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3.2. The elliptic curve and the K3-surface [04SH]

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3.2. The elliptic curve and the K3-surface

Here we consider the well-known fibrations of the elliptic curve and the K3-surface.

Let ℂ​E\mathbb{C}E be an elliptic curve, i.e. a Riemann surface of genus 1. Since ℂ​E\mathbb{C}E is topologically a torus, there is a trivial S1S^{1}-fibration λE:ℂ​E→S1\lambda_{E}:\mathbb{C}E\to S^{1}.

Suppose that the elliptic curve ℂ​E⊂ℂ​TΔ\mathbb{C}E\subset\mathbb{C}T_{\Delta} is presented as a curve in a toric surface ℂ​TΔ\mathbb{C}T_{\Delta}, where Δ\Delta is the Newton polygon of a polynomial defining ℂ​E\mathbb{C}E. By the genus formula (see [8]), Int⁡Δ\operatorname{Int}\Delta contains a unique lattice point. By Proposition 1.10 a dual Δ\Delta-complex is homotopy equivalent to a circle. It is easy to see that the fibration from Theorem 1’ coincides up to homotopy with the trivial S1S^{1}-fibration ℂ​E≈S1×S1→S1\mathbb{C}E\approx S^{1}\times S^{1}\to S^{1}.

Another famous fibration λK:ℂ​K→S2\lambda_{K}:\mathbb{C}K\to S^{2} has the K3-surface ℂ​K\mathbb{C}K as its total space. All its fibers, except for 24 of them are Lagrangian tori.

Suppose that the polygon Δ\Delta has exactly one interior lattice point. Then, by Khovanskii’s formula [8], the zero locus ℂ​K\mathbb{C}K of a generic polynomial with the Newton polygon Δ\Delta is a K3-surface. A dual Δ\Delta-complex is homotopy equivalent to a sphere S2S^{2} by Proposition 1.10.

Again, the fibration λ\lambda can be deformed to a fibration like λK\lambda_{K} by so-called shelling of Π¯\bar{\Pi} 11 1 A higher-dimensional version of such deformation will be the subject for a future paper..

In higher dimensions, if Δ\Delta is a non-singular polyhedron with a unique interior lattice point, then the corresponding hypersurface V⊂ℂ​TΔV\subset\mathbb{C}T_{\Delta} is a smooth Calabi-Yau manifold. Singular torus fibrations V→SnV\to S^{n} were constructed by Zharkov [17]. Ruan [13] noted that such fibrations can be made Lagrangian.

Theorem 1’ constructs in this case a stratified torus fibration over a polyhedral complex homotopy equivalent to SnS^{n}.

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