Skip to main content
7 of 14
It turned out to be a silly question

Bounding the modular discriminant of an elliptic curve

Suppose that we are given an elliptic curve $E$ over a number field $K$ and a finite morphism $\pi:E\longrightarrow \mathbf{P}^1_K$ of degree $d$. Assume $\pi$ is unramified outside a finite set $S=\{s_1,\ldots,s_n\}\subset\mathbf{P}^1_K$ of points. Fix an embedding $K\longrightarrow \mathbf{C}$.

Consider $X=E\times_K \mathbf{C}$. We can write $X=\mathbf{C}/ (\mathbf{Z}+\tau \mathbf{Z})$, where $\tau$ is an element in the complex upper half plane. We define $\Vert \Delta\Vert(X) =\vert (\Im \tau)^6 \vert q\prod_{k=1}^\infty (1-q^k)^{24}$, where we write $q=\exp(2\pi i \tau)$ as usual. This is called the modular discriminant.

I would like to bound $\Vert\Delta\Vert(X)$ from below and above using constants depending on $d$ and the ramification of $\pi$. (Note that Hurwitz gives that $2d = \deg R$ with $R$ the ramification divisor.)

For example, the bound could be something like
$$ \vert \log \Vert \Delta \Vert(X) \vert \leq 1020323 \cdot d^{10} \cdot s_1\cdot \ldots \cdot s_n\cdot \vert S\vert^6. $$

Example. If $\pi$ is the Weierstrass function then it is etale above $\{0,1,\infty,s\}$ and of degree $2$. The modular discriminant is given by $\Vert\Delta\Vert(X) = s^2(s-1)^2$. This is precisely what I'm looking for.

EDIT: I think I just answered my own question! One simply performs some automorphisms of the projective line and uses the above formula. Whoooops.