For every positive integer , the number is divisible by :
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testing n= 1 gives an answer of 2009. Now I shall prove that the expression is divisible by 2009 for all n by using modular arithmetic, as follows... 2 0 0 9 = 7 2 × 4 1 → 3 1 0 0 n − 6 5 0 n − 5 5 8 n + 1 1 7 n ≡ 6 n − 6 n − 5 n + 5 n ≡ 0 m o d ( 7 ) → 3 1 0 0 n − 6 5 0 n − 5 5 8 n + 1 1 7 n ≡ 2 5 n − 3 5 n − 2 5 n + 3 5 n ≡ 0 m o d ( 4 1 ) ∴ 2 0 0 9 ∣ ( 3 1 0 0 n − 6 5 0 n − 5 5 8 n + 1 1 7 n ) a s r e q u i r e d