L21A
       1. Definition: Gauss Curvature and Mean Curvature
       2. Definition: Elliptic, Hyperbolic, Planar and Parabolic Point
 
L21B
       1. Definition: Umbilical Point
       2. Example: Study the Gauss Map on {2z=x^2＋y^2} at (0,0,0)
 
L21C
       1. Proposition: A Connected Surface with Every Point Being Umbilical
           Point Is a Piece of Plane or Sphere