Number theoryDifficulty 5.2AIME, harderProve itSaudi Arabia
Prove that there exists a positive integer n such that the last digits of n3 are □201320132013.
Solution
Notice that a=201320132013 and 1012 are coprime. By Euler's theorem aϕ(1012)≡1mod(1012) Let b=aϕ(1012)−1. We have ab≡1mod(1012). Because b and 1012 are coprime, again by Euler's theorem a≡abϕ(1012)≡bϕ(1012)−1mod(1012). But ϕ(1012)−1=213×511−1≡(−1)13×(−1)11−1≡0mod(3). Therefore, there exists a positive integer m such that ϕ(1012)−1=3m. Let n=bm. We have n3=bϕ(1012)−1≡201320132013mod(1012).
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