Eta Products and Models for Modular Curves
Eta Products and Models for Modular Curves
Eta Products and Models for Modular Curves
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
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.