July 14th, 2013, 12:15 AM  #1 
Newbie Joined: Jun 2012 Posts: 13 Thanks: 0  specific search
I have two products P1 = x1*x2*x3*...xn P2 = y1*y2*y3* ...yn xi,yi integers I want to make abs(P1/P2  1) the best smallest value by changing only one pair xi,yj in the products. f.e. x1*y3*x3.... and y1*y2*x2* ... I have to search for the best pair. I can do that by calculating all the possible values P'1,P'2 by taking all the possible pairs but I want to do that without doing the multiplications, perhaps by examining all the values xi/yj. How can I search for the best xi.yj. Thanks for any resonse 
July 14th, 2013, 05:14 AM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: specific search
You want to swap one pair, right? Take the quotient Q = P1/P2. If you swap xi and yj you get Q' = Q * yj^2/xi^2, and so you want yj^2/xi^2 to be as close as possible to 1/Q and thus xi/yj to be as close as possible to sqrt(Q). For each x, find the y closest (geometrically) to x/sqrt(Q). The pair producing the smallest geometric distance is the right one to choose. 
July 15th, 2013, 05:30 AM  #3 
Newbie Joined: Jun 2012 Posts: 13 Thanks: 0  Re: specific search
Thanks, CRGreathouse !


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