Hi Friso,
I understand that you wish to plot the symbolic equations related to 'e'(eigenvector) against 'r' and 'theta'.
Please refer to the following workaround.
S = 5; %Define S;
a = 5; %Define a;
syms r theta
sigma_rr = (S/2)*(1-(a/r)^2)+(S/2)*(1-4*((a/r)^2)+3*(a/r)^4)*cos(2*theta);
sigma_00 = (S/2)*(1+(a/2)^2)-(S/2)*(1+3*(a/3)^4)*cos(2*theta);
sigma_0r = -(S/2)*(1+(2*(a/r)^2)-3*(a/r)^4)*sin(2*theta);
A = [sigma_rr, sigma_0r ; sigma_0r, sigma_00];
B = [cos(theta), sin(theta) ; -sin(theta), cos(theta)];
C = transpose(B);
D = C*B*A;
e = eig(D);
[V,E] = eig(D);
count = 0;
for i = [1 2]
subplot(2,1,i);
ezplot(e(i,1),[-10,10],[-2*pi,2*pi]);
end
