Eratosthenes Sieve for Prime Numbers

March 5, 2015

In Algebra 1B, students have been working on factoring.  In order to assist them with finding factors, they used Eratosthenes Sieve to find all the prime numbers from 1 to 100.  A sieve is a strainer.  Eratosthenes came up with a method for straining out the composite number in such a way that all that remained were the prime numbers.  A prime number is only divisible by itself and one.  By using the image above, the students know that the first prime number is two.  They circle the two and then cross off every second number.  The next prime number is three.  They circle the three and then cross off every third number.  They continue this process until they have cycled through the numbers.  The numbers that are left are all the prime numbers.