ScalingStacks

Example 5.26 . [02TP]

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Example 5.26.

Let Σ\Sigma be a complete fan in NℝN_{\mathbb{R}} and XΣX_{\Sigma} the corresponding toric variety. Recall the description of the projective space ℙr\mathbb{P}^{r} as a toric variety given in Example 4.3. Let H:N→ℤrH\colon N\to\mathbb{Z}^{r} be a linear map such that, for each σ∈Σ\sigma\in\Sigma there exist τ∈ΣΔr\tau\in\Sigma_{\Delta^{r}} with H⁡(σ)⊂τH(\sigma)\subset\tau. Let p∈ℙ0r​(K)p\in\mathbb{P}^{r}_{0}(K). Then we have an equivariant morphism φp,H:XΣ→ℙr\varphi_{p,H}\colon X_{\Sigma}\to\mathbb{P}^{r}. Consider the support function ΨΔr\Psi_{\Delta^{r}} on ΣΔr\Sigma_{\Delta^{r}}. Then LΨΔr=𝒪ℙr​(1)L_{\Psi_{\Delta^{r}}}=\mathcal{O}_{\mathbb{P}^{r}}(1). Write L=φp,H∗​LΨΔrL=\varphi^{\ast}_{p,H}L_{\Psi_{\Delta^{r}}}, s=φp,H∗​sΨΔrs=\varphi^{\ast}_{p,H}s_{\Psi_{\Delta^{r}}} and Ψ=H∗​ΨΔr\Psi=H^{\ast}\Psi_{\Delta^{r}}. Thus (L,s)=(LΨ,sΨ)(L,s)=(L_{\Psi},s_{\Psi}).

Set A=H+valK⁡(p)A=H+{\operatorname{val}}_{K}(p) for the affine map. Let ∥⋅∥\|\cdot\| be the metric on LanL^{{\text{\rm an}}} induced by the canonical metric of 𝒪​(DΨΔr)an\mathcal{O}(D_{\Psi_{\Delta^{r}}})^{{\text{\rm an}}} and let ψ\psi be the function associated to it by Proposition 5.16. By Proposition 5.24, ψ=A∗​ΨΔr\psi=A^{\ast}\Psi_{\Delta^{r}}. This is a piecewise affine concave function on NℝN_{\mathbb{R}} with rec⁡(ψ)=Ψ\operatorname{rec}(\psi)=\Psi that can be made explicit as follows.

Let {e1,…,er}\{e_{1},\dots,e_{r}\} be the standard basis of ℤr\mathbb{Z}^{r} and let {e1∨,…,er∨}\{e_{1}^{\vee},\dots,e_{r}^{\vee}\} be the dual basis. Write mi=ei∨∘H∈Mm_{i}=e_{i}^{\vee}\circ H\in M and li=ei∨​(valK⁡(p))∈ℝl_{i}=e_{i}^{\vee}({\operatorname{val}}_{K}(p))\in\mathbb{R}. Then

Ψ\displaystyle\Psi =min⁡{0,m1,…,mr}\displaystyle=\min\{0,m_{1},\dots,m_{r}\}
ψ\displaystyle\psi =min⁡{0,m1+l1,…,mr+lr}\displaystyle=\min\{0,m_{1}+l_{1},\dots,m_{r}+l_{r}\}

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