Back in 200 BC a Greek mathematician named Eratosthenes developed a simple step-by-step method

for determining which numbers are prime.

This method is called "Eratosthenes' Sieve".

The Eratosthenes' Sieve

A sieveis a strainer

that allows small particles to pass through

while catching the larger particles.

In the case of Eratosthenes' Sieve,

are the Prime Numbers.

the small particles that pass through

are all the non-prime numbers

and the large particles that remain

Eratosthenes

He was born in Cirene (actually Shahhat, Libia, which is in Africa)

He studied in Alexandria

Arquimedes was his friend

He died at the age of 80.

He was a great mathematician, philosopher,

poet and astronomer.

People knew him as the second Plato or

because he was always second in

every subject he studied.

His method to find out the prime numbers can be divided into 5 simple steps.

But wait a minute,

folks!

What are Prime Numbers?

Numbers Divisible Only by Themselves and 1

Any number that has only two factors, itself and 1.

Is the number 1 a prime number?

No, it isn´t. The number 1 is NOT considered Prime

Because it only has one factor , itself.

Why?

What is a sieve?

What can we do with Eratosthenes´ sieve?

Let´s do it!

We can find out which of the numbers from 1 to 100 are prime

Make a table of the numbers 1 to 100 organized into 10 rows

The Eratosthenes' Sieve - Step 1

The Eratosthenes' Sieve - Step 2

Cross out the number 1 because by definition it is NOT Prime number

The Eratosthenes' Sieve - Step 3

Circle the number 2 and cross out all the multiples of 2

(or all the even numbers)

The Eratosthenes' Sieve - Step 4

Look for the following number that is not crossed out. Circle the number and cross out all the multiples of that number.

The Eratosthenes' Sieve - Step 5

Repeat step 4 until you get to the number 7.

Finished?

All of the circled numbers are PRIME NUMBERS!

Now you can watch a simulation of the sieve...Pay attention!