L4A 
       1.Theorem: A Compact Surface Has an Elliptic Point
 
L4B
       1.Theorem: A Compact Connected Surface with Constant
          Gauss Curvature Is a Sphere
       2.Lemma: Three Conditions of a Point to Be an Umbilical Point
       3.Proof: A Compact Connected Surface with Constant Gauss Curvature Is
          a Sphere
 
L4C
       1.Proof: A Compact Connected Surface with Constant Gauss Curvature
          Is a Sphere (cont.)
       2.Proof: Three Conditions of a Point to Be an Umbilical Point