This is maybe the first question I actually need to know the answer to!
Let N be a positive integer such that mathbbH/Gamma(N) has genus zero. Then the function field of mathbbH/Gamma(N) is generated by a single function. When N=2, the cross-ratio lambda is such a function. A point of mathbbH/Gamma(2) at which lambda=lambda0 is precisely an elliptic curve in Legendre normal form
y2=x(x−1)(x−lambda0)
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 mathbbH/Gamma(3) at which gamma=gamma0 is precisely an elliptic curve in Hesse normal form
x3+y3+1+gamma0xy=0
where (I think) the points (omega,0),(omega3,0),(omega5,0) (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 mathbbH/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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