Curves of Lamé
These are curves of Lamé, generalizations of the circle given by
the formula xn + yn = 1.
When n = 2, you get a circle; when n is a bit greater
than 2, you get Piet Hein’s “supercircles”. He liked the effect you
got when n was roughly e, which is the default here, but
you can change the exponent in the drop-down box to see the different
kinds of curves you can get.