Tschirnhausen cubic

(Redirected from L'Hospital's Cubic)

In algebraic geometry, the Tschirnhausen cubic, or Tschirnhaus' cubic is a plane curve defined, in its left-opening form, by the polar equation

Tschirnhausen cubic, case of a = 1

where sec is the secant function.

History

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The curve was studied by von Tschirnhaus, de L'Hôpital, and Catalan. It was given the name Tschirnhausen cubic in a 1900 paper by Raymond Clare Archibald, though it is sometimes known as de L'Hôpital's cubic or the trisectrix of Catalan.

Other equations

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Put  . Then applying triple-angle formulas gives

 
 
 
 

giving a parametric form for the curve. The parameter t can be eliminated easily giving the Cartesian equation

 .

If the curve is translated horizontally by 8a and the signs of the variables are changed, the equations of the resulting right-opening curve are

 
 

and in Cartesian coordinates

 .

This gives the alternative polar form

 .

Generalization

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The Tschirnhausen cubic is a Sinusoidal spiral with n = −1/3.

References

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  • J. D. Lawrence, A Catalog of Special Plane Curves. New York: Dover, 1972, pp. 87-90.
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  • Weisstein, Eric W. "Tschirnhausen Cubic". MathWorld.
  • "Tschirnhaus' Cubic" at MacTutor History of Mathematics archive
  • Tschirnhausen cubic at mathcurve.com