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.
- Dilworth's Lemma
- Dilworth's Theorem
- Dimension
- Dimensionality Theorem
- Dimension Axiom
- Dimension Invariance Theorem
- Diminished Polyhedron
- Diminished Rhombicosidodecahedron
- Dini Expansion
- Dini's Surface
- Dini's Test
- Dinitz Problem
- Diocles's Cissoid
- Diophantine Equation
- Diophantine Equation--10th Powers
- Diophantine Equation--5th Powers
- Diophantine Equation--6th Powers
- Diophantine Equation--7th Powers
- Diophantine Equation--8th Powers
- Diophantine Equation--9th Powers
- Diophantine Equation--Cubic
- Diophantine Equation--Linear
- Diophantine Equation nth Powers
- Diophantine Equation--Quadratic
- Diophantine Equation--Quartic