Dupin's Indicatrix
A pair of conics obtained by expanding an equation in Monge's Form 
in a Maclaurin Series
This gives the equation
Amazingly, the radius of the indicatrix in any direction is equal to the Square Root of the Radius ofCurvature in that direction (Coxeter 1969).
References
Coxeter, H. S. M. ``Dupin's Indicatrix'' §19.8 in Introduction to Geometry, 2nd ed. New York: Wiley, pp. 363-365, 1969.
- Heptakaidecagon
- Heptaparallelohedron
- Heptomino
- Herbrand's Theorem
- Hereditary Representation
- Heredity
- Hermann Grid Illusion
- Hermann-Hering Illusion
- Hermann-Mauguin Symbol
- Hermann's Formula
- Hermite Constants
- Hermite Differential Equation
- Hermite-Gauss Quadrature
- Hermite Interpolation
- Hermite-Lindemann Theorem
- Hermite Polynomial
- Hermite Quadrature
- Hermite's Interpolating Polynomial
- Hermite's Theorem
- Hermitian Form
- Hermitian Matrix
- Hermitian Operator
- Hermit Point
- Heronian Tetrahedron
- Heronian Triangle