二次互反律(Quadratic Reciprocity Law)
\[
\Bigl(\frac{p}{q}\Bigr)\Bigl(\frac{q}{p}\Bigr)=(-1)^{\frac{p-1}{2}\cdot\frac{q-1}{2}}
\]
Answer
\[
\Bigl(\frac{p}{q}\Bigr)\Bigl(\frac{q}{p}\Bigr)=(-1)^{\frac{p-1}{2}\cdot\frac{q-1}{2}}
\]