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Apr 6, 2022 at 10:46 comment added Joe Silverman @FrançoisBrunault Thanks, that's probably the paper that I was trying to recall when I mentioned Mordell's name.
Apr 6, 2022 at 10:08 vote accept did
Apr 6, 2022 at 9:01 comment added François Brunault This method has been used before, not with the aim to find equations of $X_1(N)$, but to determine elliptic curves over $\mathbb{Q}$ with a given torsion subgroup. For example Billing and Mahler in "On exceptional points on cubic curves" (Journal of the LMS, vol. 15, 1940) showed that an elliptic curve over $\mathbb{Q}$ cannot have a rational point of order $11$. In the course of doing so, they prove in Lemma 2 (p. 41) that a certain cubic curve with explicit equation has exactly 5 rational points, the curve being of course $X_1(11)$.
Apr 5, 2022 at 20:55 history answered Joe Silverman CC BY-SA 4.0