

disp(' ')
 disp(' A case of a critical point where the discriminat = 0 ')
 disp('  ')
 disp('        f(x,y) = x^3 - xy^2   ')
 
 f = inline('x.^3 - x.*y.^2  ');
 surf(X,Y,f(X,Y)); shading interp;
 hold on
 levels = linspace(-6,6, 21);
 contour3(X,Y,f(X,Y), levels,'k')
 rotate3d on
 pause

 figure
 contour(X,Y,f(X,Y), levels)
 disp(' Put in a another quarter to continue  ')
 pause
 close all

