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Number theory Difficulty 5.0 AIME Prove it Russia

A 100-digit positive integer nn is called unusual if the last 100 digits of the decimal representation of n3n^3 form the decimal representation of nn, but the last 100 digits of the decimal representation of n2n^2 do not. Prove that there are at least two unusual 100-digit numbers. (V. Senderov)

Solution

For example, such numbers are n1=101001=999n_1 = 10^{100} - 1 = 99\ldots9 and n2=1010021=499n_2 = \frac{10^{100}}{2} - 1 = 49\ldots9. Indeed, the numbers
n13n1=(n1+1)n1(n11)=10100n1(n11) n_1^3 - n_1 = (n_1 + 1)n_1(n_1 - 1) = 10^{100} \cdot n_1(n_1 - 1)
and
n23n2=(n2+1)n2(n21)=10100n2n212 n_2^3 - n_2 = (n_2 + 1)n_2(n_2 - 1) = 10^{100} \cdot n_2 \cdot \frac{n_2 - 1}{2}
are divisible by 1010010^{100}; this means that ni3n_i^3 ends with nin_i. On the other hand, the numbers n13n1=n1(n11)n_1^3 - n_1 = n_1(n_1 - 1) and n23n2=n2(n21)n_2^3 - n_2 = n_2(n_2 - 1) are not divisible by 5 (and even less by 1010010^{100}); therefore, ni2n_i^2 does not end with nin_i.

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