Number theoryDifficulty 5.1AIME, harderProve itIran
Prove that for every natural number m, there exists a natural number N such that for every natural number b that 2≤b≤1389, the sum of digits of N in base b, is more than m.
Solution
We claim that N=(1389!)m+1−1 has the desired properties. Let 2≤b≤1389, we know that there are at least m+1 zeros in the rightmost of (1389!)m+1 in base b, because (1389!)m+1 is divisible by bm+1. Hence, the m+1 rightmost digits of (1389!)m+1−1 is b−1, then sum of the digits of N in base b is at least (m+1)(b−1) which is greater than or equal to m+1.
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