Number theoryDifficulty 5.5AIME, harderProve itUkraine
Find all positive integers n, such that 11n−1 is divisible by 10n−1.
Solution
Since 10n−1=9⋅(10n−1+⋯+10+1) we obtain that 11n−1 is divisible by 9. Considering the residues modulo 9: 11n−1≡2n−1(mod9) we get n=6k. But then 106k−1 is divisible by 106−1. Hence it is divisible by 103+1 and 10+1=11 which is not possible.
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Source: MathNet,
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