Envelopes, based on circles and rectangles

Inspired by Solving for Xmas: how to make mathematical Christmas cards

36 Triangles

36 points i around the shape each connected to points i +/- 12. On the circle, this results in all equilateral triangles.

Cardioid Shape

72 points i around the shape each connected to point i * 2 (mod 72).

Nephroid Shape

108 points i around the shape each connected to point i * 3 (mod 108).

i-Squared Shape

90 points i around the shape each connected to point i ^ 2 (mod 90).

i-Cubed Shape

180 points i around the shape each connected to point i ^ 3 (mod 180).

i ^ 4 Shape

18 points i around the shape each connected to point i ^ 4 (mod 18).