@tomleslie 

Thank you very much.
Could you explain this part of the code:

 r:=0.1:
  display( [ plot3d( [f,T], t = pt[1]-r .. pt[1]+r, u = pt[2]-r .. pt[2]+r, color = [red, green]),
             pointplot3d( [pt[], eval(T, [t=pt[1], u=pt[2]])], symbol=solidsphere, symbolsize=20, color=blue)
           ]
         );

Appreciate it!