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2.4. The model torus fibrations as Kähler manifolds [03EY]

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2.4. The model torus fibrations as Kähler manifolds

The (d−1)(d-1)-torus fibration W⁡(λ,θ)W(\lambda,\theta) (more naturally, a 𝕋d−1\mathbb{T}^{d-1}-torsor) will depend on additional phase multi-parameter θ:={θv},v∈vert⁡(S)\theta:=\{\theta_{v}\},v\in\operatorname{vert}(S), where all θv\theta_{v} have values in ℝ/ℤ\mathbb{R}/\mathbb{Z}. First, we form (trivial) affine torus bundles over the affine open sets by identifying the fibers with the affine tori

𝕋v​(θv):={n∈𝕋:⟨v,n⟩+θv≡0modℤ} over ​Uv,\displaystyle\mathbb{T}_{v}(\theta_{v}):=\{n\in\mathbb{T}\ :\ \langle v,n\rangle+\theta_{v}\equiv 0\mod\mathbb{Z}\}\quad\text{ over }U_{v},
𝕋/w:=(ℝd/w)/(ℤd/w) over ​Vw.\displaystyle\mathbb{T}/w:=(\mathbb{R}^{d}/w)/(\mathbb{Z}^{d}/w)\quad\text{ over }V_{w}.

The gluing maps are independent of a base point in an overlap Uv∩VwU_{v}\cap V_{w} and given there by the natural projection 𝕋v​(θv)→𝕋/w\mathbb{T}_{v}(\theta_{v})\to\mathbb{T}/w. This defines the torsor π:W⁡(λ,θ)→Σ\D\pi:W(\lambda,\theta)\to{\Sigma\backslash D}.

The topology of the total space of the torsor is determined by the combinatorics of Σ\Sigma, i.e., independent of λ\lambda and θ\theta as long as λ\lambda is in the right secondary cone SC⁡(S)\operatorname{SC}(S). In particular, all W⁡(λ,θ)W(\lambda,\theta) are diffeomorphic to each other, though not canonically.

Since the linear parts of the transition maps are the same for the base and for the fibers, the tangent space at any point in W⁡(λ,θ)W(\lambda,\theta) splits canonically as

TW⁡(λ,θ)≅TΣ\D⊕TΣ\D.T_{W(\lambda,\theta)}\cong T_{\Sigma\backslash D}\oplus T_{\Sigma\backslash D}.

This allows to define a canonical (integrable) almost complex structure on W⁡(λ,θ)W(\lambda,\theta) as

Jw=(0𝟙−𝟙0).J_{w}=\left(\begin{array}[]{cc}0&\mathbbm{1}\\ -\mathbbm{1}&0\\ \end{array}\right).

Given a Riemannian metric gi​jg_{ij} on YY, one can define the pullback metric π∗​(gi​j)\pi^{*}(g_{ij}) on W⁡(λ,θ)W(\lambda,\theta) which is, in fact, Kähler. If, in addition, gi​jg_{ij} satisfy the real Monge-Ampère equation, the induced metric on W⁡(λ,θ)W(\lambda,\theta) is Ricci-flat.

There is another slightly different description of the torus bundle over a Kähler affine manifold YY (cf. [KS01]), which is useful when considering limiting behavior of Calabi-Yau degenerations. We define a torus fibration Wϵ​(λ)W_{\epsilon}(\lambda) as the quotient of the total space of the tangent bundle T⁡(Σ\D)T({{\Sigma\backslash D}}) by the integral lattice spanned by {ϵ​∂∂yi}\left\{\epsilon\frac{\partial}{\partial y_{i}}\right\}, where yiy_{i} are the affine coordinates. This torus bundle carries canonical complex structure and the pullback metric which comes from the splitting TWϵ​(λ)≅TΣ\D⊕TΣ\DT_{W_{\epsilon}(\lambda)}\cong T_{\Sigma\backslash D}\oplus T_{\Sigma\backslash D} as before.

But in order to make a connection with the previous picture and with the geometry of toric hypersurfaces we need to twist the complex structure on Wϵ​(λ)W_{\epsilon}(\lambda) by the element of H1​(Σ\D,𝕋d−1)H^{1}({\Sigma\backslash D},\mathbb{T}^{d-1}) associated with the phase parameters θv\theta_{v}. The resulting Kähler manifold Wϵ​(λ,θ)W_{\epsilon}(\lambda,\theta) can be canonically identified with W⁡(ϵ−1​λ,θ)W(\epsilon^{-1}\lambda,\theta) constructed by the first method starting with the ϵ\epsilon-rescaled Kähler affine structure.

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