function [x] = plotellipse(A,x0,c)
% determine the points to plot an ellispe in two dimensions, 
% described by (x-x0)'*A*(x-x0) = c, where A is symmetric

dtheta = 0.1;
[u,d] = eig(A);
x = [];
d = inv(sqrt(d));
for theta = 0:dtheta:2*pi
  w = sqrt(c)*[cos(theta); sin(theta)];
  z = d*w;
  x = [x u*z + x0];
end
x = [x x(:,1)];