More on my love affair with primes. This one is a terrific, infallible test for prime numbers:
For any prime number P
(P-1)! + 1 = x
X is divisible by P (the original prime)
I’d never heard this one before. Made me sit up in my chair when the video started rolling. Primes always pass this test, composites always fail it. The video goes on to show a small group of primes where the result is divisible by P twice. Enjoy the vid and math on.