Thursday, 15 May 2008

nt.number theory - Modular curves of genus zero and normal forms for elliptic curves

This is maybe the first question I actually need to know the answer to!



Let $N$ be a positive integer such that $mathbb{H}/Gamma(N)$ has genus zero. Then the function field of $mathbb{H}/Gamma(N)$ is generated by a single function. When $N = 2$, the cross-ratio $lambda$ is such a function. A point of $mathbb{H}/Gamma(2 )$ at which $lambda = lambda_0$ is precisely an elliptic curve in Legendre normal form



$$y^2 = x(x - 1)(x - lambda_0)$$



where the points $(0, 0), (1, 0)$ constitute a choice of basis for the $2$-torsion. When $N = 3$, there is a modular function $gamma$ such that a point of $mathbb{H}/Gamma(3)$ at which $gamma = gamma_0$ is precisely an elliptic curve in Hesse normal form



$$x^3 + y^3 + 1 + gamma_0 xy = 0$$



where (I think) the points $(omega, 0), (omega^3, 0), (omega^5, 0)$ (where $omega$ is a primitive sixth root of unity) constitute a choice of basis for the $3$-torsion.



Question: Does this picture generalize? That is, for every $N$ above does there exist a normal form for elliptic curves which can be written in terms of a generator of the function field of $mathbb{H}/Gamma(N)$ and which "automatically" equips the $N$-torsion points with a basis? (I don't even know if this is possible when $N = 1$, where the Hauptmodul is the $j$-invariant.) If not, what's special about the cases where it is possible?

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