# height function

###### Definition 1

Let $A$ be an abelian group  . A height function on $A$ is a function $h\colon A\to\mathbb{R}$ with the properties:

1. 1.

For all $Q\in A$ there exists a constant $C_{1}$, depending on $A$ and $Q$, such that for all $P\in A$:

 $h(P+Q)\leq 2h(P)+C_{1}$
2. 2.

There exists an integer $m\geq 2$ and a constant $C_{2}$, depending on $A$, such that for all $P\in A$:

 $h(mP)\geq m^{2}h(P)-C_{2}$
3. 3.

For all $C_{3}\in\mathbb{R}$, the following set is finite:

 $\{P\in A:h(P)\leq C_{3}\}$

Examples:

1. 1.

For $t=p/q\in\mathbb{Q}$, a fraction in lower terms, define $H(t)=\max\{\mid p\mid,\mid q\mid\}$. Even though this is not a height function as defined above, this is the prototype of what a height function should look like.

2. 2.

Let $E$ be an elliptic curve  over $\mathbb{Q}$. The function on $E(\mathbb{Q})$, the points in $E$ with coordinates in $\mathbb{Q}$, $h_{x}\colon E(\mathbb{Q})\to\mathbb{R}$ :

 $h_{x}(P)={{\log H(x(P)),\quad if\ P\neq 0}\brace{0,\quad if\ P=0}}$

is a height function ($H$ is defined as above). Notice that this depends on the chosen Weierstrass model of the curve.

3. 3.

The canonical height of $E/\mathbb{Q}$ (due to Neron and Tate) is defined by:

 $h_{C}(P)=1/2\lim_{N\to\infty}4^{(-N)}h_{x}([2^{N}]P)$

where $h_{x}$ is defined as in (2).

Finally we mention the fundamental theorem of “descent”, which highlights the importance of the height functions:

###### Theorem 1 (Descent)

Let $A$ be an abelian group and let $h\colon A\to\mathbb{R}$ be a height function. Suppose that for the integer $m$, as in property (2) of height, the quotient group  $A/mA$ is finite. Then $A$ is finitely generated   .

## References

• 1 Joseph H. Silverman, . Springer-Verlag, New York, 1986.
 Title height function Canonical name HeightFunction Date of creation 2013-03-22 13:49:09 Last modified on 2013-03-22 13:49:09 Owner alozano (2414) Last modified by alozano (2414) Numerical id 5 Author alozano (2414) Entry type Definition Classification msc 14H52 Related topic EllipticCurve Related topic RankOfAnEllipticCurve Related topic ArithmeticOfEllipticCurves Related topic CanonicalHeightOnAnEllipticCurve Defines height function Defines canonical height Defines descent theorem