The "trefoil" curve is the name given by Cundy and Rollett (1989, p. 72) to the quartic plane curve given by the equation
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(1)
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As such, it is a simply a modification of Kepler's folium with 
and 
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(2)
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obtained by dropping the coefficients 2.
The area enclosed by the trefoil curve is
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(3)
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the geometric centroid 
of the enclosed region is
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(4)
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(5)
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and the area moment of inertia elements by
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(6)
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(7)
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(8)
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(E. Weisstein, Feb 3, 2018).
