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For nonzero integral $D$, the elliptic curve $E_D:y^2=x^3-D^{2}x$ has rational torsion precisely $O,(0,0),(D,0),(-D,0)$. At every good prime $p\equiv 3(\mathrm{mod}4)$, cancellation of the Legendre symbols at $x$ and $-x$ gives $\#E_D({\mathbb{F}}_p)=p+1$. The torsion-freeness of the formal group over Qp for odd p makes the rational torsion subgroup inject into this group. The Dirichlet theorem on primes in arithmetic progressions excludes odd factors of its order and bounds its two-primary part by four. The four displayed rational 2-torsion points then give equality.

The two reduced positive definite integral forms of discriminant $-35$ areFor a prime $p>7$, the first represents $p$ exactly whenand the second represents $p$ exactly when both Legendre symbols are $-1$.

At an odd good prime, use a model $y^2=x^3-d^{2}x$ with $d$ a unit. If $p\equiv 3(\mathrm{mod}4)$, pairing $x$ with $-x$ cancels their Legendre symbols, so the trace is zero. If $p\equiv 1(\mathrm{mod}4)$, the four distinct rational 2-torsion points force the group order to be divisible by four, whereas $p+1$ is two modulo four. The trace cannot vanish. This proves the converse without an explicit complex-multiplication character-sum evaluation.