ScalingStacks

Example 4.44 . [02QM]

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Example 4.44.

The projective morphisms associated to 𝕋\mathbb{T}-Cartier divisors generated by global sections can also be made explicit in terms of the lattice points of the associated polytope. Consider a complete toric variety XΞ£X_{\Sigma} of dimension nn equipped with a 𝕋\mathbb{T}-Cartier divisor DΞ¨D_{\Psi} generated by global sections. Let m0,…,mrβˆˆΞ”Ξ¨βˆ©Mm_{0},\dots,m_{r}\in\Delta_{\Psi}\cap M be such that conv⁑(m0,…,mr)=ΔΨ\operatorname{conv}(m_{0},\dots,m_{r})=\Delta_{\Psi}. These vectors determine an H-representation Ξ¨=mini=0,…,r⁑mi\Psi=\min_{i=0,\dots,r}m_{i}. Let H:Nℝ→ℝrH\colon N_{\mathbb{R}}\to\mathbb{R}^{r} be the linear map defined by H⁑(u)=(mi​(u)βˆ’m0​(u))i=1,…,rH(u)=(m_{i}(u)-m_{0}(u))_{i=1,\dots,r}. By Lemma 3.79, Ξ¨=Hβˆ—β€‹Ξ¨Ξ”r+m0\Psi=H^{\ast}\Psi_{\Delta^{r}}+m_{0}.

In ℝr\mathbb{R}^{r} we consider the fan ΣΔr\Sigma_{\Delta^{r}}, whose associated toric variety is β„™r\mathbb{P}^{r}. One easily verifies that, for each ΟƒβˆˆΞ£\sigma\in\Sigma, there is Οƒβ€²βˆˆΞ£Ξ”r\sigma^{\prime}\in\Sigma_{\Delta^{r}} with H⁑(Οƒ)βŠ‚Οƒβ€²H(\sigma)\subset\sigma^{\prime}. Let p=(p0:…:pr)p=(p_{0}:\dots:p_{r}) be an arbitrary rational point of the principal open subset of β„™r\mathbb{P}^{r}. The equivariant morphism Ο†p,H:Xβ†’β„™Kr\varphi_{p,H}\colon X\to\mathbb{P}^{r}_{K} can be written explicitly as (p0Ο‡m0:…:prΟ‡mr)(p_{0}\chi^{m_{0}}:\dots:p_{r}\chi^{m_{r}}). Moreover, DΞ¨=Ο†p,Hβˆ—β€‹DΨΔr+div⁑(Ο‡βˆ’m0)D_{\Psi}=\varphi_{p,H}^{\ast}D_{\Psi_{\Delta^{r}}}+\operatorname{div}(\chi^{-m_{0}}).

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