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Eta Products and Models for Modular Curves

Eta Products and Models for Modular Curves

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Lemma: (C ∗ p/q d , 〈ζq〉) ∼ (C ∗ p/q d 1 , 〈ζq 1〉) if <strong>and</strong> only if<br />

q gcd(d,N/d) = q gcd(d,N/d)<br />

1<br />

.<br />

Pf/ Let λ be a primitive N th root of unity. So ζ = λ jN/r <strong>for</strong> some j with<br />

(j, r) = 1. Then the two elliptic curves are isomorphic if <strong>and</strong> only if<br />

q 1 = (λ) kN/d q <strong>for</strong> some k.<br />

First suppose 〈λ jN/r q〉 = 〈λ jN/r λ kN/d q〉. Then<br />

jN/r = (jN/r + kN/d)(1 + ld) + mN<br />

0 = kN/d + ldjN/r + ldkN/d + mN.<br />

Thus, we see that d ∣ ∣ (kN/d). So<br />

d<br />

gcd(d,N/d)<br />

∣ k.<br />

There<strong>for</strong>e q gcd(d,N/d)<br />

1<br />

= λ kN gcd(d,N/d)/d q d = q d .<br />

The other direction is similar.

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