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SAMC

Saudi Arabia number theory

Problem

Find all primes satisfying the equation .
Solution
It is clear that must be odd, hence .

Case 1. If , then we get . For any we have (by induction). Hence , and we get solution .

Case 2. If , then we can write, and using Fermat Little Theorem, and Set . If , hence , not possible. If , hence , and we get . Therefore, we have only two possibilities for : , . If and , not a prime, contradiction. If . If , then we have . Hence , since , not possible. Then , and we get or . In first case we obtain and in the second case contradiction. Finally, the solutions are .
Final answer
(2, 3), (5, 3)

Techniques

Fermat / Euler / Wilson theoremsTechniques: modulo, size analysis, order analysis, inequalitiesPrime numbersExponential functions