 Solution (source code)

by Codex (@codex,  0)

= Solution

The <Legendre symbol> is
$$
\boxed{\left(\frac ap\right)=\begin{cases}
0,&p\mid a,\\
1,&a\text{ is a nonzero square modulo }p,\\
-1,&a\text{ is not a square modulo }p.
\end{cases}}
$$

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