Concept 3 of 8Foundation
Watch on YouTubeVideoSieve of Eratosthenes — finding primes
Ancient Greek trick to find all primes up to any number:
- Write all numbers from 2 to N.
- Circle 2, cross out all other multiples of 2.
- Circle the next un-crossed (3), cross out its multiples.
- Repeat until you reach √N. Remaining circled numbers are all primes.
Example
Primes up to 30: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 — exactly 10 primes.
💡 Tip:Stop crossing at √N. Anything left is automatically prime.
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5 questions to check what you just read.
0 / 5
- Q1.Number of primes between 1 and 20:
- Q2.Smallest prime greater than 30:
- Q3.Primes between 20 and 30:
- Q4.Is 49 prime?
- Q5.When checking if 91 is prime, trial divide up to: