 Discriminant criterion for a quadratic congruence modulo an odd prime (source code)

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= Discriminant criterion for a quadratic congruence modulo an odd prime

For $p\nmid a$, the congruence $aX^2+bX+c=0\pmod p$ has a solution exactly when its discriminant $b^2-4ac$ is zero or a quadratic residue modulo $p$.

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