ScalingStacks

2. Statement of the results [04S9]

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2. Statement of the results

Let V⊂ℂ​ℙn+1V\subset{\mathbb{C}}{\mathbb{P}}^{n+1} be a smooth hypersurface of degree dd. We choose homogeneous coordinates [Z0:…:Zn+1][Z_{0}:\dots:Z_{n+1}] so that VV is transverse to coordinate hyperplanes Zj=0Z_{j}=0 and all their intersections. The complement of the coordinate hyperplanes in ℂ​ℙn+1{\mathbb{C}}{\mathbb{P}}^{n+1} is (ℂ∗)n+1(\mathbb{C}^{*})^{n+1}. Denote V∘=V∩(ℂ∗)n+1{V}^{\circ}=V\cap(\mathbb{C}^{*})^{n+1}. Then the hypersurface V∘∈(ℂ∗)n+1{V}^{\circ}\in(\mathbb{C}^{*})^{n+1} is given by equation f⁡(z)=0f(z)=0, where

z=(z1,…,zn+1)=(Z1/Z0,…,Zn+1/Z0)z=(z_{1},\dots,z_{n+1})=(Z_{1}/Z_{0},\dots,Z_{n+1}/Z_{0})

stands for affine coordinates in (ℂ∗)n+1(\mathbb{C}^{*})^{n+1} and ff is a polynomial with the Newton polyhedron Δd\Delta_{d} from Example 5. Recall that we denote the real nn-dimensional torus with Tn=S1×⋯×S1T^{n}=S^{1}\times\dots\times S^{1}.

Theorem 1.

For every maximal dual Δd\Delta_{d}-complex Π\Pi there exists a stratified TnT^{n}-fibration λ:V→Π¯\lambda:V\to\bar{\Pi} . This fibration satisfies to the following properties

  • •

    the induced map λ∗:Hn​(Π¯,ℤ)→Hn​(V,ℤ)\lambda^{*}:H^{n}(\bar{\Pi};\mathbb{Z})\to H^{n}(V;\mathbb{Z}) is injective, where Hn​(Π¯,ℤ)≈ℤpgH^{n}(\bar{\Pi};\mathbb{Z})\approx\mathbb{Z}^{p_{g}}, pg=hn,0p_{g}=h^{n,0} is the geometric genus of VV;

  • •

    for each primitive piece UjU_{j} of Π\Pi (see Definition 5) the inverse image λ−1​(Uj)\lambda^{-1}(U_{j}) is an open pair-of-pants 𝒫n\mathcal{P}_{n}.

  • •

    for each nn-cell ee of Π¯\bar{\Pi} there exists a point x∈ex\in e such that the fiber λ−1​(x)\lambda^{-1}(x) is a Lagrangian nn-torus Tn⊂VT^{n}\subset V;

  • •

    there exist Lagrangian embedding ϕk:Sn→V\phi_{k}:S^{n}\to V, k=1,…,pgk=1,\dots,p_{g} such that the cycles λ⁡(ϕk​(Sn))\lambda(\phi_{k}(S^{n})) form a basis of Hn​(Π¯)H_{n}(\bar{\Pi}).

Maximal dual Δd\Delta_{d}-complexes exist for every degree dd and every dimension nn.

This theorem admits a straightforward generalization to toric varieties other than ℂ​ℙn+1{\mathbb{C}}{\mathbb{P}}^{n+1}. Let Δ\Delta be a bounded convex lattice polyhedron such that all singularities of the toric variety ℂ​TΔ\mathbb{C}T_{\Delta} are isolated. Note that the isolated singular points of ℂ​TΔ\mathbb{C}T_{\Delta} necessarily correspond to some vertices of Δ\Delta. Consider the space (ℂ∗)#⁡(Δ∩ℤn+1)(\mathbb{C}^{*})^{\#(\Delta\cap\mathbb{Z}^{n+1})} of all polynomials of the type f⁡(z)=∑j∈Δaj​zjf(z)=\sum\limits_{j\in\Delta}a_{j}z^{j} such that aj≠0a_{j}\neq 0. Then for a generic choice of a polynomial ff from this space the closure VV in ℂ​TΔ\mathbb{C}T_{\Delta} of the zero set of ff is a smooth hypersurface transverse to all toric subvarieties ℂ​TΔ′\mathbb{C}T_{\Delta^{\prime}} corresponding to the faces Δ′⊂Δ\Delta^{\prime}\subset\Delta. All such VV are diffeomorphic and, if we equip them with the symplectic form from ℂ​TΔ\mathbb{C}T_{\Delta}, are symplectomorphic varieties.

Theorem 1’.

For every maximal dual Δ\Delta-complex Π\Pi there exists a stratified TnT^{n}-fibration λ:V→Π¯\lambda:V\to\bar{\Pi}. This fibration satisfies to the following properties

  • •

    the induced map λ∗:Hn​(Π¯,ℤ)→Hn​(V,ℤ)\lambda^{*}:H^{n}(\bar{\Pi};\mathbb{Z})\to H^{n}(V;\mathbb{Z}) is injective, where Hn​(Π¯,ℤ)≈ℤpgH^{n}(\bar{\Pi};\mathbb{Z})\approx\mathbb{Z}^{p_{g}}, pg=hn,0p_{g}=h^{n,0} is the geometric genus of VV;

  • •

    for each primitive piece UjU_{j} of Π\Pi (see Definition 5) the inverse image λ−1​(Uj)\lambda^{-1}(U_{j}) is an open pair-of-pants 𝒫n\mathcal{P}_{n}.

