Find the lowest positive value of which satisfies the congruence above.
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Since 2 0 1 5 = 5 × 1 3 × 3 1 , we can check 9 2 2 0 1 6 modulo 5, 13, and 31 separately. Since 9 2 = 3 1 × 3 − 1 = 1 3 × 7 + 1 , we have
9 2 2 0 1 6 ≡ ( − 1 ) 2 0 1 6 ≡ 1 ( m o d 3 1 )
and
9 2 2 0 1 6 ≡ 1 2 0 1 6 ≡ 1 ( m o d 1 3 ) .
Since g cd ( 9 2 , 5 ) = 1 , by Fermat's Little Theorem,
9 2 2 0 1 6 ≡ ( 9 2 4 ) 5 0 4 ≡ 1 5 0 4 ≡ 1 ( m o d 5 ) .
Because 9 2 2 0 1 6 is 1 modulo 5, 13, and 31, it must also be 1 modulo 2015, so that is our answer.
(Posting solutions to 3 year old problems FTW ⌣ ¨ )