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 Discussion: All Topics Topic: Sieve of Eratosthenes Related Item: http://mathforum.org/mathtools/tool/411/

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 Subject: sieve of Eratosthenes Author: George Reese Date: Feb 22 2004
http://mathforum.org/mathtools/tool.html?id=411

Hi Jeanette,
As you've seen, it removes all the multiples of a number. So if you start with 2
and make your way up the numbers, you keep reaching primes (those numbers that
haven't been deleted). As you continue, the distance between numbers begins to
spread out, but each time your reach a number, you know it must be prime because
you have removed all the multiples of the ones before it.
Clear as mud? ;)
George

On Feb 22, 2004, Jeanette E. Horng wrote:

When I first saw "Sieve of Eratosthenes," I honestly was intrigued by the name
itself!  I wasn't so sure what this was and thought it may help me as a future
teacher to know it :)

So as I opened this tool, read the directions, and played around with the
buttons.  It seemed to be a good tool to help students understand where the
multiples of each number lies.

My first thought for applying this tool was to use it finding prime numbers.
Turns out that the purpose of the Sieve of Eratosthenes was to "initially
[assume] that all numbers are prime, and then [go] back and [mark] composite
numbers as not prime, using the fact that a factor of a prime cannot be prime"
(I found this on a random website).  I got somewhat confused and was wondering
how it shows that numbers are prime?  There may be a connection that I simply am
missing!

Thanks :)

Jeanette