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Number theory Difficulty 4.8 AIME Prove it Saudi Arabia

Prove that 20142014 divides 53n5557n53+4n53 n^{55} - 57 n^{53} + 4 n for all integer nn.

Solution

Notice first that 2014=2×19×532014 = 2 \times 19 \times 53. Applying Fermat's little theorem for the prime numbers 2,19,532, 19, 53 we obtain:
53n5557n53+4nnn+00mod253n5557n53+4n15n17(n19)20+4n15n19+4n19n0mod19and 53n5557n53+4n04n+4n0mod53 \begin{gathered} 53 n^{55} - 57 n^{53} + 4 n \equiv n - n + 0 \equiv 0 \quad \bmod 2 \\ 53 n^{55} - 57 n^{53} + 4 n \equiv 15 n^{17} (n^{19})^{2} - 0 + 4 n \equiv 15 n^{19} + 4 n \equiv 19 n \equiv 0 \quad \bmod 19 \\ \text{and } 53 n^{55} - 57 n^{53} + 4 n \equiv 0 - 4 n + 4 n \equiv 0 \quad \bmod 53 \end{gathered}
for all integer nn. Therefore 20142014 divides 53n5557n53+4n53 n^{55} - 57 n^{53} + 4 n for all integer nn.

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