L22_A

       1. Proposition: A Connected Surface with Every Point

           Being Umbilical Point Is a Piece of Plane or Sphere (cont.)

L22_B

       1. Proposition: A Connected Surface with Every Point

           Being Umbilical Point Is a Piece of Plane or Sphere (cont.)

       2. Definition: Asymptotic Direction and Asymptotic Curve

       3. Example: Straight Line

L22_C

       1. Example: Straight Line (cont.) and Curve with Positive Curvature

       2. Observation: There Is NO Asymptotic Direction at an Elliptic Point

       3. Definition: Dupin Indicatrix