 Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2009/c/c1/ii/solution

p > 1

is a prime number if and only if

(p - 1)! \equiv - 1 (mod p) .

In particular this holds for every prime, including

p = 2

 New to topics? Read the docs here!