Maths Olympiad Prep

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, 2010

Number theory Difficulty 4.8 AIME Prove it Estonia

Does there exist a prime number pp such that both p3+2008p^3 + 2008 and p3+2010p^3 + 2010 are primes as well?

Solution

Let pp be any prime number. If pp is not divisible by 77, then p3p^3 is congruent to either 11 or 1-1 modulo 77. Since 20081(mod7)2008 \equiv -1 \pmod{7} and 20101(mod7)2010 \equiv 1 \pmod{7}, either of the numbers p3+2008p^3 + 2008 and p3+2010p^3 + 2010 is divisible by 77 and hence composite. If pp is divisible by 77, then p=7p = 7 and p3+2010=73+2010=2353=13181p^3 + 2010 = 7^3 + 2010 = 2353 = 13 \cdot 181 is composite, too.

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