  • •

    for each nn-cell ee of Π¯\bar{\Pi} there exists a point x∈ex\in e such that the fiber λ−1​(x)\lambda^{-1}(x) is a Lagrangian nn-torus Tn⊂VT^{n}\subset V;

  • •

    there exist Lagrangian embeddings ϕk:Sn→V\phi_{k}:S^{n}\to V, k=1,…,pgk=1,\dots,p_{g} such that the cycles λ⁡(ϕk​(Sn))\lambda(\phi_{k}(S^{n})) form a basis of Hn​(Π¯)H_{n}(\bar{\Pi}).

By Remark 1.9 not all convex lattice polyhedra have maximal dual complexes. However, in the case of Δd\Delta_{d} (the polyhedra corresponding to the projective space), such subdivisions exists for any dd. Maximal subdivisions also exist for products of different Δd\Delta_{d} (this corresponds to hypersurfaces in the product of projective spaces). It is conjectured that for any lattice polyhedron Δ\Delta there exists a sufficiently large integer NN that N​ΔN\Delta (the result of scaling of Δ\Delta by NN) has a maximal subdivision.

The next theorem describes the behavior of the fibration λ\lambda with respect to a complex structure on VV. Recall that, unlike the smooth and symplectic structures, the complex structure on VV depends on the polynomial ff and not just on Δ\Delta.

Recall that a map λ:V→Π¯\lambda:V\to\bar{\Pi} is called a totally real fibration if for any z∈Vz\in V the tangent space to the fiber through zz is totally real i.e. contains no positive-dimensional complex subspaces (as long as the fiber is smooth near zz). We say that a hypersurface V⊂ℂ​TΔV\subset\mathbb{C}T_{\Delta} is defined over ℝ\mathbb{R} if it can be obtained as the closure of the zero set of a polynomial f:(ℂ∗)n+1→ℂf:(\mathbb{C}^{*})^{n+1}\to\mathbb{C} whose coefficients are real.

Theorem 2.

For every maximal dual Δ\Delta-complex Π\Pi there exists a smooth hypersurface V⊂ℂ​TΔV\subset\mathbb{C}T_{\Delta} defined over ℝ\mathbb{R} such that the map λ\lambda from Theorem 1’ preserves the real structure of VV, i.e. λ∘conj=λ\lambda\circ\operatorname{conj}=\lambda. Furthermore, λ\lambda is a totally real fibration.

Theorems 1’ and 2 can be extended further to polyhedra Δ\Delta corresponding to toric varieties with non-isolated singularities. However, in order to do that, one has to modify the definition of stratified fibrations to include singular total spaces VV. We do not do that. In the next theorem we no longer have any restrictions on the convex lattice polyhedron Δ\Delta, but its statement concerns only the toric, non-singular, part V∘⊂(ℂ∗)n+1{V}^{\circ}\subset(\mathbb{C}^{*})^{n+1} of the hypersurface VV.

Theorem 3.

For every maximal dual Δ\Delta-complex Π\Pi there exists a stratified TnT^{n}-fibration λ∘:V∘→Π\lambda^{\circ}:{V}^{\circ}\to\Pi. This fibration satisfies to the following properties

  • •

    the induced map (λ∘)∗:Hn​(Π,ℤ)→Hn​(V∘,ℤ)(\lambda^{\circ})^{*}:H^{n}(\Pi;\mathbb{Z})\to H^{n}({V}^{\circ};\mathbb{Z}) is injective, where Hn​(Π,ℤ)≈ℤpgH^{n}(\Pi;\mathbb{Z})\approx\mathbb{Z}^{p_{g}}, pg=hn,0p_{g}=h^{n,0} is the geometric genus of VV;

  • •

    for each primitive piece UjU_{j} of Π\Pi (see Definition 5) the inverse image (λ∘)−1​(Uj)(\lambda^{\circ})^{-1}(U_{j}) is an open pair-of-pants 𝒫n\mathcal{P}_{n}.

  • •

    for each nn-cell ee of Π\Pi there exists a point x∈ex\in e such that the fiber (λ∘)−1​(x)(\lambda^{\circ})^{-1}(x) is a Lagrangian nn-torus Tn⊂VT^{n}\subset V;

  • •

    there exist Lagrangian embeddings ϕk:Sn→V∘\phi_{k}:S^{n}\to{V}^{\circ}, k=1,…,pgk=1,\dots,p_{g} such that the cycles λ∘​(ϕk​(Sn))\lambda^{\circ}(\phi_{k}(S^{n})) form a basis of Hn​(Π)H_{n}(\Pi).

Remark 2.1.

These theorems generalize to complete intersections. The base of the fibration in this case is the intersection of the maximal dual balanced polyhedra for the corresponding hypersurfaces (we have to choose them in a mutually general position).

From a different point of view the base is dual to a maximal mixed lattice subdivision of the Newton polyhedra of the participating equations. The primitive pieces for complete intersections are products of the primitive pieces for hypersurfaces. Sturmfels’ generalization [14] of the patchworking technique allows to produce in this case the Lagrangian lifts of the base cycles.

This generalization will be the subject of a future paper.

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