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 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 of over an open set , the continuous function is plurisubharmonic on . In turn, this means that for any morphism , where is the closed unit disk in ,
Assume that 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 which then is a positive measure on . There are two ways to define this current. The first one works locally and proceeds by induction : if , for a local non-vanishing section of , one defines a sequence of closed positive currents by the formulae , ,…, and is defined to be . What makes this construction work is the fact that at each step, is a well defined current (product of a continuous function and of a positive current), and one has to prove that 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 is a line bundle with a continuous semi-positive metric , then there exists a sequence of smooth semi-positive metrics on the line bundle which converges uniformly to the initial metric : for any local section , converges uniformly to on compact sets. The curvature current is then the limit of the positive currents , and the measure is the limit of the measures . (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 on . This metric is defined as follows : if is an open set, and is a section of on corresponding to an analytic function on which is homogeneous of degree , then for any , one has
The associated measure on is as follows, cf. [58, 43] : the subset of all points such that for all is naturally identified with the polycircle (map to ) ; take the normalized Haar measure of this compact group and push it onto .