ScalingStacks

5.4 Special Lagrangian fibration in the generic region [00TY]

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5.4 Special Lagrangian fibration in the generic region

In the setting of section 5.2, the very strong regularity bounds in the generic region leads to the existence of special Lagrangian TnT^{n}-fibrations thereon.

Theorem 5.13.

For any fixed compact K⊂ℛK\subset\mathcal{R}, for s≫1s\gg 1 depending on KK, there is a special Lagrangian (SLag) TnT^{n}-fibration on an open subset of XsX_{s} containing Us,KU_{s,K}.

Remark 5.14.

By considering a compact exhaustion of ℛ\mathcal{R}, we can choose KK so that the region Us,KU_{s,K} occupies a percentage of the total measure on XsX_{s} arbitrarily close to 1.

Proof.

Since KK is a compact subset in the open set ℛ\mathcal{R}, we can find an open set 𝒰⊂K\mathcal{U}\subset K properly contained in ℛ\mathcal{R}. This ensures that the smooth convergence in Thm. 5.6 happens uniformly on a slightly larger set Us,K′U_{s,K^{\prime}} than Us,KU_{s,K}. We assume s≫1s\gg 1 as ususal.

Consider a coordinate region (s−1​Log)−1​(B⁡(x,r⁡(x))CLOSE(s^{-1}\text{Log})^{-1}(B(x,r(x)) contained in this larger set, which is topologically Tn×B⁡(x,r⁡(x))T^{n}\times B(x,r(x)). Here the TnT^{n} is well defined as a homology cycle independent of the coordinates. We define the phase angles θs\theta_{s} by requiring ∫Tne−1​θs​Ω>0.\int_{T^{n}}e^{\sqrt{-1}\theta_{s}}\Omega>0. We consider the rescaled CY metrics (s2​gC​Y,s,s2​ωC​Y,s)(s^{2}g_{CY,s},s^{2}\omega_{CY,s}), so the diameter of TnT^{n} fibres are now of order O⁡(1)O(1) by (32)(33). Within any log scale, these rescaled CY structures are C∞C^{\infty}-close to the standard flat structures in section 2.7 up to constant factors. By construction the Kähler forms are exact in these coordinate charts. Thus by Zhang’s result surveyed in section 2.7, within any log scale, we can construct a SLag TnT^{n}-fibration with phase θs\theta_{s}, whose fibres are very small C∞C^{\infty}-perturbations of the fibres of the map (ℂ∗)n→ℝn(\mathbb{C}^{*})^{n}\to\mathbb{R}^{n},

Log:(zm1,…,zmn)→(log⁡|zm1|,…​log⁡|zmn|).\text{Log}:(z^{m_{1}},\ldots,z^{m_{n}})\to(\log|z^{m_{1}}|,\ldots\log|z^{m_{n}}|).

Observe that on overlapping charts, the Log-fibres with respect to one chart are very small C∞C^{\infty}-perturbations of the Log-fibres of the other chart. Then the uniqueness part of Zhang’s argument shows that on overlapping charts the SLag TnT^{n}-fibrations are in fact defined independent of charts. (It is the local universal family of SLags within the perturbative regime.) Thus the local constructions glue to a SLag fibration on a subset of XsX_{s} containing Us,KU_{s,K} as required. ∎

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