https://www.mersenne.org/primes/ https://www.mersenne.org/primes/ For Mersennes at least, which right now are the largest primes. A prime is known once you've

https://www.mersenne.org/primes/ https://www.mersenne.org/primes/ For Mersennes at least, which right now are the largest primes. A prime is known once you've shown it is prime, i.e. no factors other than 1 and itself.