Finding Primes

The sieve, and infinitely many primes

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• The Sieve of Eratosthenes finds primes by crossing out multiples.
• To test whether $n$ is prime, you only need to try primes up to $\sqrt{n}$.
• Euclid: there are infinitely many primes.

Steps

1. The Sieve of Eratosthenes
2. Why stop at 7?
3. Prime test
4. Do primes ever run out?
5. Primes below 100
6. A careful reading