ScalingStacks

Heights [01K2]

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Heights

Consider line bundles L¯0,…,L¯n\overline{L}_{0},\dots,\overline{L}_{n} with admissible adelic metrics. Let ZZ be a subvariety of XX of dimension kk and s0,…,sks_{0},\dots,s_{k} invertible meromorphic sections of L0,…,LkL_{0},\dots,L_{k} whose divisors hace no common intersection point on ZZ. For any v∈M⁡(F)v\in M(F), we have recalled in Sections 1.2, 1.2 and 1.3 the definitions of the local height pairing

(div^⁡(s0)​…​div^⁡(sk)|Z)v(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|Z)_{v}

where the index vv indicates the corresponding place of FF. The global height is the sum, over all v∈M⁡(F)v\in M(F), of these local heights :

(div^⁡(s0)​…​div^⁡(sk)|Z)=∑v∈M⁡(F)(div^⁡(s0)​…​div^⁡(sk)|Z)v.(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|Z)=\sum_{v\in M(F)}(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|Z)_{v}.

It inherits from the local heights their multilinear symmetric character.

Let us replace sks_{k} by another invertible meromorphic section f​skfs_{k}. Then,

(div^⁡(s0)​…​div^⁡(f​sk)|Z)\displaystyle(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(fs_{k})|Z) =∑v∈M⁡(F)(div^⁡(s0)​…​div^⁡(f​sk)|Z)v\displaystyle=\sum_{v\in M(F)}(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(fs_{k})|Z)_{v}
=∑v∈M⁡(F)(div^⁡(s0)​…​div^⁡(sk)|Z)v\displaystyle=\sum_{v\in M(F)}(\mathop{\widehat{\operatorname{div}}}(s_{0})\dots\mathop{\widehat{\operatorname{div}}}(s_{k})|Z)_{v}
+∑v∈M⁡(F)∫Xvlog|f|−1c1(L¯0)…c1(L¯k−1)δZv.\displaystyle\qquad+\sum_{v\in M(F)}\int_{X_{v}}\log\left|{f}\right|^{-1}c_{1}(\overline{L}_{0})\dots c_{1}(\overline{L}_{k-1})\delta_{\mathrm{Z}_{v}}.

In particular, if ZZ is a point z∈X⁡(F)z\in X(F), then δZv=δz\delta_{\mathrm{Z}_{v}}=\delta_{z} is the Dirac mass at zz and

(div^⁡(s0)|Z)=∑v∈M⁡(F)log⁡‖s0‖v−1​(z).(\mathop{\widehat{\operatorname{div}}}(s_{0})|Z)=\sum_{v\in M(F)}\log\left\|{s_{0}}\right\|^{-1}_{v}(z).

Let us observe that it is independent on the choice of the chosen meromorphic section s0s_{0}, provided it is regular at zz. Any other section has the form f​s0fs_{0}, for some invertible meromorphic function ff on XX. Then,

(div^⁡(f​s0)|Z)\displaystyle(\mathop{\widehat{\operatorname{div}}}(fs_{0})|Z) =∑v∈M⁡(F)log⁡‖f​s0‖v−1​(z)\displaystyle=\sum_{v\in M(F)}\log\left\|{fs_{0}}\right\|^{-1}_{v}(z)
=∑v∈M⁡(F)log⁡‖s0‖v−1​(z)+∑v∈M⁡(F)log⁡|f|v−1​(z)\displaystyle=\sum_{v\in M(F)}\log\left\|{s_{0}}\right\|^{-1}_{v}(z)+\sum_{v\in M(F)}\log\left|{f}\right|_{v}^{-1}(z)
=(div^⁡(f​s0)|Z)\displaystyle=(\mathop{\widehat{\operatorname{div}}}(fs_{0})|Z)

since, by the product formula, the second term vanishes.

By induction on the dimension of ZZ, and using the commutativity of the local height pairings, it follows that the global height only depends on the metrized line bundles, and not on the actual chosen sections s0,…,sks_{0},\dots,s_{k}. We denote it by

(c^1​(L¯0)​…​c^1​(L¯k)|Z).({\widehat{c}}_{1}(\overline{L}_{0})\dots{\widehat{c}}_{1}(\overline{L}_{k})|Z).

Again, it is multilinear symmetric in the metrized line bundles L¯0,…,L¯k\overline{L}_{0},\dots,\overline{L}_{k}. By the same argument, it only depends on their isomorphism classes in Pic¯ad​(X)\overline{\operatorname{Pic}}_{\text{ad}}(X).

It satisfies a projection formula : for any morphism f:Y→Xf\colon Y\rightarrow X and any kk-dimensional subvariety ZZ of YY,

(c^1​(f∗​L¯0)​…​c^1​(f∗​L¯k)|Z)=(c^1​(L¯0)​…​c^1​(L¯k)|f∗​(Z)CLOSE,({\widehat{c}}_{1}(f^{*}\overline{L}_{0})\dots{\widehat{c}}_{1}(f^{*}\overline{L}_{k})|Z)=({\widehat{c}}_{1}(\overline{L}_{0})\dots{\widehat{c}}_{1}(\overline{L}_{k})|f_{*}(Z),

where the cycle f∗​(Z)f_{*}(Z) is defined as deg⁡(Z/f⁡(Z))​f​(Z)\deg(Z/f(Z))f(Z) if ZZ and f⁡(Z)f(Z) have the same dimension, so that f:Z→f⁡(Z)f\colon Z\rightarrow f(Z) is generically finite, of some degree deg⁡(Z/f⁡(Z))\deg(Z/f(Z)). If ZZ and f⁡(Z)f(Z) don’t have the same dimension, one sets f∗​(Z)=0f_{*}(Z)=0.

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