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PIRIFORM QUARTIC
Curve studied by Wallis in 1685 and Bonnet in 1844.
From the Latin Pirum "pear". Other names: drop of water, pegtop. 
Cartesian equation:
, or .
For b = a / 2, the equation can be written:. Cartesian parametrization: with , . Rational quartic. Area: . 
Given a point P describing the circle (C)
with diameter [OA] (where A is the point with coordinates
(a, 0)), let Q be the point on the line x = b with
same ordinate as P. The piriform quartic is the locus of the point
M on the line (OQ) with same abscissa as P.
In other words, the piriform quartics are the antihyperbolisms of the circle with respect to a point O on this circle and a line perpendicular to the diameter one end of which is O. 
Note that the piriform quartics for any value of b
are images of the curve obtained when a = b by a scaling
along Oy.
They are special cases of tear
curves.
Up to scaling, the piriform quartic is a plane projection of the pancake curve x=cos(u),y=sin(u),z=sin(2u). 
A tube with a bore shaped like a piriform quartic is a representation of the Klein bottle.
See also the double drop of water, the kieroids, and the spherical cycloids.
See also here
the "true" profile of the drop of water.



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© Robert FERRÉOL 2017