The online computer game “Is this prime?” tests a player’s knowledge of prime numbers—and just surpassed 2,999,999 attempts. Give it a whirl.

In one of the most hilarious papers we have read this year, two scientists challenged some famous quantum factorization ...

The smallest such number is 561: For any integer b, b561 – b is always a multiple of 561, even though 561 is not prime.

So 2, 3, 5, 7, 11, and so on are prime numbers. The number 12, for example, wouldn’t be prime because you can divide it by other numbers, like 2 and 3. And as you count up and up, prime numbers become ...