Wikipedia
Quadrifolium
The quadrifolium (also known as four-leaved clover) is a type of rose curve with n=2. It has the polar equation:
r = cos(2θ),
with corresponding algebraic equation
(x + y) = (x − y).
Rotated by 45°, this becomes
r = sin(2θ)
with corresponding algebraic equation
(x + y) = 4xy.
In either form, it is a plane algebraic curve of genus zero.
The dual curve to the quadrifolium is
(x − y) + 837(x + y) + 108xy = 16(x + 7y)(y + 7x)(x + y) + 729(x + y).
The area inside the curve is $\tfrac 12 \pi$, which is exactly half of the area of the circumcircle of the quadrifolium. The length of the curve is about 9.6884.