Local Tamagawa theorem for square-free Elkies $16D$, with Cassels on $3$-primary Sha orders
The $3$-isogeny $\varphi\colon E_{16D}\to E_{-432D}$, for square-free Elkies $D$, has Tamagawa ratio determined at the prime $3$. We prove that $c(E')/c(E)$ equ