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Number theory Difficulty 4.8 AIME Prove it China

Find all positive integers nn such that 20n+220n + 2 can divide 2003n+20022003n + 2002. (posed by Wu Weichao)

Solution

It is easy to see that nn is an even number. Let n=2mn = 2m, then from
40m+22003×2m+200240m + 2 \mid 2003 \times 2m + 2002
we can get 20m+12003m+100120m + 1 \mid 2003m + 1001.
But 2003m+1001=100(20m+1)+3m+9012003m + 1001 = 100(20m + 1) + 3m + 901,
so 20m+13m+90120m + 1 \mid 3m + 901.
And when 3m+90120m+1=1,2,3,4\frac{3m+901}{20m+1} = 1, 2, 3, 4, mm is not a positive integer.
Therefore,
3m+90120m+15,m89697<10. \begin{aligned} \frac{3m+901}{20m+1} &\ge 5, \\ m &\le \frac{896}{97} < 10. \end{aligned}
Hence m9m \le 9. But after checking one by one for m=1,2,3,,9m = 1, 2, 3, \dots, 9, we know that 20m+13m+90120m + 1 \nmid 3m + 901 for m=1,2,3,,9m = 1, 2, 3, \dots, 9.
Therefore, there is no positive integer nn satisfying the condition.

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