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.
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