ScalingStacks

Example 7.11 . [0302]

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Example 7.11.

The discrete Legendre transform enables us to reproduce Batyrev duality [5]. Let Δ⊆Mℝ\Delta\subseteq M_{\mathbb{R}} be a reflexive polytope, ∇⊆Nℝ\nabla\subseteq N_{\mathbb{R}} the polar dual, and assume 0∈Δ0\in\Delta is the unique interior point. We then obtain two toric degenerations given by the equations

s0+t​∑m∈M∩Δcm​sm=0,s0+t​∑n∈N∩∇cn​sn=0s_{0}+t\sum_{m\in M\cap\Delta}c_{m}s_{m}=0,\quad\quad s_{0}+t\sum_{n\in N\cap\nabla}c_{n}s_{n}=0

in ℙΔ×𝔸1\mathbb{P}_{\Delta}\times\mathbb{A}^{1} and ℙ∇×𝔸1\mathbb{P}_{\nabla}\times\mathbb{A}^{1} respectively, with sms_{m} (sns_{n}) the section of 𝒪ℙΔ​(1)\mathcal{O}_{\mathbb{P}_{\Delta}}(1) corresponding to mm (the section of 𝒪ℙ∇​(1)\mathcal{O}_{\mathbb{P}_{\nabla}}(1) corresponding to nn). It is easy to check that the dual intersection complexes of these two degenerations are given as follows. For the first degeneration, B=∂∇B=\partial\nabla with polyhedral decomposition given by the proper faces of ∇\nabla. The fan structure at each vertex vv is given by projection Uv↪Nℝ→Nℝ/ℝ​vU_{v}\hookrightarrow N_{\mathbb{R}}\rightarrow N_{\mathbb{R}}/\mathbb{R}v. For the second degeneration, one uses Δ\Delta instead of ∇\nabla. One can then check that if one polarizes the two degenerations using 𝒪ℙΔ​(1)\mathcal{O}_{\mathbb{P}_{\Delta}}(1) and 𝒪ℙ∇​(1)\mathcal{O}_{\mathbb{P}_{\nabla}}(1) respectively, then the corresponding triples (B,𝒫,φ)(B,\mathscr{P},\varphi) are Legendre dual. Thus Batyrev duality is a special case of this general approach to a mirror construction.

For a much more general construction which works for the Batyrev-Borisov construction [6] of mirrors of complete intersection Calabi-Yaus in toric varieties, see [22].

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