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Re: symbolic solver question
Posted:
Dec 12, 2012 9:22 AM


"Paul Mennen" <nospam@mennen.org> wrote in message news:ka9ob8$r68$1@newscl01ah.mathworks.com... > 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?
Because you're passing a string into SOLVE and so it can't "see" the variable A you've defined in your workspace. It's treating it as a general symbolic value. Depending on the version of MATLAB you're using, either:
solve(det(s*eye(4)A)==0, s)
or:
solve(det(s*eye(4)A), s)
should work. I would recommend using EIG instead of this SOLVE call particularly if you're going to work with larger matrices.
 Steve Lord slord@mathworks.com To contact Technical Support use the Contact Us link on http://www.mathworks.com



