Fermat's Little Theorem

p is a prime, a be any integer, $a\not\equiv 0(\mathrm{mod}p)$
We have $a^{p-1}\equiv 1(\mathrm{mod}p)$

Example

$3^{2018}(\mathrm{mod}17)=3^{126\ast 16+2}=3^2=9$

It is a special case for Euler phi