Maths Olympiad Prep

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, 2016

Number theory Difficulty 5.3 AIME, harder Prove it Hong Kong

Find all natural numbers nn such that nn, n2+10n^2 + 10, n22n^2 - 2, n3+6n^3 + 6 and n5+36n^5 + 36 are all prime numbers.

Solution

The only solution is n=7n = 7.
Observe that
nn(mod 7),n2+10(n+2)(n2)(mod 7),n22(n+3)(n3)(mod 7),n3+6(n1)(n2+n+1)(mod 7),n5+36(n+1)(n4n3+n2n+1)(mod 7). \begin{aligned} n &\equiv n \quad (\text{mod } 7), \\ n^2 + 10 &\equiv (n + 2)(n - 2) \quad (\text{mod } 7), \\ n^2 - 2 &\equiv (n + 3)(n - 3) \quad (\text{mod } 7), \\ n^3 + 6 &\equiv (n - 1)(n^2 + n + 1) \quad (\text{mod } 7), \\ n^5 + 36 &\equiv (n + 1)(n^4 - n^3 + n^2 - n + 1) \quad (\text{mod } 7). \end{aligned}
Since n3n-3, n2n-2, n1n-1, nn, n+1n+1, n+2n+2 and n+3n+3 are 7 consecutive integers, one of them must be a multiple of 7. It follows that one of the numbers is equal to 7, or otherwise it cannot be a prime. Clearly, n2+10>7n^2 + 10 > 7 and n5+36>7n^5 + 36 > 7.
If n=7n=7, the 5 numbers are 7, 59, 47, 349, 16843, all of which are primes.

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