Deltoid

   x = a(2cos t + cos 2t) 

y = a(2sin t - sin 2t)

References

• J. Dennis Lawrence, A Catalog of Special Plane Curves, pp. 131-135.

• 2d Curves : http://www.2dcurves.com/roulette/rouletted.html

• Famous Curves Index : http://www-gap.dcs.st-and.ac.uk/~history/Curves/Tricuspoid.html