Number theoryDifficulty 7.7National olympiad, round 2Find the answer
Find all prime numbers p for which there exists a unique a∈{1,2,…,p} such that a3−3a+1 is divisible by p.
A number or a short expression. Spacing and $ signs are ignored.
Solution
We show that p=3 is the only prime that satisfies the condition. Let f(x)=x3−3x+1. As preparation, let's compute the roots of f(x). By Cardano's formula, it can be seen that the roots are 2Re32−1+(2−1)2−(3−3)3=2Re3cos32π+isin32π={2cos92π,2cos94π,2cos98π} where all three values of the complex cubic root were taken. Notice that, by the trigonometric identity 2cos2t=(2cost)2−2, the map φ(x)=x2−2 cyclically permutes the three roots. We will use this map to find another root of f, when it is considered over Fp. Suppose that f(a)=0 for some a∈Fp and consider g(x)=x−af(x)=x−af(x)−f(a)=x2+ax+(a2−3). We claim that b=a2−2 is a root of g(x). Indeed, g(b)=(a2−2)2+a(a2−2)+(a2−3)=(a+1)⋅f(a)=0. By Vieta's formulas, the other root of g(x) is c=−a−b=−a2−a+2. If f has a single root then the three roots must coincide, so a=a2−2=−a2−a+2. Here the quadratic equation a=a2−2, or equivalently (a+1)(a−2)=0, has two solutions, a=−1 and a=2. By f(−1)=f(2)=3, in both cases we have 0=f(a)=3, so the only choice is p=3. Finally, for p=3 we have f(1)=−1,f(2)=3 and f(3)=19, from these values only f(2) is divisible by 3, so p=3 satisfies the condition.
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