3.2. Mahler measures and heights of divisors [01K5]
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3.2. Mahler measures and heights of divisors
In this section, we assume that is a projective geometricall integral smooth curve of positive genus over . For any place , let be the corresponding analytic curve.
Let be an invertible meromorphic function on . Let us view it as an invertible meromorphic section of the trivial metrized line bundle . Let be any line bundle on with an admissible adelic metric. Then,
Moreover, according to Theorem 1.3 of [19] (see Section 1.3),
In other words, this furnishes an integral formula for the height (relative to ) of any divisor which is rationally equivalent to :
Néron–Tate heights
We want to apply this formula to a specific metrized line bundle on . The Jacobian of is an Abelian variety of dimension . We also choose a divisor of degree on and correspondingly fix an embedding of into . (For this, we may need to enlarge the ground field .) Finally, we let be the theta divisor of , defined as the image of by the map .
As described above, the line bundle admits a canonical metrization ; this induces a metrization on its inverse image on . The metrized line bundle gives rise to the (theta) Néron–Tate height on . Consequently, decomposing , we obtain
where is the fixed divisor of degree on .
Canonical measures
Since has degree , the measure on has total mass ; let us define a measure of total mass on by
When is archimedean, the measure is the Arakelov measure on the Riemann surface . Let us recall its definition. Consider an orthonormal basis of , i.e., a basis satisfying the relations
Then,
Let us now assume that is ultrametric. By a theorem of Heinz [40], the metric on the line bundle coincides with the canonical metric defined by Zhang [57] using the reduction graph of the minimal regular model of . This allows in particular to compute the measure : the reader will find in [20, 57, 5]) a quite explicit formula for , involving the physical interpretation of the graph as an electric network.
Superelliptic curves
The formulas of this section combine to the following : if is a divisor of an invertible meromorphic function on ,
As pointed out by R. De Jong [22], the case of superelliptic curves is particularly interesting. Indeed, such curves are presented as a ramified -covering of the projective line, which is totally ramified over the point at infinity, given by an equation , where is a polynomial of degree , prime to . One has .
Let us take for the divisor the single point over the point at infinity. For each point in , is a rational function on which has a single pole of order at infinity, and which vanishes along the fiber of . The group of automorphisms of acts transitively on this fiber, and respects the metrics, so that all of these points have the same Néron-Tate height. This implies the following formula
of [22]. The elliptic Mahler measure, defined by [27, 26] as a Shnirelman integral is therefore a natural integral when viewed on Berkovich spaces.