# Martin's Blog

## The matrix lemma for elliptic curves

Posted by Martin Orr on Friday, 25 May 2012 at 14:00

Let be a principally polarised abelian variety of dimension over . We can associate with a complex matrix called the period matrix which roughly speaking describes a basis for the image of in (actually it is not really the period matrix as it is only defined up to the action of on the Siegel upper half space; we can make it nearly unique by forcing it to be in a particular fundamental domain).

The matrix lemma says that, if is defined over a number field, then the entries of the imaginary part of the period matrix cannot be too large with respect to the height of (Faltings height or modular height).

Matrix lemma. (Masser 1987) Let be a principally polarised abelian variety of dimension over a number field . Let be the period matrix for in the standard fundamental domain of the Siegel upper half space. There is a constant depending only on such that all the entries of satisfy

Last time I used a lower bound for the lengths of non-zero periods in the proof of the isogeny theorem. This follows from the matrix lemma as we can easily relate lengths of periods and the period matrix.

Today I will prove the matrix lemma for elliptic curves. The general proof requires various facts about Siegel modular forms and also uses a funny choice of level structure due to Igusa (I do not understand why). But the basic structure of the proof is already visible in the elliptic curves case and we can be concrete about the modular forms involved, using only facts I learned in Part III.

The lemma is valid taking to be either the stable Faltings height of or the Weil height of a point representing in the moduli space of ppAVs of dimension . This is because Faltings proved a bound for the difference between the two heights. The proof of the matrix lemma uses the height from the moduli space. But in applying it to the isogeny theorem, we need the Faltings height because we need the bound for varieties related by an isogeny , and so far as I know this can only be proven using the Faltings height.

I was a bit disappointed when I realised this because I had thought that the Masser-Wüstholz isogeny theorem gave a proof of Finiteness Theorem I independent of Faltings' work. Of course the comparison between heights is not all of Faltings' proof, but according to Milne, "Technically, this is by far the hardest part of the proof."

In order to define the height of a point in the moduli space we need to choose an embedding of the moduli space into projective space. I think we have to choose the right embedding for the lemma to work (and also for the comparison with the Faltings height to work) but I am not very clear about that -- I think this might be related to the Igusa level structure which I mentioned.

For today, I am sticking to elliptic curves, where there is nothing to worry about. We can simply use the Weil height of the -invariant.

### The proof

Let be an elliptic curve over the number field . Let be a complex number in the upper half plane such that generate the period lattice for . We can choose in the standard fundamental domain defined by

The period lemma asserts that there is an absolute constant such that We will prove this by bounding in two ways, one involving and the other involving .

Recall that where and are modular forms of weight 12, being a certain multiple of and being a cusp form.

In the compactified fundamental domain, vanishes only at . We have ; indeed has no zeroes except at the cusp. So is non-zero in some neighbourhoods of and . Hence there is a constant such that

Cusp forms decay exponentially as we approach the cusp i.e. there are constants such that (This follows from the existence of -expansions.)

Combining these two inequalities, we get that

Since is defined over the number field , the same is true of . Using the definition of the Weil height, it is not hard to prove Liouville's inequality:

By basic properties of heights, so

Hence

Combining the two bounds for , we get that which simplifies to the desired