ScalingStacks

Measures (admissible metrics) [01J1]

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Measures (admissible metrics)

Let us now return to semi-positive metrized line bundles L¯0,…,L¯k−1\overline{L}_{0},\dots,\overline{L}_{k-1}, approximated by smooth semi-positive metrized line bundles L¯j(m)\overline{L}_{j}^{(m)}. I claim that for any kk-dimensional variety Z⊂X\mathrm{Z}\subset\mathrm{X}, the sequence of measures c1​(L¯0(m))​…​c1​(L¯k−1(m))​δZc_{1}(\overline{L}_{0}^{(m)})\dots c_{1}(\overline{L}_{k-1}^{(m)})\delta_{\mathrm{Z}} converges to a measure on X\mathrm{X}.

To prove the claim, we may assume that L¯0,…,L¯k−1\overline{L}_{0},\dots,\overline{L}_{k-1} have sections s0,…,sk−1s_{0},\dots,s_{k-1} without common zeroes on Z\mathrm{Z}. Let also consider a smooth function φ\varphi on X\mathrm{X} ; let L¯k\overline{L}_{k} be the trivial line bundle with the section sk=1s_{k}=1, metrized in such a way that ‖sk‖=e−φ\left\|{s_{k}}\right\|=e^{-\varphi}. Then, one has

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

writing L¯k\overline{L}_{k} has the quotient of two ample metrized line bundles, we deduce from the existence of the local height pairing for admissible metrics that these integrals converge when m→∞m\rightarrow\infty. Consequently, the sequence of measures (c1​(L¯0(m))​…​c1​(L¯k−1(m))​δZ)m(c_{1}(\overline{L}^{(m)}_{0})\dots c_{1}(\overline{L}^{(m)}_{k-1})\delta_{\mathrm{Z}})_{m} converges to a positive linear form on the space of smooth functions. By a theorem of Gubler ([34], Theorem 7.12), which builds on the Stone-Weierstraß theorem and the compactness of the Berkovich space X\mathrm{X}, the space of smooth functions is dense in the space of continuous complex functions on X\mathrm{X}. A positivity argument, analogous to the proof that positive distributions are measures, then implies that our linear form is actually a positive measure which deserves the notation

c1​(L¯0)​…​c1​(L¯k−1)​δZ.c_{1}(\overline{L}_{0})\dots c_{1}(\overline{L}_{k-1})\delta_{\mathrm{Z}}.

We then extend this definition by linearity to the case of arbitrary admissible line bundles. The total mass of this measure is again the multidegree of Z{\mathrm{Z}} with respect to the line bundles LjL_{j} (for 0≤j≤k−10\leq j\leq k-1).

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