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Superelliptic curves [01K9]

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Superelliptic curves

The formulas of this section combine to the following : if div⁡(f)=∑nP​P\operatorname{div}(f)=\sum n_{P}P is a divisor of an invertible meromorphic function on XX,

∑nP​h^Θ​([P−D])=∑v∈M⁡(F)∫Xvlog⁡|f⁡(x)|v​d​μv​(x).\sum n_{P}\widehat{h}_{\Theta}([P-D])=\sum_{v\in M(F)}\int_{X_{v}}\log\left|{f(x)}\right|_{v}\,\mathrm{d}\mu_{v}(x).

As pointed out by R. De Jong [22], the case of superelliptic curves is particularly interesting. Indeed, such curves are presented as a ramified μN\mu_{N}-covering x:X→𝐏1x\colon X\rightarrow{\mathbf{P}}^{1} of the projective line, which is totally ramified over the point at infinity, given by an equation yN=a⁡(x)y^{N}=a(x), where aa is a polynomial of degree m>Nm>N, prime to NN. One has g=12​(N−1)​(m−1)g=\frac{1}{2}(N-1)(m-1).

Let us take for the divisor DD the single point OO over the point at infinity. For each point PP in X⁡(F)X(F), x−x⁡(P)x-x(P) is a rational function on XX which has a single pole of order NN at infinity, and which vanishes along the fiber x−1​(x​(P))x^{-1}(x(P)) of xx. The group of automorphisms of XX 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

h^Θ​(P−O)=1N​∑v∈M⁡(F)∫Xvlog⁡|x−x⁡(P)|v​μv\widehat{h}_{\Theta}(P-O)=\frac{1}{N}\sum_{v\in M(F)}\int_{X_{v}}\log\left|{x-x(P)}\right|_{v}\mu_{v}

of [22]. The elliptic Mahler measure, defined by [27, 26] as a Shnirelman integral is therefore a natural integral when viewed on Berkovich spaces.

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