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LEMNISCATE OF GERONO, or EIGHT
Curve studied by Grégoire de St Vincent in 1647
and Cramer in 1750.
Name given by Aubry in 1895. CamilleChristophe Gerono, 1799  1891: French mathematician. 

Cartesian parametrization:
().
Cartesian equation:
or or
also .

The lemniscate
of Gerono is a special case of besace
(see this page for a construction) and of Lissajous
curve (cf. the parametrization: ).
The lemniscate of Gerono is the antihyperbolism of a circle with respect to its centre and a tangent. 

It can also be obtained by the Newton transformation from two tangent circles, as illustrated opposite: 

Another construction, due to L. I. Magnus: while describing a circle, a point P is projected on Q on a diameter, the point Q is projected on R on (OM); the lemniscate of Gerono is the locus of the point M of [PQ] such that QM = QR. 

The equation shows that it can be obtained as a polyzomal curve, median of the parabolas and . 

As are all Lissajous curves, the lemniscate of Gerono
is the projection of two sinusoidal
crowns:
1) Projection on xOy of the pancake
curve, parametrized by: .
2) Projection on xOy of the Viviani
window: .
More generally, the lemniscate of Gerono is a view of
the horse fetter,
intersection of a sphere and a tangent cylinder.
The lemniscate of Gerono can be obtained from the lemniscate of Bernoulli in the following way: trace on the sphere with centre O and radius a the curve (Viviani's curve) the south pole stereographic projection of which is the lemniscate of Bernoulli, and project this curve perpendicularly on xOy. 

A difference between the lemniscate of Gerono and that of Bernoulli: the first one has six vertices (4 maxima of curvature and two minima) as the opposite view with its evolute shows; the lemniscate of Bernoulli only has two vertices, at both ends. 

See also:
 the Tannery pear, rotation of a
halfeight around its axis.
 the Klein
bottle, a representation of which is based on an eight, as well as
the pseudo crosscap.
Rotation surfaces around the axes of symmetry.
1) Around the axis passing through the vertices,
equation, and Cartesian parametrization: ,. 

2) Around the other axis,
equation, and Cartesian parametrization: , .


If the equation is changed into , we get the red curve opposite, with polar equation , and parametrization: . Its reunion with the lemniscate of Gerono gives a curve close to the bow tie.  
If is changed into we also get a curve with the shape of an eight, also parametrized by . 

The image by a dilatation of this lemniscate, is sometimes named "lemniscate of Montferrier" ; cartesian equation : . On the right for a = 2b. 

See here how
to "thicken" an eight: .
See also the biaxial inverse
of an eight.
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© Robert FERRÉOL 2022