Higher Reciprocity, Cassels Pairings, and Selmer Towers for the 3/5 Congruent Number Problem

We study the arithmetic of the elliptic curves $$A_m:y^2=x(x-m)(x+4m)$$ attached to the $3\mathrm{/}5$ congruent number problem. A difference of ternary representation numbers controls the relevant central $L$-values. For $p\equiv 11(\mathrm{m}\mathrm{o}\mathrm{d}40)$ the ordinary Cassels pairing degenerates; we construct an explicit $4$-cover and show that the next Cassels–Tate pairing is governed by the normalized representation defect, equivalently by a factorial character, a Pell symbol, and a class number congruence. For composite parameters we compute the ordinary Cassels matrices of four twists and, in the two prime case, a degree $1024$ governing field for their joint distribution. The same higher descent extends to larger radicals: a second rational pushout determines the full ${\mathrm{\Lambda }}^{\mathrm{\prime }}$ row of the next pairing. We also determine two explicit Selmer towers with four dimensional ordinary radical, and all finite $2$-power Selmer groups when the ordinary radical is one dimensional.