What is a prime number?

The first primes are 2, 3, 5, 7, 11 and 13; 2 is the only even prime. Numbers that are not prime, other than 1, are called composite, and every whole number greater than 1 can be written as a unique product of primes. Around 300 BC, Euclid proved that there are infinitely many primes. Primes are essential to modern cryptography: systems such as RSA rely on the fact that multiplying two large primes is easy but factoring the result back is extremely hard. The largest known prime, found in 2024, has more than 41 million digits.