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4. Reconstruction of the complex hypersurface from a balanced polyhedron Π [04SW]

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4. Reconstruction of the complex hypersurface from a balanced polyhedron Π\Pi

Theorem 1’ can be treated as a pair-of-pants decomposition for VV. We can use this presentation to reconstruct VV from Π\Pi. This allows to interpret a maximal balanced polyhedral complex Π\Pi as the complex encoding the gluing pattern of pairs-of-pants in order to get VV. Here is the way to reconstruct VV from Π\Pi.

For each vertex vjv_{j} of Π\Pi take a copy Q¯j\bar{Q}_{j} of 𝒫¯n\bar{\mathcal{P}}_{n}. This copy can be identified with the localized hyperplane Q¯n⊂(ℂ∗)n+1\bar{Q}^{n}\subset(\mathbb{C}^{*})^{n+1}. Recall that by Proposition 1.24 ∂1(Q¯j)\partial_{1}(\bar{Q}_{j}) consists of n+2n+2 components. Each such component corresponds to a 1-cell of Π\Pi adjacent to vjv_{j}.

Let ej​ke_{jk} be a 1-cell of Π\Pi connecting the vertices vjv_{j} and vkv_{k}. For each such 1-cell we identify the closures FjF_{j} and FkF_{k} of the corresponding components of ∂1(Q¯j)\partial_{1}(\bar{Q}_{j}) and ∂1(Q¯k)\partial_{1}(\bar{Q}_{k}) in the following way.

Without the loss of generality we may assume that in both copies Q¯j,Q¯k\bar{Q}_{j},\bar{Q}_{k} of Q¯n\bar{Q}^{n} the edge ej​ke_{jk} corresponds to the facet xn+1=−Rnx_{n+1}=-R_{n} of Δ⁡(Rn)\Delta(R_{n}) (see 3.4). (Note that such correspondence is given by matrices Mj,MkM_{j},M_{k} from Proposition 1.11.) We attach FjF_{j} to FkF_{k} by the map

(z1,…,zn,zn+1)↦(z1,…,zn,z¯n+1),log⁡|zn+1|=−Rn,(z_{1},\dots,z_{n},z_{n+1})\mapsto(z_{1},\dots,z_{n},\bar{z}_{n+1}),\log|z_{n+1}|=-R_{n},

where z¯n+1\bar{z}_{n+1} is the complex conjugate to zn+1z_{n+1}.

The result UU of this gluing is a manifold with boundary. The boundary comes from the unbounded cells of Π\Pi. Denote W∘=U∖∂UW^{\circ}=U\smallsetminus\partial U. The boundary is formed by the closures FF of the components of ∂1(Qj)\partial_{1}(Q_{j}) that correspond to unbounded 1-cells in Π\Pi. By Proposition 1.24 each such FF is a circle fibration over a union of lower-dimensional pairs-of-pants 𝒫n−1\mathcal{P}_{n-1}. Let WW be the result of collapsing all fibers of these fibrations on ∂U\partial U. Note that WW is canonically a smooth manifold since this procedure locally coincides with collapsing the boundary on 𝒫¯n\bar{\mathcal{P}}_{n} which results in ℂ​ℙn{\mathbb{C}}{\mathbb{P}}^{n}.

Theorem 4.

The manifold WW is diffeomorphic to VV. The manifold W∘W^{\circ} is diffeomorphic to V∘{V}^{\circ}.

Corollary 4.1.

The manifolds WW and W∘W^{\circ} depend only on the lattice polygon Δ\Delta associated to Π\Pi, not on Π\Pi itself.

Remark 4.2.

With a little more care this reconstruction process can be made in the symplectic category, i.e. the result WW of gluing can be given a natural symplectic structure. This is due to the following two reasons. The first one is that the pair-of-pants possesses a natural symplectic structure (the one which gives the standard symplectic ℂ​ℙn{\mathbb{C}}{\mathbb{P}}^{n} after the symplectic reduction of the boundary). The second one is that two pairs-of-pants get identified along a part FF of their boundary which is a symplectically flat hypersurface, it has a neighborhood F×[0,1]F\times[0,1] symplectically isomorphic to Qn−1×AQ_{n-1}\times A, where A⊂ℂ∗A\subset\mathbb{C}^{*} is an annulus. This product is consistent with the S1S^{1}-fibration F→Qn−1F\to Q_{n-1} from Proposition 1.24.

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