The negative derivative

(1) |

(2) | |||

(3) |

At each point

(4) |

**References**

Gray, A. ``The Shape Operator,'' ``Calculation of the Shape Operator,'' and ``The Eigenvalues of the Shape
Operator.'' §14.1, 14.3, and 14.4 in *Modern Differential Geometry of Curves and Surfaces.*
Boca Raton, FL: CRC Press, pp. 268-269, 274-279, 1993.

Reckziegel, H. In *Mathematical Models from the Collections of Universities and Museums*
(Ed. G. Fischer). Braunschweig, Germany: Vieweg, p. 30, 1986.

© 1996-9

1999-05-26