge d\overline{\log\xi_{i}})\wedge(dd^{c}\phi_{0})^{n-m}.

The Calabi-Yau condition on d​dc​ϕ0dd^{c}\phi_{0} and the normalization on ΩY\Omega_{Y} imply

(d​dc​ϕ0)n−m=(∫Yc1​(L0)n−m)​−1(n−m)2​ΩY∧Ω¯Y.(dd^{c}\phi_{0})^{n-m}=(\int_{Y}c_{1}(L_{0})^{n-m})\sqrt{-1}^{(n-m)^{2}}\Omega_{Y}\wedge\overline{\Omega}_{Y}.

The conclusion is that

Lemma 2.1.

As long as the NA MA equation holds

det(D2​u)​(∑∂u∂xi​di)n−m=const,\det(D^{2}u)(\sum\frac{\partial u}{\partial x_{i}}d_{i})^{n-m}=\text{const},

then the generalized Calabi ansatz d​dc​udd^{c}u is a Calabi-Yau metric, in this case of proportional line bundles.

Remark 2.2.

It is understood that uu is strictly convex, and ∑∂u∂xi​di\sum\frac{\partial u}{\partial x_{i}}d_{i} is positive. These two conditions guarantee the metric is positive definite.

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