ScalingStacks

Example 5.44 . [02UC]

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Example 5.44.

Let N=ℤ2N=\mathbb{Z}^{2} and consider the fan Σ\Sigma generated by e0=(−1,−1)e_{0}=(-1,-1), e1=(1,0)e_{1}=(1,0) and e2=(0,1)e_{2}=(0,1). Then XΣ=ℙ2X_{\Sigma}=\mathbb{P}^{2}. The virtual support function Ψ=0\Psi=0 corresponds to the trivial line bundle 𝒪ℙ2\mathcal{O}_{\mathbb{P}^{2}}. Consider the function

ψ⁡(x,y)={0, if ​x≤0,x, if ​0≤x≤1,1, if ​1≤x.\psi(x,y)=\begin{cases}0,&\text{ if }x\leq 0,\\ x,&\text{ if }0\leq x\leq 1,\\ 1,&\text{ if }1\leq x.\\ \end{cases}

Then rec⁡(ψ)=Ψ\operatorname{rec}(\psi)=\Psi, but ψ\psi does not extend to a continuous function on NΣN_{\Sigma} and therefore it does not determine a model of (XΣ,𝒪)(X_{\Sigma},\mathcal{O}). By contrast, let Σ′\Sigma^{\prime} be the fan obtained subdividing Σ\Sigma by adding the edge corresponding to e′=(0,−1)e^{\prime}=(0,-1). Then XΣ′X_{\Sigma^{\prime}} is isomorphic to a blow-up of ℙ2\mathbb{P}^{2} at one point. The function ψ\psi extends to a continuous function on NΣ′N_{\Sigma^{\prime}} and it corresponds to a toric model of (XΣ′,𝒪)(X_{\Sigma^{\prime}},\mathcal{O}).

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