Calculator can't help you (not fully! maybe partially)

The remainder when 1 9 92 19^{92} is divided by 92 is X 2 X^{2} . Find the remainder when X 92 X^{92} is divided by 92.


The answer is 9.

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1 solution

By Euler's theorem , as 19 19 and 92 92 are coprime, we have that

1 9 ϕ ( 92 ) 1 ( m o d 92 ) \large 19^{\phi(92)} \equiv 1 \pmod{92} , where ϕ ( n ) \phi(n) is Euler's totient function .

Now as 92 = 4 × 23 92 = 4 \times 23 and these two factors are coprime, we have that

ϕ ( 92 ) = ϕ ( 4 ) × ϕ ( 23 ) = 2 × 22 = 44 \phi(92) = \phi(4) \times \phi(23) = 2 \times 22 = 44 . (Note that ϕ ( p ) = p 1 \phi(p) = p - 1 for any prime p p .)

So 1 9 92 = 1 9 2 44 + 4 1 9 4 ( m o d 92 ) 49 ( m o d 92 ) \large 19^{92} = 19^{2*44 + 4} \equiv 19^{4} \pmod{92} \equiv 49 \pmod{92} ,

where a calculator did come into play, (as noted in the title of the question). So X 2 = 49 X = 7 X^{2} = 49 \Longrightarrow X = 7 .

Now as 7 7 and 92 92 are also coprime, we can follow the same steps as above to find that

7 92 = 7 2 44 + 4 7 4 ( m o d 92 ) 9 ( m o d 92 ) \large 7^{92} = 7^{2*44 + 4} \equiv 7^{4} \pmod{92} \equiv \boxed{9} \pmod{92} .

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