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Semi-positive continuous metrics [01IN]

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Semi-positive continuous metrics

More generally, both the curvature and the Poincaré–Lelong equation make sense for metrized line bundles with arbitrary (continuous) metrics, except that c1​(L¯)c_{1}(\overline{L}) has to be considered as a current. The notion a semi-positivity can even be extended to this more general case, because it can be tested by duality : a current is positive if its evaluation on any nonnegative differential form is nonnegative. Alternatively, semi-positive (continuous) metrized line bundles are characterized by the fact that for any local frame ss of L¯\overline{L} over an open set U\mathrm{U}, the continuous function log⁡‖s‖−1\log\left\|{s}\right\|^{-1} is plurisubharmonic on U\mathrm{U}. In turn, this means that for any morphism φ:D¯→U\varphi\colon\overline{D}\rightarrow\mathrm{U}, where D¯=D¯​(0,1)\overline{D}=\overline{D}(0,1) is the closed unit disk in 𝐂{\mathbf{C}},

log⁡‖s‖−1​(φ⁡(0))≤12​π​∫02​πlog⁡‖s‖−1​(φ⁡(ei​θ))​𝑑θ.\log\left\|{s}\right\|^{-1}(\varphi(0))\leq\frac{1}{2\pi}\int_{0}^{2\pi}\log\left\|{s}\right\|^{-1}(\varphi(e^{i\theta}))\mathrm{d}\theta.

Assume that L¯\overline{L} is semi-positive. Although products of currents are not defined in general (not more than products of distributions), the theory of Bedford–Taylor [8, 7] and Demailly [24, 25] defines a current c1​(L¯)nc_{1}(\overline{L})^{n} which then is a positive measure on X\mathrm{X}. There are two ways to define this current. The first one works locally and proceeds by induction : if u=log⁡‖s‖−1u=\log\left\|{s}\right\|^{-1}, for a local non-vanishing section ss of LL, one defines a sequence (Tk)(T_{k}) of closed positive currents by the formulae T0=1T_{0}=1, T1=ddc⁡uT_{1}=\mathop{\mathrm{d}\mathrm{d}^{c}}u,…, Tk+1=ddc⁡(u​Tk)T_{k+1}=\mathop{\mathrm{d}\mathrm{d}^{c}}(uT_{k}) and c1​(L¯)n=ddc⁡(u)nc_{1}(\overline{L})^{n}=\mathop{\mathrm{d}\mathrm{d}^{c}}(u)^{n} is defined to be TnT_{n}. What makes this construction work is the fact that at each step, u​TkuT_{k} is a well defined current (product of a continuous function and of a positive current), and one has to prove that Tk+1T_{k+1} is again a closed positive current. The other way, which shall be the one akin to a generalization in the ultrametric framework, consists in observing that if LL is a line bundle with a continuous semi-positive metric ‖⋅‖\left\|{\cdot}\right\|, then there exists a sequence of smooth semi-positive metrics ‖⋅‖k\left\|{\cdot}\right\|_{k} on the line bundle LL which converges uniformly to the initial metric : for any local section ss, ‖s‖k\left\|{s}\right\|_{k} converges uniformly to ‖s‖\left\|{s}\right\| on compact sets. The curvature current c1​(L¯)c_{1}(\overline{L}) is then the limit of the positive currents c1​(L¯k)c_{1}(\overline{L}_{k}), and the measure c1​(L¯)nc_{1}(\overline{L})^{n} is the limit of the measures c1​(L¯k)nc_{1}(\overline{L}_{k})^{n}. (We refer to [43] for the global statement ; to construct the currents, one can in fact work locally in which case a simple convolution argument establishes the claim.)

An important example of semi-positive metric which is continuous, but not smoth, is furnished by the Weil metric on the line bundle 𝒪⁡(1)\mathscr{O}(1) on 𝐏n​(𝐂){\mathbf{P}}^{n}({\mathbf{C}}). This metric is defined as follows : if U⊂𝐏n​(𝐂)\mathrm{U}\subset{\mathbf{P}}^{n}({\mathbf{C}}) is an open set, and ss is a section of 𝒪⁡(1)\mathscr{O}(1) on UU corresponding to an analytic function FsF_{s} on π−1​(U)⊂𝐂∗n+1\pi^{-1}(\mathrm{U})\subset{\mathbf{C}}^{n+1}_{*} which is homogeneous of degree 11, then for any (x0,…,xn)∈π−1​(U)(x_{0},\dots,x_{n})\in\pi^{-1}(\mathrm{U}), one has

‖s‖W=|Fs​(x0,…,xn)|max⁡(|x0|,…,|xn|CLOSE.\left\|{s}\right\|_{\mathrm{W}}=\frac{\left|{F_{s}(x_{0},\dots,x_{n})}\right|}{\max(\left|{x_{0}}\right|,\dots,\left|{x_{n}}\right|}.

The associated measure c1​(𝒪⁡(1)¯W)nc_{1}(\overline{\mathscr{O}(1)}_{\mathrm{W}})^{n} on 𝐏n​(𝐂){\mathbf{P}}^{n}({\mathbf{C}}) is as follows, cf. [58, 43] : the subset of all points [x0:…:xn]∈𝐏n(𝐂)[x_{0}:\dots:x_{n}]\in{\mathbf{P}}^{n}({\mathbf{C}}) such that |xj|=|xk|\left|{x_{j}}\right|=\left|{x_{k}}\right| for all j,kj,k is naturally identified with the polycircle 𝐒1n\mathbf{S}_{1}^{n} (map [x0:…:xn][x_{0}:\dots:x_{n}] to (x1/x0,…,xn/x0)(x_{1}/x_{0},\dots,x_{n}/x_{0})) ; take the normalized Haar measure of this compact group and push it onto 𝐏n​(𝐂){\mathbf{P}}^{n}({\mathbf{C}}).

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