LESSON 2 Sifting for Primes

Mr. Moreno's class was working on reports for mathematics class. The students got to work on their topic, prime numbers. Ana began, “I found some great information on a famous Greek mathematician named Eratosthenes.”

“Era . . . who?” asked Lin.

“You pronounce his name 'Air - uh - TAHS - thuh - neez,' ” replied Mr. Moreno.

Ana continued, “He was a librarian in Alexandria, Egypt. In about 240 BCE, he discovered a way to find prime numbers. They call his method the Sieve of Eratosthenes.

This book says that Eratosthenes had etched a list of consecutive numbers on a table made on a metal plate. He considered each number one by one and marked each of its multiples with a small hole. The numbers that were not marked were the prime numbers. In the end, it looked like a sieve. Since it was used to separate prime numbers from nonprime numbers, it got the name, Sieve of Eratosthenes. He presented this table to a king in Egypt.”