MATLAB: Sigma notation with matrices

matricessymsum

I am trying to model a differential equation including , where C is a 4 by 4 matrix, i are rows, and j are vectors. The sigma notation is crucial towards using the interaction coefficient with the proper elements in the matrix. I don't at all know what I'm doing, and have been using symsum so far but have gotten absolutely nowhere. How do I code this without getting "Subscript indices must either be real positive integers or logicals errors"?

Best Answer

Taking a wild guess: 
x = rand(1,4);                                      % Create Vector
C = rand(4);                                        % Create Matrix
for i = 1:4
    for j = 1:4
        E(i,j) = C(i,j)*x(i)*x(j);                  % Multiplications
    end
end
S = sum(E(:));                                      % Summation
There are likely more efficient ways to do this, however I am choosing to keep this as transparent as possible. Note that symsum is not intended to do this type of summation.
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