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DOUBLEHEART CURVE
Curve studied by Grégoire de Saint
Vincent in 1647 and Cramer in 1750.
Homemade nickname: rabbit's ears. 
Cartesian equation:
, i.e. .
Quartic of genus 1. 
The double hearts are the polyzomal curves associated to two tangent circles, here the circles and . 

They are obtained by the slidercrank mechanism as follows: 

Opposite, photograph of a real mechanism tracing this curve thanks to this method.  
Another curve shaped like rabbit's ears, due to Sébastien Kerner: 

Not to be mistaken for the Viviani curve, including the regular bifolium. They are rational quartics. 

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© Robert FERRÉOL, Jacques MANDONNET 2017