A 100-digit positive integer n is called unusual if the last 100 digits of the decimal representation of n3 form the decimal representation of n, but the last 100 digits of the decimal representation of n2 do not. Prove that there are at least two unusual 100-digit numbers. (V. Senderov)
Solution
For example, such numbers are n1=10100−1=99…9 and n2=210100−1=49…9. Indeed, the numbers n13−n1=(n1+1)n1(n1−1)=10100⋅n1(n1−1) and n23−n2=(n2+1)n2(n2−1)=10100⋅n2⋅2n2−1 are divisible by 10100; this means that ni3 ends with ni. On the other hand, the numbers n13−n1=n1(n1−1) and n23−n2=n2(n2−1) are not divisible by 5 (and even less by 10100); therefore, ni2 does not end with ni.
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