ScalingStacks

Example : projective space [01IV]

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Example : projective space

Let us consider the smooth metric on 𝒪⁡(1)\mathscr{O}(1) associated to the model (𝔛,𝒪⁡(1)​,1)(\mathfrak{X},\mathscr{O}(1),1) of (PKn,𝒪⁡(1))(\mathrm{P}^{n}_{K},\mathscr{O}(1)). Let 𝔘i\mathfrak{U}_{i} be the formal open subset of PKn\mathrm{P}^{n}_{K} defined by the non-vanishing of the homogeneous coordinate xix_{i}. Over, 𝔘i\mathfrak{U}_{i}, 𝒪⁡(1)\mathscr{O}(1) has a global non-vanishing section, namely the one associated to the homogeneous polynomial XiX_{i}. The generic fiber UiU_{i} of 𝔘i\mathfrak{U}_{i} in the sense of algebraic geometry is an affine space, with coordinates zj=xj/xiz_{j}=x_{j}/x_{i}, for 0≤j≤n0\leq j\leq n, and j≠ij\neq i. However, its generic fiber Ui\mathrm{U}_{i} in the sense of rigid geometry is the nn-dimensional polydisk in this affine space defined by the inequalities |zj|≤1\left|{z_{j}}\right|\leq 1. We thus observe that for any x∈(𝔘i)Kx\in(\mathfrak{U}_{i})_{K},

‖Xi‖​(x)=1=1max⁡(|z0|,…,|zi−1|​,1,|zi+1|,…,|zn|)=|xi|max⁡(|x0|,…,|xi|)=‖Xi‖W​(x).\left\|{X_{i}}\right\|(x)=1=\frac{1}{\max(\left|{z_{0}}\right|,\dots,\left|{z_{i-1}}\right|,1,\left|{z_{i+1}}\right|,\dots,\left|{z_{n}}\right|)}=\frac{\left|{x_{i}}\right|}{\max(\left|{x_{0}}\right|,\dots,\left|{x_{i}}\right|)}=\left\|{X_{i}}\right\|_{\mathrm{W}}(x).

In other words, the Weil metric on 𝒪⁡(1)\mathscr{O}(1) is a smooth metric.

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