Wednesday, December 9, 2015

elementary number theory - Find remainder when $777^{777}$ is divided by $16$


Find remainder when $777^{777}$ is divided by $16$.





$777=48\times 16+9$. Then $777\equiv 9 \pmod{16}$.



Also by Fermat's theorem, $777^{16-1}\equiv 1 \pmod{16}$ i.e $777^{15}\equiv 1 \pmod{16}$.



Also $777=51\times 15+4$. Therefore,



$777^{777}=777^{51\times 15+4}={(777^{15})}^{51}\cdot777^4\equiv 1^{15}\cdot 9^4 \pmod{16}$ leading to $ 81\cdot81 \pmod{16} \equiv 1 \pmod{16}$.



But answer given for this question is $9$. Please suggest.

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