Odd-prime obstructions for the congruent-number isogeny coverings
= Odd-prime obstructions for the congruent-number isogeny coverings
{title2=$p\equiv3\pmod4$}
For the curve $Y^2=X^3+4p^2X$ with $p\equiv3\pmod4$, the classes $p$ and $2p$ in <two-isogeny descent> lead to $N^2=p(U^4+4V^4)$ and $N^2=2p(U^4+V^4)$. A primitive solution forces $p\mid N$ and $p\nmid V$. The resulting congruences make $-1$ a square modulo $p$, a contradiction. Thus these coverings have no rational points.