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Multiply Together, Get Cubic (Posted on 2007-06-21) Difficulty: 4 of 5
Determine all possible integer pairs (p,q) such that p+q+s=pqs, where s=gcd(p,q) and gcd denotes the greatest common divisor.

No Solution Yet Submitted by K Sengupta    
Rating: 3.0000 (1 votes)

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Some Thoughts Some steps? | Comment 3 of 8 |
If gcd(p,q)=s, then we can write p=as, q=bs, with gcd(a,b)=1. Replacing, as+bs+s= abs, so a+bs+s= abs, and then (ab-1)s-bs-a=0. The positive solution is (b+√(b^4+4a(ab-1)))/(2ab-2), so necessarily b^4+4a(ab-1) must equal r for integer r. (This is not a sufficient condition, by the way.)

I checked Charlie's results, and fortunately they agree with this condition, but I couldn't solve it directly, or prove that there are no more possibilities.

Edited on June 21, 2007, 11:18 am
  Posted by Federico Kereki on 2007-06-21 11:17:35

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