# Néron–Tate height

(Redirected from Canonical height)

In number theory, the Néron–Tate height (or canonical height) is a quadratic form on the Mordell-Weil group of rational points of an abelian variety defined over a global field. It is named after André Néron and John Tate.

## Definition and properties

Néron defined the Néron–Tate height as a sum of local heights.[1] Although the global Néron–Tate height is quadratic, the constituent local heights are not quite quadratic. Tate (unpublished) defined it globally by observing that the logarithmic height ${\displaystyle h_{L}}$ associated to a symmetric invertible sheaf ${\displaystyle L}$ on an abelian variety ${\displaystyle A}$ is “almost quadratic,” and used this to show that the limit

${\displaystyle {\hat {h}}_{L}(P)=\lim _{N\rightarrow \infty }{\frac {h_{L}(NP)}{N^{2}}}}$

exists, defines a quadratic form on the Mordell-Weil group of rational points, and satisfies

${\displaystyle {\hat {h}}_{L}(P)=h_{L}(P)+O(1),}$

where the implied ${\displaystyle O(1)}$ constant is independent of ${\displaystyle P}$.[2] If ${\displaystyle L}$ is anti-symmetric, that is ${\displaystyle [-1]^{*}L=L}$, then the analogous limit

${\displaystyle {\hat {h}}_{L}(P)=\lim _{N\rightarrow \infty }{\frac {h_{L}(NP)}{N}}}$

converges and satisfies ${\displaystyle {\hat {h}}_{L}(P)=h_{L}(P)+O(1)}$, but in this case ${\displaystyle {\hat {h}}_{L}}$ is a linear function on the Mordell-Weil group. For general invertible sheaves, one writes ${\displaystyle L^{\otimes 2}=(L\otimes [-1]^{*}L)\otimes (L\otimes [-1]^{*}L^{-1})}$ as a product of a symmetric sheaf and an anti-symmetric sheaf, and then

${\displaystyle {\hat {h}}_{L}(P)={\frac {1}{2}}{\hat {h}}_{L\otimes [-1]^{*}L}(P)+{\frac {1}{2}}{\hat {h}}_{L\otimes [-1]^{*}L^{-1}}(P)}$

is the unique quadratic function satisfying

${\displaystyle {\hat {h}}_{L}(P)=h_{L}(P)+O(1)\quad {\mbox{and}}\quad {\hat {h}}_{L}(0)=0.}$

The Néron–Tate height depends on the choice of an invertible sheaf on the abelian variety, although the associated bilinear form depends only on the image of ${\displaystyle L}$ in the Néron–Severi group of ${\displaystyle A}$. If the abelian variety ${\displaystyle A}$ is defined over a number field K and the invertible sheaf is symmetric and ample, then the Néron–Tate height is positive definite in the sense that it vanishes only on torsion elements of the Mordell-Weil group ${\displaystyle A(K)}$. More generally, ${\displaystyle {\hat {h}}_{L}}$ induces a positive definite quadratic form on the real vector space ${\displaystyle A(K)\otimes \mathbb {R} }$.

On an elliptic curve, the Néron-Severi group is of rank one and has a unique ample generator, so this generator is often used to define the Néron–Tate height, which is denoted ${\displaystyle {\hat {h}}}$ without reference to a particular line bundle. (However, the height that naturally appears in the statement of the Birch–Swinnerton-Dyer conjecture is twice this height.) On abelian varieties of higher dimension, there need not be a particular choice of smallest ample line bundle to be used in defining the Néron–Tate height, and the height used in the statement of the Birch–Swinnerton-Dyer conjecture is the Néron–Tate height associated to the Poincaré line bundle on ${\displaystyle A\times {\hat {A}}}$, the product of ${\displaystyle A}$ with its dual.

## The elliptic and abelian regulators

The bilinear form associated to the canonical height ${\displaystyle {\hat {h}}}$ on an elliptic curve E is

${\displaystyle \langle P,Q\rangle ={\frac {1}{2}}{\bigl (}{\hat {h}}(P+Q)-{\hat {h}}(P)-{\hat {h}}(Q){\bigr )}.}$

The elliptic regulator of E/K is

${\displaystyle \operatorname {Reg} (E/K)=\det {\bigl (}\langle P_{i},P_{j}\rangle {\bigr )}_{1\leq i,j\leq r},}$

where P1,…,Pr is a basis for the Mordell-Weil group E(K) modulo torsion (cf. Gram determinant). The elliptic regulator does not depend on the choice of basis.

More generally, let A/K be an abelian variety, let B ≅ Pic0(A) be the dual abelian variety to A, and let P be the Poincaré line bundle on A × B. Then the abelian regulator of A/K is defined by choosing a basis Q1,…,Qr for the Mordell-Weil group A(K) modulo torsion and a basis η1,…,ηr for the Mordell-Weil group B(K) modulo torsion and setting

${\displaystyle \operatorname {Reg} (A/K)=\det {\bigl (}\langle P_{i},\eta _{j}\rangle _{P}{\bigr )}_{1\leq i,j\leq r}.}$

(The definitions of elliptic and abelian regulator are not entirely consistent, since if A is an elliptic curve, then the latter is 2r times the former.)

