ScalingStacks

Proposition 4.99 . [02SG]

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Proposition 4.99.

Let Π\Pi be a complete SCR polyhedral complex in NℝN_{\mathbb{R}} and ψ\psi an H-lattice concave function on Π\Pi. Set Σ=rec⁡(Π)\Sigma=\operatorname{rec}(\Pi) and Ψ=rec⁡(ψ)\Psi=\operatorname{rec}(\psi). Let σ∈Σ\sigma\in\Sigma and mσ∈Mm_{\sigma}\in M such that Ψ|σ=mσ|σ\Psi|_{\sigma}=m_{\sigma}|_{\sigma}. Let πσ:Nℝ→N​(σ)ℝ\pi_{\sigma}\colon N_{\mathbb{R}}\to N(\sigma)_{\mathbb{R}} be the projection and πσ∨:M​(σ)ℝ→Mℝ\pi^{\vee}_{\sigma}\colon M(\sigma)_{\mathbb{R}}\to M_{\mathbb{R}} the dual inclusion. Then

(4.100) (ψ−mσ)​(σ)=(πσ)∗​(ψ−mσ),(\psi-m_{\sigma})(\sigma)=(\pi_{\sigma})_{\ast}(\psi-m_{\sigma}),

Hence the restriction of the divisor Dψ−mσD_{\psi-m_{\sigma}} to 𝒱⁡(σ){\mathcal{V}}(\sigma) corresponds to the H-lattice concave function (πσ)∗​(ψ−mσ)(\pi_{\sigma})_{\ast}(\psi-m_{\sigma}). Dually,

(4.101) (ψ−mσ)​(σ)∨=(πσ∨+mσ)∗​ψ∨.(\psi-m_{\sigma})(\sigma)^{\vee}=(\pi^{\vee}_{\sigma}+m_{\sigma})^{\ast}\psi^{\vee}.

In other words, the Legendre-Fenchel dual of (ψ−mσ)​(σ)(\psi-m_{\sigma})(\sigma) is the restriction of ψ∨\psi^{\vee} to the face FσF_{\sigma} translated by −mσ-m_{\sigma}.

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