Tranguloid TrefoilRendered by Paul BourkeSurface by Roger Bagula April 2002 

x = 2 sin(3 u) / (2 + cos(v))
y = 2 (sin(u) + 2 sin(2 u)) / (2 + cos(v + 2 pi / 3)) z = (cos(u)  2 cos(2 u)) (2 + cos(v)) (2 + cos(v + 2 pi / 3)) / 4
pi <= u <= pi 