Find all prime numbers p for which there exists a unique a ∈ {1, 2, . . . , p} such that $a^3 − 3a + 1$ is divisible by p.
When I see the solution of this problem , the first think I surprised was we need to find the root of this equation, and I think this isn't related to some normal solution .
My question is how root of this cubic equation is related to the solution divisible by p ? enter image description here