What is the remainder when 1 3 1000 13^{1000} divided by 19 19

1 3 1000 m o d 19 = ? \large 13^{1000} \bmod{19} = ?


The answer is 6.

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1 solution

Chew-Seong Cheong
Sep 22, 2018

1 3 1000 1 3 1000 m o d ϕ ( 19 ) (mod 19) Since gcd ( 13 , 19 ) = 1 , Euler’s theorem applies. 1 3 1000 m o d 18 (mod 19) Euler’s tottient function ϕ ( 19 ) = 18 1 3 10 (mod 19) ( 19 6 ) 10 (mod 19) 6 10 (mod 19) 2 10 3 10 (mod 19) 2 5 2 5 9 5 (mod 19) 32 1 8 5 (mod 19) 13 ( 19 1 ) 5 (mod 19) 13 (mod 19) 6 (mod 19) \begin{aligned} 13^{1000} & \equiv 13^{1000 \color{#3D99F6}\bmod \phi(19)} \text{ (mod 19)} & \small \color{#3D99F6} \text{Since }\gcd(13,19) = 1\text{, Euler's theorem applies.} \\ & \equiv 13^{1000 \color{#3D99F6}\bmod 18} \text{ (mod 19)} & \small \color{#3D99F6} \text{Euler's tottient function }\phi(19) = 18 \\ & \equiv 13^{10} \text{ (mod 19)} \\ & \equiv (19-6)^{10} \text{ (mod 19)} \\ & \equiv 6^{10} \text{ (mod 19)} \\ & \equiv 2^{10}\cdot 3^{10} \text{ (mod 19)} \\ & \equiv 2^5\cdot 2^5\cdot 9^5 \text{ (mod 19)} \\ & \equiv 32\cdot18^5 \text{ (mod 19)} \\ & \equiv 13\cdot (19-1)^5 \text{ (mod 19)} \\ & \equiv -13 \text{ (mod 19)} \\ & \equiv \boxed 6 \text{ (mod 19)} \end{aligned}

@Aly Ahmed , it should be 1 3 1000 m o d 19 = ? 13^{1000} \bmod 19 = ? . Use \mod or \bmod. Note that 5 m o d 3 = 2 5 \bmod 3 = 2 but 5 2 ( m o d 3 ) 5 \equiv 2 \pmod 3 . Congruence \equiv "\equiv" is used instead of equal sign = = .

Chew-Seong Cheong - 2 years, 8 months ago

Easy peasy ...got hard problem

Magnas Bera - 1 year, 11 months ago

Please explain how you written the first line

Vaibhav Kak9329 - 2 days, 3 hours ago

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Hope that the following help.

Chew-Seong Cheong - 1 day, 21 hours ago

@Vaibhav Kak9329 , do you know that the Community section will be removed by July 2? Join the discussion at https://brilliant.org/discussions/thread/brilliant-community/ to know more.

Chew-Seong Cheong - 1 day, 21 hours ago

I have understood

Vaibhav Kak9329 - 1 day, 4 hours ago

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