We stated that the number 561 is a Carmichael number, but we never checked that a561 ≡ a (mod 561) for every value of a. (a) The number 561 factors as 3 · 11 · 17. First use Fermat’s little theorem to prove that a561 ≡ a (mod 3), a561 ≡ a (mod 11), and a561 ≡ a (mod 17) for every value of a.

Then explain why these three congruences imply that a561 ≡ a (mod 561) for every value of a.