Moderate 2015

92 2016 a ( m o d 2015 ) \Large \color{#3D99F6}{92}^{\color{#20A900}{2016}} \equiv \color{#EC7300}{a} \pmod{\color{#D61F06}{2015}}

Find the lowest positive value of a a which satisfies the congruence above.


The answer is 1.

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2 solutions

Steven Yuan
Mar 11, 2018

Since 2015 = 5 × 13 × 31 , 2015 = 5 \times 13 \times 31, we can check 9 2 2016 92^{2016} modulo 5, 13, and 31 separately. Since 92 = 31 × 3 1 = 13 × 7 + 1 , 92 = 31 \times 3 - 1 = 13 \times 7 + 1, we have

9 2 2016 ( 1 ) 2016 1 ( m o d 31 ) 92^{2016} \equiv (-1)^{2016} \equiv 1 \! \! \! \! \pmod{31}

and

9 2 2016 1 2016 1 ( m o d 13 ) . 92^{2016} \equiv 1^{2016} \equiv 1 \! \! \! \! \pmod{13}.

Since gcd ( 92 , 5 ) = 1 , \gcd(92, 5) = 1, by Fermat's Little Theorem,

9 2 2016 ( 9 2 4 ) 504 1 504 1 ( m o d 5 ) . 92^{2016} \equiv (92^4)^{504} \equiv 1^{504} \equiv 1 \! \! \! \! \pmod{5} .

Because 9 2 2016 92^{2016} is 1 modulo 5, 13, and 31, it must also be 1 \boxed{1} modulo 2015, so that is our answer.

(Posting solutions to 3 year old problems FTW ¨ \ddot \smile )

Jaimin Pandya
May 17, 2015

what did you mean mod?

Willy Manzanas - 6 years ago

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Mod is short name of Modulo . You will get more information about mod from : http://en.wikipedia.org/wiki/Modular_arithmetic

Jaimin Pandya - 6 years ago

okay thanks

Willy Manzanas - 6 years ago

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