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symbolic solver question
Posted:
Dec 12, 2012 6:04 AM


Here is my matlab session:
>> syms s a A >> A = [0 0 2*a a; 0 0 a 2*a; 1 0 0 0; 0 1 0 0]; >> eig(A) ans = (a)^(1/2) (a)^(1/2) 3^(1/2)*(a)^(1/2) 3^(1/2)*(a)^(1/2)
OK, so far so good. I asked for the eigenvalues of the matrix A and it shows the 4 eigenvalues that I expected. So then for the sake of understanding the solver, I though I would try to compute the eigenvalues by setting the determinant of (sIA) equal to zero: >> solve('det(s*eye(4)A) = 0',s) Warning: Explicit solution could not be found.
Why doesn't this work? After some experimentation I found this works: >> solve([char(det(s*eye(4)A)) '= 0'],s) ans = (a)^(1/2) (a)^(1/2) 3^(1/2)*(a)^(1/2) 3^(1/2)*(a)^(1/2)
So as you can see the solver can compute the eigenvalues correctly, but this form of having to convert the determinant to a character array seems kind of awkward. Is that necessary, or is there a simpler way of entering this equation into the solver?
Thanks ~Paul



