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Remark 4.83 . [02RY]

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Remark 4.83.

Let Σ\Sigma be a complete fan in NℝN_{\mathbb{R}} and Ψ\Psi a virtual support function on Σ\Sigma. Let (𝒳,D,e)({\mathcal{X}},D,e) be a toric model of (XΣ,DΨ)(X_{\Sigma},D_{\Psi}). Then, by Theorem 4.81, there exists a complete SCR polyhedral complex Π\Pi in NℝN_{\mathbb{R}} with rec⁡(Π)=Σ\operatorname{rec}(\Pi)=\Sigma and a rational piecewise affine function ψ\psi on Π\Pi such that e​ψe\psi is an H-lattice function, rec⁡(ψ)=Ψ\operatorname{rec}(\psi)=\Psi and (𝒳,D,e)=(𝒳Π,De​ψ,e)({\mathcal{X}},D,e)=({\mathcal{X}}_{\Pi},D_{e\psi},e). Moreover, if (𝒳′,D′,e′)({\mathcal{X}}^{\prime},D^{\prime},e^{\prime}) is another toric model that gives the function ψ′\psi^{\prime}, then both models are equivalent if and only if ψ=ψ′\psi=\psi^{\prime}. Thus, to every toric model we have associated a rational piecewise affine function ψ\psi on Π\Pi such that rec⁡(ψ)=Ψ\operatorname{rec}(\psi)=\Psi. Two equivalent models give rise to the same function.

The converse is not true. Given a rational piecewise affine function ψ\psi, with rec⁡(ψ)=Ψ\operatorname{rec}(\psi)=\Psi, we can find a complete SCR polyhedral complex Π\Pi such that ψ\psi is piecewise affine on Π\Pi. But, in general rec⁡(Π)\operatorname{rec}(\Pi) does not agree with Σ\Sigma. What we can expect is that Σ′:=rec⁡(Π)\Sigma^{\prime}:=\operatorname{rec}(\Pi) is a refinement of Σ\Sigma. Therefore the function ψ\psi gives us an equivalence class of toric models of (XΣ′,DΨ)(X_{\Sigma^{\prime}},D_{\Psi}). But ψ\psi may not determine an equivalence class of toric models of (XΣ,DΨ)(X_{\Sigma},D_{\Psi}). In Corollary 5.43 in next section we will give a necessary condition for a function ψ\psi to define an equivalence class of toric models of (XΣ,DΨ)(X_{\Sigma},D_{\Psi}) and in Example 5.44 we will exhibit a function that does not satisfy this necessary condition. By contrast, as we will see in Theorem 4.97, the concave case is much more transparent.

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