ScalingStacks

Proposition 8.26 . [02YM]

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Proposition 8.26.

In the above setting, one has :

degπ’ͺℙ⁑(E)​(1)⁑(ℙ⁑(E))\displaystyle\deg_{{\mathcal{O}}_{\mathbb{P}(E)}(1)}(\mathbb{P}(E)) =βˆ‘i0,…,irβˆˆβ„•i0+β‹―+ir=na0i0​…​arir\displaystyle=\sum_{{i_{0},\dots,i_{r}\in{\mathbb{N}}}\atop{i_{0}+\dots+i_{r}=n}}a_{0}^{i_{0}}\dots a_{r}^{i_{r}}
hπ’ͺℙ⁑(E)​(1)¯⁑(ℙ⁑(E))\displaystyle\operatorname{h}_{{\overline{{\mathcal{O}}_{\mathbb{P}(E)}(1)}}}(\mathbb{P}(E)) =(βˆ‘i0,…,irβˆˆβ„•i0+β‹―+ir=n+1a0i0​…​arir)​hπ’ͺβ„™n​(1)¯⁑(β„™n)\displaystyle=\left(\sum_{{i_{0},\dots,i_{r}\in{\mathbb{N}}}\atop{i_{0}+\dots+i_{r}=n+1}}a_{0}^{i_{0}}\dots a_{r}^{i_{r}}\right)\operatorname{h}_{{\overline{{\mathcal{O}}_{\mathbb{P}^{n}}(1)}}}(\mathbb{P}^{n})
+βˆ‘i0,…,irβˆˆβ„•i0+β‹―+ir=na0i0…arirAn,r(i0,…,ir),\displaystyle\kern 99.58464pt+\sum_{{i_{0},\dots,i_{r}\in{\mathbb{N}}}\atop{i_{0}+\dots+i_{r}=n}}a_{0}^{i_{0}}\dots a_{r}^{i_{r}}A_{n,r}(i_{0},\dots,i_{r}),

where An,r​(i0,…,ir)=βˆ‘m=0r(im+1)β€‹βˆ‘j=im+2n+r+112​jA_{n,r}(i_{0},\dots,i_{r})=\sum_{m=0}^{r}(i_{m}+1)\sum_{j=i_{m}+2}^{n+r+1}\frac{1}{2j}. In particular, the height of ℙ⁑(E)\mathbb{P}(E) is a positive rational number.

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