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Find the remainder when $444^{444^{444}}$ is divided by $7$.

My approach :

$E(7) = 6 $

$444^{444} \pmod 6 = 0$

so , $444^0 \pmod 7 = 1$

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    Sorry!,i didn't notice that.2012-06-27

3 Answers 3

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Yes your approach is indeed correct. I assume by $E(7)$ you mean the Euler totient function $\phi(7)$.

By Euler's theorem/ Fermat's little theorem, we have $444^{6} \equiv 1 \pmod{7}$ Now $6$ divides $444$. Hence, $444^{444} = 6M$. Hence, $444^{444^{444}} = 444^{6M} = \left( 444^6\right)^M \equiv 1^M \pmod{7} \equiv 1 \pmod{7}$

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    Yes I meant the same.Thanks sir @marvis.2012-06-27
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$a^{\phi(p)}=1(\mod p)$.Here $444^{444}=0(\mod \phi(7)) \implies 444^{444}=k*\phi(7)$ for some integer k.Then, $444^{444^{444}}(\mod 7)= 444^{k*\phi(7)}(\mod 7) = (444^{\phi(7)})^k(\mod 7)=1^k(\mod 7) = 1 $

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step 1 : Try to bring big no's into a form so that we get remainder of $-1 $ or $1$ , so that it will be easy for us to simplify

$444 = 4*111 $ as $ \frac {111}7 $ gives a remainder of $-1$ . So our goal of converting a bigger number to number which gives remainder $1$ or $-1$ is attained and as $ -1^{even} = 1$

so $ 444^x /7 = (4^x * 111^x )/7 = 4^x / 7 $ $( 111^x \ divided \ by\ 7\ \ \ gives \ remainder \ of \ -1^x\ which\ is\ equal\ to\ 1\ as\ x\ is\ even\ )$

Where $ x = 444^{444} (even) $ So

$4^x= 4^{444^{444}} = 2^{888^{444}} $

step 2 : Observe the pattern of the remainders

$ 2^0 mod 7 = 1 $

$ 2^1 mod 7 = 2 $

$ 2^2 mod 7 = 4 $


$ 2^3 mod 7 = 1 $

$ 2^4 mod 7 = 2 $

$2^5 mod 7 = 4 $


so here, for a period of $ 3 $ remainder $ 1 $ repeats

here $ 888 $ is a multiple of $3$ , so

$ 2^{888}\ mod \ 7 =\ 1 $

$ 1^{444} \ mod \ 7 = \ 1 $

Hence Answer is $ 1 $

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    Correction made ... Thank You very much . This gave an opportunity to know about MAth Jax .2012-10-25