Matlab eigenvectors of symbolic matrix not working

[Maxong091]

Hi all,

I'm trying to find the eigenvectors of a symbolic 3x3 rotation matrix in MATLAB, it appears to work for some inputs but not all, for example:

A =[ cos(q), -sin(q), 0]
[ sin(q), cos(q), 0]
[ 0, 0, 1]

[V,lambda]=eig(A)

V =[ 0, -i, i]
[ 0, 1, 1]
[ 1, 0, 0]

lambda =[ 1, 0, 0]
[ 0, cos(q) - sin(q)*i, 0]
[ 0, 0, cos(q) + sin(q)*i]

this works fine, however if I try something which is not a rotation about a unit axis I get the following error:

A =[ cos(q), -sin(q), 0]
[ 0, 0, -1]
[ sin(q), cos(q), 0]

>> [V,lambda]=eig(A)
Warning: basis of eigenspace for eigenvalue cos(q)/3 - (cos(q)^2/3 + cos(q)^3/27 + sin(q)^2/2 + ((cos(q)/3 - cos(q)^2/9)^3 +
(cos(q)^3/27 + cos(q)^2/3 + sin(q)^2/2)^2)^(1/2))^(1/3)/2 + (cos(q)/3 - cos(q)^2/9)/(2*(cos(q)^2/3 + cos(q)^3/27 + sin(q)^2/2 +
((cos(q)/3 - cos(q)^2/9)^3 + (cos(q)^3/27 + cos(q)^2/... [linalg::eigenvectors]
? Error using ==> mupadmex
Error in MuPAD command: Unable to find explicit eigenvectors.

Error in ==> sym.sym>sym.mupadmexnout at 2003
out = mupadmex(fcn,args{:});

Error in ==> sym.eig at 68
[V,D,p] = mupadmexnout('mllib::eigenvectors',A);


Any help would be appreciated
Thanks
Max

 

[viscousflow]

eig does not support symbolic. Consider using maple. Or wolfram Alpha http://www.wolframalpha.com/