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Double Sum Using Matlab 
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#1
Feb2112, 10:15 PM

P: 20

Hi,
I have this double summation expression to solve as part of matlab code I am writing. I have searched matlab no syntax that can do it. Please assist. Q=ƩƩxixjaij i.e double sum of xi xj aij, i=1:n, j=1:n Please assist me Thanks adeeyo 


#2
Feb2212, 03:44 PM

Sci Advisor
P: 6,077

What do mean by solve? What you have written is simply an expression. To solve something you need to describe what is known and what you are trying to find.



#3
Feb2212, 05:12 PM

P: 20

Thanks Mathman,
What I mean is this. I wrote a matlab code for that expression and manual as seen below but got different answer. I don't know what is wrong either with matlab code or manual expression or both. Q=∑∑xixjaij the first sigma has i=1:n, the second sigma j=1:n for i=1:n for j=1:n Q=sum(sum(x(i).*x(j).*(a(i, j)))); end end Mannual Q=x(1)*x(1)*(a(1)*a(1))+x(1)*x(2)*(a(1)*a(2))+x(1)*x(3)*a(1)*a(3)+x(2)* x(1)*a(2)*a(1))+x(2)*x(2)*a(2)*a(2))+x(2)*x(3)*a(2)*a(3)*+x(3)*x(1)*a(3 )*a(1))+x(3)* x(2)*a(3)*a(2))+x(3)*x(3)*a(3)*a(3)); Thanks 


#4
Feb2312, 03:45 PM

Sci Advisor
P: 6,077

Double Sum Using Matlab
From what I recall about coding: You need to set Q = 0.0 before starting. Then the operating instruction should be Q=Q+x(i)*x(j)*a(i, j). To save a little time you could multiply by x(i) outside the j loop.



#5
Feb2412, 04:57 PM

P: 35

if a is nxn and x is nx1 try Q = sum(sum(a.*repmat(x,[1 n]).*repmat(x',[n 1])))



#6
Feb2412, 05:03 PM

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P: 4,500

i=1, j=1: sum(sum(x(1)*x(1)*a(1,1))) = x(1)*x(1)*a(1,1) so Q=x(1)*x(1)*a(1,1) i=1,j=2: sum(sum(x(1)*x(2)*a(1,2))) = x(1)*x(2)*a(1,2) so Q=x(1)*x(2)*a(1,2) NOT x(1)*x(1)*a(1,1)+x(1)*x(2)*a(1,2) If you replace the line with Q=Q+x(i)*x(j)*a(i,j) you should get the right answer 


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