The elliptic and abelian regulators appear in the Birch–Swinnerton-Dyer conjecture.

## Lower bounds for the Néron–Tate height

There are two fundamental conjectures that give lower bounds for the Néron–Tate height. In the first, the field K is fixed and the elliptic curve E/K and point P ∈ E(K) vary, while in the second, the elliptic Lehmer conjecture, the curve E/K is fixed while the field of definition of the point P varies.

• (Lang)[3]     ${\displaystyle {\hat {h}}(P)\geq c(K)\log \max {\bigl \{}\operatorname {Norm} _{K/\mathbb {Q} }\operatorname {Disc} (E/K),h(j(E)){\bigr \}}\quad }$ for all ${\displaystyle E/K}$ and all nontorsion ${\displaystyle P\in E(K).}$
• (Lehmer)[4]     ${\displaystyle {\hat {h}}(P)\geq {\frac {c(E/K)}{[K(P):K]}}}$ for all nontorsion ${\displaystyle P\in E({\bar {K}}).}$

In both conjectures, the constants are positive and depend only on the indicated quantities. (A stronger form of Lang's conjecture asserts that ${\displaystyle c}$ depends only on the degree ${\displaystyle [K:\mathbb {Q} ]}$.) It is known that the abc conjecture implies Lang's conjecture, and that the analogue of Lang's conjecture over one dimensional characteristic 0 function fields is unconditionally true.[3][5] The best general result on Lehmer's conjecture is the weaker estimate ${\displaystyle {\hat {h}}(P)\geq c(E/K)/[K(P):K]^{3+\epsilon }}$ due to Masser.[6] When the elliptic curve has complex multiplication, this has been improved to ${\displaystyle {\hat {h}}(P)\geq c(E/K)/[K(P):K]^{1+\epsilon }}$ by Laurent.[7] There are analogous conjectures for abelian varieties, with the nontorsion condition replaced by the condition that the multiples of ${\displaystyle P}$ form a Zariski dense subset of ${\displaystyle A}$, and the lower bound in Lang's conjecture replaced by ${\displaystyle {\hat {h}}(P)\geq c(K)h(A/K)}$, where ${\displaystyle h(A/K)}$ is the Faltings height of ${\displaystyle A/K}$.

## Generalizations

A polarized algebraic dynamical system is a triple (V,φ,L) consisting of a (smooth projective) algebraic variety V, a self-morphism φ : V → V, and a line bundle L on V with the property that ${\displaystyle \phi ^{*}L=L^{\otimes d}}$ for some integer d > 1. The associated canonical height is given by the Tate limit[8]

${\displaystyle {\hat {h}}_{V,\phi ,L}(P)=\lim _{n\to \infty }{\frac {h_{V,L}(\phi ^{(n)}(P))}{d^{n}}},}$

where φ(n) = φ o φ o … o φ is the n-fold iteration of φ. For example, any morphism φ : PNPN of degree d > 1 yields a canonical height associated to the line bundle relation φ*O(1) = O(d). If V is defined over a number field and L is ample, then the canonical height is non-negative, and

${\displaystyle {\hat {h}}_{V,\phi ,L}(P)=0~~\Longleftrightarrow ~~P~{\rm {is~preperiodic~for~}}\phi .}$

(P is preperiodic if its forward orbit P, φ(P), φ2(P), φ3(P),… contains only finitely many distinct points.)

## References

1. ^ Néron, André (1965). "Quasi-fonctions et hauteurs sur les variétés abéliennes". Ann. of Math. (in French). 82: 249–331. doi:10.2307/1970644. MR 0179173.
2. ^ Lang (1997) p.72
3. ^ a b Lang (1997) pp.73–74
4. ^ Lang (1997) pp.243
5. ^ Hindry, Marc; Silverman, Joseph H. (1988). "The canonical height and integral points on elliptic curves". Invent. Math. 93 (2): 419–450. doi:10.1007/bf01394340. MR 0948108. Zbl 0657.14018.
6. ^ Masser, David W. (1989). "Counting points of small height on elliptic curves". Bull. Soc. Math. France. 117 (2): 247–265. MR 1015810.
7. ^ Laurent, Michel (1983). "Minoration de la hauteur de Néron-Tate" [Lower bounds of the Nerón-Tate height]. In Bertin, Marie-José. Séminaire de théorie des nombres, Paris 1981–82 [Seminar on number theory, Paris 1981–82]. Progress in Mathematics (in French). Birkhäuser. pp. 137–151. ISBN 0-8176-3155-0. MR 0729165.
8. ^ Call, Gregory S.; Silverman, Joseph H. (1993). "Canonical heights on varieties with morphisms". Compositio Mathematica. 89 (2): 163–205. MR 1255693.

General references for the theory of canonical heights