A point **p** on a Regular Surface is said to be planar if the Gaussian Curvature
and (where is the Shape Operator), or equivalently,
both of the Principal Curvatures and are 0.

**References**

Gray, A. *Modern Differential Geometry of Curves and Surfaces.* Boca Raton, FL: CRC Press, p. 280, 1993.

© 1996-9

1999-05-25