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.

A basic feature of number theory, prime numbers are also a fundamental building block of computer science, from hashtables to cryptography.

Prime numbers are a central topic of study in math. Despite being an object of fascination for millennia, there are still a lot of unsolved problems involving primes. Prime numbers are one of the ...