 
The bifolium ^{1)} has been studied by Longchamps
(1886) and Brocard (1887), and is also named the double
folium.
The bifolium can be constructed as
follows.
Given a circle C through O, A(0, a) and B(1, 0), and a line m through O that
cuts C in Q. Let R be the projection of Q on the xaxis, and P the projection of Q on m.
Then the bifolium is the collection of points P, where m varies in the given circle C.
Special cases are:
The bifolium for a and for a are each other's reflection in the xaxis.
notes
1) In German: Zweiblatt.
