First supplementary law for quadratic reciprocity
= First supplementary law for quadratic reciprocity
Euler's criterion at $a=-1$ gives
$$
\left(\frac{-1}{p}\right)=(-1)^{(p-1)/2}.
$$
= First supplementary law for quadratic reciprocity
Euler's criterion at $a=-1$ gives
$$
\left(\frac{-1}{p}\right)=(-1)^{(p-1)/2}.
$$