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RE:[PrimeNumbers]Dumb Question

Please read:

x<=(n/2)+2

for:

x<(n/2)+2

Jon Perry

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-----Original Message-----

From: Jon Perry [mailto:perry@...]

Sent: 01 February 2002 18:38

To: Prime Numbers List

Subject: RE: [PrimeNumbers] Dumb question

If n=pq, and x=p+q, we can say x>=2sqrt(n), and x<(n/2)+2.

I don't think there is anything else we can infer.

>I suppose it's dumb because given the sum of the two

If 20=p+q, what are p and q? And from this, what is pq?

>factors it is trivial to work out the factors and so can't be done.

Jon Perry

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-----Original Message-----

From: David Litchfield [mailto:Mnemonix@...]

Sent: 01 February 2002 18:02

To: Prime Numbers List

Subject: [PrimeNumbers] Dumb question

This is probably going to sound like a really dumb question with the answer

being a resounding no, but, is there anyway, given a number which is the

result of multiplying two primes together, to ascertain what the sum of

those two factors is? I suppose it's dumb because given the sum of the two

factors it is trivial to work out the factors and so can't be done.

TIA,

David

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Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/ > If n=pq, and x=p+q, we can say x>=2sqrt(n), and x<(n/2)+2.

Hmm. Shame.

>

> I don't think there is anything else we can infer.

>

Well, the reason I say it's trivial is for this. Assume the number you've

> >I suppose it's dumb because given the sum of the two

> >factors it is trivial to work out the factors and so can't be done.

>

> If 20=p+q, what are p and q? And from this, what is pq?

been given is say 51 and we want to work out p and q.

If we knew p + q = 20 we could say

20 / 2 = 100

100 - 51 = 49

sqrt(100) = 10

sqrt(49) = 7

10 - 7 = 3

10 + 7 = 17

17 * 3 = 51

All numbers whose factors add up to give n share the same perfect squares as

per fermat

e.g. p + q = 20

3 * 17 = 51, 3 + 17 = 20, 20 / 2 = 100, 100 - 51 = 49

5 * 15 = 75, 5 + 15 = 20, 20 / 2 = 100, 100 - 75 = 25

7 * 13 = 91, 7 + 13 = 20, 20 / 2 = 100, 100 - 91 = 9

9 * 11 = 99, 9 + 11 = 20, 20 / 2 = 100, 100 - 99 = 1

Cheers,

David Litchfield