Too Many 4s

What is the remainder when 444 4 4444 4444^{4444} is divided by 9 9 ?


The answer is 7.

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2 solutions

Kenny Lau
Jul 9, 2014

From Euler's theorem, 444 4 4444 m o d 9 = ( 4444 m o d 9 ) 4444 m o d ϕ ( 9 ) m o d 9 = ( 16 m o d 9 ) 4444 m o d 6 m o d 9 = 7 4 m o d 9 = 4 9 2 m o d 9 = 4 2 m o d 9 = 16 m o d 9 = 7 \begin{array}{l} 4444^{4444}\mod9 \\=(4444\mod9)^{4444\mod\phi(9)}\mod9 \\=(16\mod9)^{4444\mod6}\mod9 \\=7^4\mod9 \\=49^2\mod9 \\=4^2\mod9 \\=16\mod9 \\=7 \end{array}

Note: 4444's digit sum is 16, therefore 4444 mod 9 = 16 mod 9

Kenny Lau - 6 years, 11 months ago

Note: ϕ ( 9 ) = 9 × ( 1 1 3 ) = 6 \phi(9)=9\times\left(1-\frac13\right)=6

Kenny Lau - 6 years, 11 months ago

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Could not understand this statement above, what's that?

Sagnik Saha - 6 years, 10 months ago

Same :) & thus upvoted!

Krishna Ar - 6 years, 11 months ago

First note that 444 4 4444 = ( 9 493 + 7 ) 4444 7 4444 ( m o d 9 ) 4444^{4444} = (9 * 493 + 7)^{4444} \equiv 7^{4444} \pmod{9} .

Next, note that 7 3 = 343 = 9 38 + 1 7^{3} = 343 = 9 * 38 + 1 , and so 7 3 1 ( m o d 9 ) 7^{3} \equiv 1 \pmod{9} .

Finally, note that 4444 = 3 1481 + 1 4444 = 3 * 1481+ 1 , and so

444 4 4444 7 3 1481 + 1 1 7 ( m o d 9 ) 4444^{4444} \equiv 7^{3 * 1481 + 1} \equiv 1*7 \pmod{9} ,

i.e., the desired remainder is 7 \boxed{7} .

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