L17_A
        1. Example: Is There a Regular Surface with the Given
            Differentiable Functions E, F, G, e, f, g (cont.)
        2. Covariant Derivative

L17_B
        1. General Formula of the Covariant Derivative
        2. Example: Covariant Derivate of a Vector Field on a
            Plane
        3. Parallel Vector Field
        4. Proposition: There Exists a Unique Parallel Vector
            Field along a Curve with Given Initial Value

L17_C
        1. Proposition: The Inner Product of Two Parallel Vector
            Fields Is Constant 
        2. Example: The Tangent Vector Field of a Meridian Is a
            Parallel Vector Field    on a Sphere