# Difference between revisions of "Talk:Prime numbers"

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why not use <hask>(\x -> n > x*x)</hask> --[[User:JohannesAhlmann|Johannes Ahlmann]] 21:18, 5 February 2007 (UTC) |
why not use <hask>(\x -> n > x*x)</hask> --[[User:JohannesAhlmann|Johannes Ahlmann]] 21:18, 5 February 2007 (UTC) |
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+ | LOL! That is ''indeed'' what I meant. |
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+ | It turns out my comment above is correct - the <hask>takeWhile</hask> filtering in <hask>factors</hask> is in fact unecessary. The function works just fine without it. (Notice I have made some edits to correct the multiple bugs in the <hask>primes</hask> function. Oops!) |
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+ | Now the only use of <hask>takeWhile</hask> is in the <hask>is_prime</hask> function, which could be changed to 'give up' the search a lot faster and hence confirm large primes with much less CPU time and RAM usage. Maybe I'll wrap my brain around that later. |
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+ | [[User:MathematicalOrchid|MathematicalOrchid]] 10:17, 6 February 2007 (UTC) |

## Revision as of 10:17, 6 February 2007

Here's an interesting question: will the program go faster if we replace all those `(n >)`

expressions with `(\x -> floor (sqrt n) > x)`

?

On one hand, a composite integer cannot possess a factor greater than its square root.

On the other hand, since the list we're looking through contains all possible prime numbers, we are guaranteed to find a factor or an exact match eventually, so do we need the `takeWhile`

at all?

Throwing this over to somebody with a bigger brain than me...

MathematicalOrchid 16:41, 5 February 2007 (UTC)

a composite can indeed have factors greater than its square root, and indeed most do. what you mean is that a composite will definitely have at least one factor smaller-equal than its square root.

why not use `(\x -> n > x*x)`

--Johannes Ahlmann 21:18, 5 February 2007 (UTC)

LOL! That is *indeed* what I meant.

It turns out my comment above is correct - the `takeWhile`

filtering in `factors`

is in fact unecessary. The function works just fine without it. (Notice I have made some edits to correct the multiple bugs in the `primes`

function. Oops!)

Now the only use of `takeWhile`

is in the `is_prime`

function, which could be changed to 'give up' the search a lot faster and hence confirm large primes with much less CPU time and RAM usage. Maybe I'll wrap my brain around that later.

MathematicalOrchid 10:17, 6 February 2007 (UTC)