A curve is simple if it does not cross itself.
2529: Unsolved Math Problems - explain xkcd
Chapter 2 Pi in Mathematics and the Physical World in: Pi (π) in Nature, Art, and Culture
Surface Of Constant Width, curve Of Constant Width, franz Reuleaux, Curvilinear, Reuleaux triangle, Asymptote, wolfram Research, mathworld, curve, shape
The Spirograph and kinematic models: Making math touchable (and pretty)
Simple Curve -- from Wolfram MathWorld
Curvature, Riemannian, Non-Euclidean & Manifolds
How to graph the shape of a heart on a graphing calculator using multiple equations and on what type of calculators can you do this - Quora
Symbolizing Collective Remembering Otherwise
Finally We May Have a Path to the Fundamental Theory of Physics… and It's Beautiful—Stephen Wolfram Writings
Fourier Series – Mathematical Mysteries