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

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.

Today, people use complex computing networks to search for prime numbers with millions of digits. But early mathematicians were running these calculations by hand.

Math enthusiasts challenge one another to find special prime numbers, including those that are palindromes and Smarandache numbers ...