Sunday 14 April 2013

elementary number theory - Checking the remainder of $40^{1999}$ divided by $7$

I have been learning some applications to Fermat's Little theorem and I am currently solving the remainder of $40^{1999}$ divided by $7$ and would appreciate if anyone could confirm my answer of $5$.



$7$ is prime and $7\nmid 40$



So according to Fermat's Little Theorem



$40^{6} \equiv 1 \pmod{7}$




$1999/6 = 333\times 6 + 1 $



Therefore $(40^6)^{333}\times 40^1 \equiv 1^{333}\times 40 \pmod{7}$



$40^{1999} \equiv 40 \pmod{7}$



$40^{1999} \equiv 5 \pmod{7}$



The remainder of $40^{1999}$ divided by $7$ is $5$

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