function d=Hahn(t,y)
% Hahn(t,y) evaluates (a rescaled version of) the right side of the 
% 2-dimensional stability counter-example of Hahn (Stability of Motion, 
% pg. 191).  The rescaling speeds up the motion for both large and 
% small |y|.  The rescaling does not change the trajectories, 
% but the calculations are less likely to bog down numerically.

b=norm(y)*(1+norm(y)^2);
d=[y(1)^2 *(y(2)-y(1)) + y(2)^5;y(2)^2 * (y(2)-2*y(1))]/b;
% M. Day, May 8, 2012