ScalingStacks

Continuous metrics [01IS]

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Continuous metrics

Let X\mathrm{X} be a KK-analytic space in the sense of Berkovich [11]. For simplicity, we will assume that X\mathrm{X} is the analytic space associated to a proper scheme over KK. In that context, the general definition of continuous metrized line bundles given above makes sense.

Let us detail the example of the line bundle 𝒪⁡(1)\mathscr{O}(1) on the projective space PKn\mathrm{P}^{n}_{K}. A point x∈PKnx\in\mathrm{P}^{n}_{K} possesses a complete residue field ℋ⁡(x)\mathscr{H}(x) which is a complete extension of KK and homogeneous coordinates [x0:…:xn][x_{0}:\dots:x_{n}] in the field ℋ⁡(x)\mathscr{H}(x). As in complex geometry, the projective space PKn\mathrm{P}^{n}_{K} is obtained by glueing n+1n+1 copies U0,…,Un\mathrm{U}_{0},\dots,\mathrm{U}_{n} of the affine space AKn\mathrm{A}^{n}_{K}, where Ui\mathrm{U}_{i} corresponds to those points xx such that xi≠0x_{i}\neq 0. Recall also that AKn\mathrm{A}^{n}_{K} is the space of multiplicative semi-norms on the KK-algebra K⁡[T1,…,Tn]K[T_{1},\dots,T_{n}] which induce the given absolute value on KK, together with the coarsest topology such that for any semi-norm x∈AKnx\in\mathrm{A}^{n}_{K}, the map K⁡[T1,…,Tn]→𝐑K[T_{1},\dots,T_{n}]\rightarrow{\mathbf{R}} defined by f↦x⁡(f)f\mapsto x(f) is continuous. The kernel of a semi-norm xx is a prime ideal 𝔭x\mathfrak{p}_{x} of K⁡[T1,…,Tn]K[T_{1},\dots,T_{n}] and xx induces a norm on the quotient ring K⁡[T1,…,Tn]/𝔭xK[T_{1},\dots,T_{n}]/\mathfrak{p}_{x}, hence on its field of fractions K⁡(x)K(x). The completion of K⁡(x)K(x) with respect to this norm is denoted ℋ⁡(x)\mathscr{H}(x) and is called the complete residue field of xx. The images in ℋ⁡(x)\mathscr{H}(x) of the intederminates TiT_{i} are denoted Ti​(x)T_{i}(x), more generally, the image in ℋ⁡(x)\mathscr{H}(x) of any polynomial f∈K⁡[T1,…,Tn]f\in K[T_{1},\dots,T_{n}] is denoted f⁡(x)f(x) ; one has x⁡(f)=|f⁡(x)|x(f)=\left|{f(x)}\right|.

Let ff be a rational function on PKn\mathrm{P}^{n}_{K}, that is an element of ∈K⁡(T1,…,Tn)\in K(T_{1},\dots,T_{n}). It defines an actual function on the open set U\mathrm{U} of 𝐏Kn{\mathbf{P}}^{n}_{K} where its denominator does not vanish ; its value at a point x∈Ux\in U is an element of ℋ⁡(x)\mathscr{H}(x). More generally, Berkovich defines an analytic function on an open set U\mathrm{U} of PKn\mathrm{P}^{n}_{K} as a function ff on UU such that f⁡(x)∈ℋ⁡(x)f(x)\in\mathscr{H}(x) for any x∈Ux\in\mathrm{U}, and such that any point x∈Ux\in\mathrm{U} possesses a neighbourhood V⊂U\mathrm{V}\subset\mathrm{U} such that f|Vf|_{\mathrm{V}} is a uniform limit of rational functions without poles on V\mathrm{V}.

The line bundle 𝒪⁡(1)\mathscr{O}(1) can also be defined in a similar way to the classical case ; by a similar GAGA theorem, its global sections are exactly the same as in algebraic geometry and are described by homogeneous polynomials of degree 11 with coefficients in KK. If PP is such a polynomial and sPs_{P} the corresponding section, then

‖sP‖​(x)=|P⁡(x0,…,xn)|max⁡(|x0|,…,|xn|)\left\|{s_{P}}\right\|(x)=\frac{\left|{P(x_{0},\dots,x_{n})}\right|}{\max(\left|{x_{0}}\right|,\dots,\left|{x_{n}}\right|)}

where [x0:…:xn][x_{0}:\dots:x_{n}] is a system of homogeneous coordinates in ℋ⁡(x)\mathscr{H}(x) for the point xx. The function ‖sP‖\left\|{s_{P}}\right\| is continuous on PKn\mathrm{P}^{n}_{K}, by the very definition of the topology on PKn\mathrm{P}^{n}_{K}. Using the fact that 𝒪⁡(1)\mathscr{O}(1) is generated by its global sections, one deduces the existence of a continuous metric on 𝒪⁡(1)\mathscr{O}(1) satisfying the previous formula.

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