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Integrating Green functions [01J2]

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Integrating Green functions

The definition of the convergence of a sequence of measures is convergence of all integrals against a given continuous compactly supported function. In applications, however, it can be desirable to integrate against more general functions. The inductive formula (1.2.1) for the local height pairing in the complex case, is such an example, as is the interpretation of Mahler measures of polynomials as (the archimedean component of) heights. However, its analogue (Equation 1.3.1) a priori holds only when log⁡‖s0‖−1\log\left\|{s_{0}}\right\|^{-1} is continuous, that is when the section s0s_{0} has no zeroes nor poles.

The fact that it still holds in the archimedean case is a theorem of Maillot [43] building on the theory of Bedford–Taylor. We proved in [19, Th. 4.1] that this relation holds in the ultrametric case too. The proof (valid both in the ultrametric and archimedean cases) works by induction, and ultimately relies on an approximation lemma according to which any semi-positive Green function gg for a divisor DD is an increasing limit of smooth functions (gn)(g_{n}) such that, for any nn, g−gng-g_{n} is a semi-positive Green function for DD. In fact, it suffices to pose gn=min⁡(g,n​log⁡|π|−1)g_{n}=\min(g,n\log\left|{\pi}\right|^{-1}) ; then, g−gn=max⁡(0,g−n​log⁡|π|−1)g-g_{n}=\max(0,g-n\log\left|{\pi}\right|^{-1}) is the maximum of two semi-positive Green functions, hence is semi-positive. (In the archimedean case, one needs to further regularize gng_{n} ; see [19] for details.)

The symmetry of the local height pairing then implies the following analogue of the Poincaré–Lelong formula. When L¯\overline{L} is the trivial line bundle, with the metric defined by an admissible function φ\varphi, the factor c1​(L¯)c_{1}(\overline{L}) will be written ddc⁡φ\mathop{\mathrm{d}\mathrm{d}^{c}}\varphi, by analogy to the complex case.

Proposition 1.3.2.

Let φ\varphi be a smooth function on X\mathrm{X} and let L¯1,…,L¯k\overline{L}_{1},\dots,\overline{L}_{k} be admissible metrized line bundles ; let Z\mathrm{Z} be a kk-dimensional subvariety of X\mathrm{X} and let ss be an invertible meromorphic sections of L¯1\overline{L}_{1}. Then,

∫Xφ​c1​(L¯1)​…​c1​(L¯k)​δZ=∫Xφ​c1​(L¯2)​…​c1​(L¯k)​δdiv⁡(s|Z)+∫Xlog⁡‖s‖−1​ddc⁡φ​c1​(L¯2)​…​c1​(L¯k)​δZ.\int_{\mathrm{X}}\varphi c_{1}(\overline{L}_{1})\dots c_{1}(\overline{L}_{k})\delta_{\mathrm{Z}}\\ =\int_{\mathrm{X}}\varphi c_{1}(\overline{L}_{2})\dots c_{1}(\overline{L}_{k})\delta_{\operatorname{div}(s|\mathrm{Z})}+\int_{X}\log\left\|{s}\right\|^{-1}\mathop{\mathrm{d}\mathrm{d}^{c}}\varphi c_{1}(\overline{L}_{2})\dots c_{1}(\overline{L}_{k})\delta_{\mathrm{Z}}.
Démonstration.

Let L¯0\overline{L}_{0} be the trivial line bundle with global section s0=1s_{0}=1 and metric defined by φ=log⁡‖s0‖−1\varphi=\log\left\|{s_{0}}\right\|^{-1}. Let s1=ss_{1}=s and, for 2≤j≤k2\leq j\leq k, let sjs_{j} be an invertible meromorphic section of LjL_{j}. Since div⁡(s0|Z)=0\operatorname{div}(s_{0}|_{\mathrm{Z}})=0,

(div^⁡(s0)​…​div^⁡(sk)|Z)=∫Xφ​c1​(L¯1)​…​c1​(L¯k)​δZ(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|\mathrm{Z})=\int_{\mathrm{X}}\varphi c_{1}(\overline{L}_{1})\dots c_{1}(\overline{L}_{k})\delta_{\mathrm{Z}}

and

(div^⁡(s0)​div^⁡(s2)​…​div^⁡(sk)|div⁡(s|Z))=∫Xφ​c1​(L¯2)​…​c1​(L¯k)​δdiv⁡(s|Z)(\mathop{\widehat{\operatorname{div}}}(s_{0})\mathop{\widehat{\operatorname{div}}}(s_{2})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|\operatorname{div}(s|_{\mathrm{Z}}))=\int_{\mathrm{X}}\varphi c_{1}(\overline{L}_{2})\dots c_{1}(\overline{L}_{k})\delta_{\operatorname{div}(s|\mathrm{Z})}

One the other hand, the symmetry of the local height pairing implies that

(div^⁡(s0)​…​div^⁡(sk)|Z)\displaystyle(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|\mathrm{Z}) =(div^⁡(s1)​div^⁡(s0)​…​div^⁡(sk)|Z)\displaystyle=(\mathop{\widehat{\operatorname{div}}}(s_{1})\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|\mathrm{Z})
=(div^⁡(s0)​div^⁡(s2)​…​div^⁡(sk)|div⁡(s|Z))\displaystyle=(\mathop{\widehat{\operatorname{div}}}(s_{0})\mathop{\widehat{\operatorname{div}}}(s_{2})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|\operatorname{div}(s|_{\mathrm{Z}}))
+∫Xlog‖s‖−1c1(L¯0)c1(L¯2)…c1(L¯k)δZ.\displaystyle\hskip 85.35826pt{}+\int_{\mathrm{X}}\log\left\|{s}\right\|^{-1}c_{1}(\overline{L}_{0})c_{1}(\overline{L}_{2})\dots c_{1}(\overline{L}_{k})\delta_{\mathrm{Z}}.

Combining these equations, we obtain the claim. ∎

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