Eratosthenes Sieve, is used to quickly find Prime numbers. He wrote it 250 bc.
For numbers less than 10,000,000 is purported to be faster to generate the numbers rather than find them in a file. 

sub Eratosthenes_Sieve($) {
for(@a=(1)x($_[0]+1),$p=2;$p**2<$_[0];){for($j=2*$p;$j<$_
[0];$j+=$p){$a[$j]=0}for($a[++$p]){}}grep{$a[$_]}(2..$#a)
}

[download]

I've made it tricky to read, and concise, I just loved the concept.
I'm not saying give up using Pari ;), it just amazed me that this equation was found two and a quarter millenia ago, ain't Maths cool.
Don't worry your RSA keys are safe, 32 Bit encryption will render:
four thousand, two hundred and ninetyfour million nine hundred and sixtyseven thousand two hundred and ninetysix digits ;)

Update: It's been done before :(, needless to say far better too, especially meowchow..

--

Brother Frankus.

¤

In reply to Eratosthenes Sieve by frankus

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