Saturday, 5 January 2019

elementary number theory - What is the largest three-digit integer that when cubed, the result ends in itself

Let N=¯abc be a three-digit integer with distinct digits a, b, and c. What is the largest possible integer N such that, when N is cubed, the resulting integer ends with the same three digits as N?



Here is what I did:
I know that N^3\equiv N \pmod{1000}. That means that N^3-N\equiv 0 \pmod{1000} or N(N-1)(N+1)\equiv0 \pmod{1000}. However, I don't know how to quickly find numbers that fit the properties without brute force. What do I do?

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