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jmc

number theory senior

Problem

Let . It is known that divides into . If can be written in base as , where are distinct digits such that and are odd and is not divisible by , find .
Solution
Notice that ; thus divides into . Furthermore, using the sum of seventh powers factorization, it follows that divides into .

Using the divisibility criterion for , we know that must be divisible by . Thus is even and not divisible by . Also, is odd, so , where does not divide into . Thus, cannot divide into either, otherwise would not be divisible by . Then, must be equal to .

Using the divisibility criterion for , it follows that is divisible by , that is divides into . Thus, . Using the divisibility criterion for , since then the alternating sum of digits, which works out to be . Thus, is either equal to or , so .

In the former case when , summing with yields that , of which only fit the problem conditions. This yields that . However, we know that and are distinct, so we can eliminate this possibility. Thus, , of which only works. The answer is .
Final answer
129