Read the hypocycloid
writeup first. The deltoid
is a plane algebraic curve
. It is the hypocycloid such that the radius
of the fixed
is 3 times the radius of the rolling circle, a
It looks like a triangle
where the edges of the triangle have been
curved toward the centre.
An equation for the deltoid is
(x2 + y2)2 - 8ax(x2 - 3y2) + 18a2(x2 + y2) = 27a4.