Witch of Agnesi

Cartesian equation: y(x^2+a^2) = a^3 or x = at, y = a/(1+t^2)

Click below to see one of the Associated curves.

Definitions of the Associated curves

Evolute       Involute 1       Involute 2

 Inverse curve wrt origin      Inverse wrt another circle

Pedal curve wrt origin      Pedal wrt another point

Negative pedal curve wrt origin      Negative pedal wrt another point

   Caustic wrt horizontal rays      Caustic curve wrt another point

This was studied and named 'versiera' (Italian for 'she-devil' or 'witch') by Maria Agnesi in 1748 in her book Istituzioni Analitiche . It is also known as 'Cubique d'Agnesi' or 'Agnésienne'. It is thought that Agnesi confused an old Italian word meaning 'free to move' with another meaning 'witch'.

The curve had been studied earlier by Fermat and Guido Grandi in 1703.

The curve lies between y = 0 and y = a. It has points of inflection at y = 3a/4. The line y = 0 is an asymptote to the curve.

The curve can be considered as the locus of a point P defined as follows. Draw a circle C with centre at (0,a/2) through O. Draw a line from O cutting C at L and the line y = a at M. Then P has the x-coordinate of M and the y-coordinate of L.

The tangent to the Witch of Agnesi at the point with parameter p is

(p^2+1)^2y + 2px = a(3p^2+1).


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JOC/EFR February 1996