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Number theory Difficulty 5.2 AIME, harder Prove it Saudi Arabia

Prove that there exists a positive integer nn such that the last digits of n3n^{3} are 201320132013\square 201320132013.

Solution

Notice that a=201320132013a = 201320132013 and 101210^{12} are coprime. By Euler's theorem
aϕ(1012)1mod(1012) a^{\phi\left(10^{12}\right)} \equiv 1 \quad \bmod \left(10^{12}\right)
Let b=aϕ(1012)1b = a^{\phi\left(10^{12}\right) - 1}. We have ab1mod(1012)a b \equiv 1 \bmod \left(10^{12}\right). Because bb and 101210^{12} are coprime, again by Euler's theorem
aabϕ(1012)bϕ(1012)1mod(1012). a \equiv a b^{\phi\left(10^{12}\right)} \equiv b^{\phi\left(10^{12}\right) - 1} \quad \bmod \left(10^{12}\right) .
But
ϕ(1012)1=213×5111(1)13×(1)1110mod(3). \phi\left(10^{12}\right) - 1 = 2^{13} \times 5^{11} - 1 \equiv (-1)^{13} \times (-1)^{11} - 1 \equiv 0 \bmod (3) .
Therefore, there exists a positive integer mm such that ϕ(1012)1=3m\phi\left(10^{12}\right) - 1 = 3 m.
Let n=bmn = b^{m}. We have
n3=bϕ(1012)1201320132013mod(1012). n^{3} = b^{\phi\left(10^{12}\right) - 1} \equiv 201320132013 \bmod \left(10^{12}\right) .

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