Compute this manually!

If 201 4 2014 n 2014^{2014}\equiv n ( mod \text{mod} 7 7 ), where 0 n < 7 0\leq n<7 , find the value of n n .


The answer is 2.

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3 solutions

Victor Loh
Jun 27, 2014

2014 5 2014 \equiv 5 ( mod \text{mod} 7 7 ) 5 2014 n \implies 5^{2014} \equiv n ( mod \text{mod} 7 7 ).

By Fermat's Little Theorem, 5 7 1 = 5 6 1 5^{7-1}=5^{6} \equiv 1 ( mod \text{mod} 7 7 ).

Hence 5 2014 = ( 5 6 ) 335 × 5 4 n 5^{2014}=(5^{6})^{335} \times 5^{4} \equiv n ( mod \text{mod} 7 7 ) 5 4 n \implies 5^{4} \equiv n ( mod \text{mod} 7 7 ).

Since 5 4 = 625 2 5^{4}=625 \equiv \boxed{2} ( mod \text{mod} 7 7 ), we are done.

nice solution

math man - 6 years, 11 months ago
Chew-Seong Cheong
Mar 11, 2018

201 4 2014 ( 2 ) 2014 (mod 7) Note that 2014 2 (mod 7) 2 2014 (mod 7) 2 ( 2 3 ) 671 (mod 7) 2 ( 8 671 ) (mod 7) 2 ( 7 + 1 ) 671 (mod 7) 2 ( 1 671 ) (mod 7) 2 (mod 7) \begin{aligned} 2014^{2014} & \equiv (-2)^{2014} \text{ (mod 7)} & \small \color{#3D99F6} \text{Note that }2014 \equiv -2 \text{ (mod 7)} \\ & \equiv 2^{2014} \text{ (mod 7)} \\ & \equiv 2(2^3)^{671} \text{ (mod 7)} \\ & \equiv 2(8^{671}) \text{ (mod 7)} \\ & \equiv 2(7+1)^{671} \text{ (mod 7)} \\ & \equiv 2(1^{671}) \text{ (mod 7)} \\ & \equiv \boxed{2} \text{ (mod 7)} \end{aligned}

2014/7 = 287 r.5, 5^n always ends in 5, so 5mod7=2 (idk how to type the equivalence symbol)

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