Principal Curve
A curve 
on a Regular Surface 
is a principal curve Iff the velocity 
always pointsin a Principal Direction, i.e.,
where
is the Shape Operator and 
is a Principal Curvature. If aSurface of Revolution generated by a plane curve is a Regular Surface, then the Meridians and Parallels are principal curves.
References
Gray, A. ``Principal Curves'' and ``The Differential Equation for the Principal Curves.'' §18.1 and 21.1 in Modern Differential Geometry of Curves and Surfaces. Boca Raton, FL: CRC Press, pp. 410-413, 1993.
- Lune (Surface)
- Lunule
- Lusin's Theorem
- LUX Method
- Lyapunov Characteristic Exponent
- Lyapunov Characteristic Number
- Lyapunov Condition
- Lyapunov Dimension
- Lyapunov Function
- Lyapunov's First Theorem
- Lyapunov's Second Theorem
- Lyndon Word
- Lyons Group
- Léon Anne's Theorem
- Lévy Constant
- Lévy Distribution
- Lévy Flight
- Lévy Fractal
- Lévy Function
- Lévy Tapestry
- Löwenheim-Skolem Theorem
- Lüroth's Theorem
- Machine
- Machin-Like Formulas
- Machin's Formula