Register to reply 
Sum of all possible products of elements taken from couples 
Share this thread: 
#1
Dec1313, 12:11 PM

P: 12

Hello
I have N couples of real numbers higher than 1. Let's call them like (a0,b0), (a1,b1),...,(aN,bN) I have a number R <= N. I need the sum of all the possible products of N elements, chosing one from each couple but exactly R times the "b" element and NR times the "a" element. Which is the best way to do it? As an example: (2,3), (5,7), (11,13) N = 3, R = 2 I need 2x7x13 + 3x5x13 + 3x7x11 Thank you! 


#2
Dec1313, 12:45 PM

P: 1,287

As an expression I think what you want to do is:
[itex]\Sigma^{N}_{k=0} (a_{k}(\Sigma^{N}_{i=0} b_{i}))[/itex] I have no idea if there is any way to compute this other than just doing it. EDIT: Nevermind, I see you don't want "sum of all possible products of N+1 elements" but sum of all possible products of a choice of R elements from the N+1 elements. No idea, you're probably going to have to write a program for that. 


#3
Dec1713, 01:24 AM

P: 12

I got the answer from "Michael":
It is the coefficient of x^R in (a0+xb0)(a1+xb1)...(aN+xbN) 


Register to reply 
Related Discussions  
Converting volume elements to area elements  Calculus & Beyond Homework  2  
Question: Can elements above iron actually be clusters of smaller elements?  Atomic, Solid State, Comp. Physics  3  
Kronecker product on only a few elements in a matrix: How to align resulting elements  Linear & Abstract Algebra  0  
Orders of products of group elements.  Linear & Abstract Algebra  4  
Questions concerning cross products, dot products, and polar coordinates  Introductory Physics Homework  1 