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Dupin's Indicatrix


A pair of conics obtained by expanding an equation in Monge's form z=F(x,y) in a Maclaurin series

z=z(0,0)+z_1x+z_2y+1/2(z_(11)x^2+2z_(12)xy+z_(22)y^2)+...
(1)
=1/2(b_(11)x^2+2b_(12)xy+b_(22)y^2).
(2)

This gives the equation

 b_(11)x^2+2b_(12)xy+b_(22)y^2=+/-1.
(3)

Amazingly, the radius of the indicatrix in any direction is equal to the square root of the radius of curvature in that direction (Coxeter 1969).


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References

Coxeter, H. S. M. "Dupin's Indicatrix" §19.8 in Introduction to Geometry, 2nd ed. New York: Wiley, pp. 363-365, 1969.

Referenced on Wolfram|Alpha

Dupin's Indicatrix

Cite this as:

Weisstein, Eric W. "Dupin's Indicatrix." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/DupinsIndicatrix.html

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