ScalingStacks

Proof. [02SD]

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Proof.

Let (𝒳,D,e)({\mathcal{X}},D,e) be a semipositive toric model. By Theorem 4.81, to the pair (𝒳,D)({\mathcal{X}},D) corresponds a pair (Π,ψ′)(\Pi,\psi^{\prime}), where ψ′\psi^{\prime} is an H-lattice function on Π\Pi, rec⁡(Π)=Σ\operatorname{rec}(\Pi)=\Sigma and rec⁡(ψ′)=e​Ψ\operatorname{rec}(\psi^{\prime})=e\Psi. By Theorem 4.95, the function ψ′\psi^{\prime} is concave. We put ψ=1e​ψ′\psi=\frac{1}{e}\psi^{\prime}. It is clear that equivalent models produce the same function.

Conversely, let ψ\psi be a rational piecewise affine concave function. Let Π′=Π⁡(ψ)\Pi^{\prime}=\Pi(\psi). This is a rational polyhedral complex. Let Σ′=rec⁡(Π′)\Sigma^{\prime}=\operatorname{rec}(\Pi^{\prime}). This is a conic rational polyhedral complex. By Proposition 3.72, Σ′=Π⁡(Ψ)\Sigma^{\prime}=\Pi(\Psi). Since Ψ\Psi is a support function on Σ\Sigma, we deduce that Σ\Sigma is a refinement of Σ′\Sigma^{\prime}. Put Π=Π′⋅Σ\Pi=\Pi^{\prime}\cdot\Sigma (Definition 3.10). Since Π′\Pi^{\prime} is a rational polyhedral complex and Σ\Sigma is a fan, then Π\Pi is an SCR polyhedral complex. Moreover, by Lemma 3.11, we have

rec⁡(Π)=rec⁡(Π′⋅Σ)=rec⁡(Π′)⋅rec⁡(Σ)=Σ′⋅Σ=Σ.\operatorname{rec}(\Pi)=\operatorname{rec}(\Pi^{\prime}\cdot\Sigma)=\operatorname{rec}(\Pi^{\prime})\cdot\operatorname{rec}(\Sigma)=\Sigma^{\prime}\cdot\Sigma=\Sigma.

Let e>0e>0 be an integer such that e​ψe\psi is an H-lattice function. Then (𝒳Π,De​ψ,e)({\mathcal{X}}_{\Pi},D_{e\psi},e) is a toric model of (XΣ,DΨ)(X_{\Sigma},D_{\Psi}). Both procedures are inverse of each other. ∎

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