Sigma notation with matrices

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"?

Answer 1

[Star Strider]

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.

[Anna Singley]

Thanks so much! Even though this may not work perfectly, it's an improvement over brute-force typing out all possible permuatations.

[Star Strider]

As always, my pleasure!