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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 XX is a projective geometricall integral smooth curve of positive genus gg over FF. For any place v∈M⁡(F)v\in M(F), let XvX_{v} be the corresponding analytic curve.

Let ff be an invertible meromorphic function on XX. Let us view it as an invertible meromorphic section of the trivial metrized line bundle 𝒪X¯\overline{\mathscr{O}_{X}}. Let L¯\overline{L} be any line bundle on XX with an admissible adelic metric. Then,

(c^1​(L¯)​c^1​(𝒪X¯)|X)=0.({\widehat{c}}_{1}(\overline{L}){\widehat{c}}_{1}(\overline{\mathscr{O}_{X}})|X)=0.

Moreover, according to Theorem 1.3 of [19] (see Section 1.3),

(c^1​(L¯)​c^1​(𝒪X¯)|X)=(c^1​(L¯)|div⁡(f))+∑v∈M⁡(F)∫Xvlog⁡|f|v−1​c1​(L¯)v.({\widehat{c}}_{1}(\overline{L}){\widehat{c}}_{1}(\overline{\mathscr{O}_{X}})|X)=({\widehat{c}}_{1}(\overline{L})|\operatorname{div}(f))+\sum_{v\in M(F)}\int_{X_{v}}\log\left|{f}\right|_{v}^{-1}c_{1}(\overline{L})_{v}.

In other words, this furnishes an integral formula for the height (relative to L¯\overline{L}) of any divisor which is rationally equivalent to 00 :

(c^1​(L¯)|div⁡(f))=∑v∈M⁡(F)∫Xvlog⁡|f|v​c1​(L¯)v.({\widehat{c}}_{1}(\overline{L})|\operatorname{div}(f))=\sum_{v\in M(F)}\int_{X_{v}}\log\left|{f}\right|_{v}c_{1}(\overline{L})_{v}.

Néron–Tate heights

We want to apply this formula to a specific metrized line bundle on XX. The Jacobian JJ of XX is an Abelian variety of dimension gg. We also choose a divisor DD of degree 11 on XX and correspondingly fix an embedding ι\iota of XX into JJ. (For this, we may need to enlarge the ground field FF.) Finally, we let Θ\Theta be the theta divisor of JJ, defined as the image of Xg−1X^{g-1} by the map (x1,…,xg−1)↦∑j=1g−1ι⁡(xj)(x_{1},\dots,x_{g-1})\mapsto\sum_{j=1}^{g-1}\iota(x_{j}).

As described above, the line bundle 𝒪J​(Θ)\mathscr{O}_{J}(\Theta) admits a canonical metrization ; this induces a metrization on its inverse image L=ι∗​𝒪J​(Θ)L=\iota^{*}\mathscr{O}_{J}(\Theta) on XX. The metrized line bundle 𝒪J​(Θ)¯\overline{\mathscr{O}_{J}(\Theta)} gives rise to the (theta) Néron–Tate height on JJ. Consequently, decomposing div⁡(f)=∑nP​P\operatorname{div}(f)=\sum n_{P}P, we obtain

(c^1(L¯)|div(f))=∑nP[F(P):F]h^Θ(ι(P))=∑nP[F(P):F]h^Θ([P−D]),({\widehat{c}}_{1}(\overline{L})|\operatorname{div}(f))=\sum n_{P}[F(P):F]\widehat{h}_{\Theta}(\iota(P))=\sum n_{P}[F(P):F]\widehat{h}_{\Theta}([P-D]),

where DD is the fixed divisor of degree 11 on XX.

Canonical measures

Since LL has degree gg, the measure c1​(L¯)vc_{1}(\overline{L})_{v} on XvX_{v} has total mass gg ; let us define a measure of total mass 11 on XvX_{v} by

μv=1g​c1​(L¯)v.\mu_{v}=\frac{1}{g}c_{1}(\overline{L})_{v}.

When vv is archimedean, the measure μv\mu_{v} is the Arakelov measure on the Riemann surface Xv​(𝐂)X_{v}({\mathbf{C}}). Let us recall its definition. Consider an orthonormal basis (ω1,…,ωg)(\omega_{1},\dots,\omega_{g}) of H0​(X,ΩX1)H^{0}(X,\Omega^{1}_{X}), i.e., a basis satisfying the relations

∫Xv​(𝐂)ωj∧ωk¯=δj,k={1if j=k ;0otherwise.\int_{X_{v}({\mathbf{C}})}\omega_{j}\wedge\overline{\omega_{k}}=\delta_{j,k}=\begin{cases}1&\text{if $j=k$ ;}\\ 0&\text{otherwise.}\end{cases}

Then,

μv=1g​∑j=1gωj∧ωj¯.\mu_{v}=\frac{1}{g}\sum_{j=1}^{g}\omega_{j}\wedge\overline{\omega_{j}}.

Let us now assume that vv is ultrametric. By a theorem of Heinz [40], the metric on the line bundle L¯\overline{L} coincides with the canonical metric defined by Zhang [57] using the reduction graph of the minimal regular model of XX. This allows in particular to compute the measure μv\mu_{v} : the reader will find in [20, 57, 5]) a quite explicit formula for μv\mu_{v}, involving the physical interpretation of the graph as an electric network.

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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