All paths admit trajectoids

Abstract

In a recent paper published in Nature, Y.I. Sobolev et al. introduced the concept of trajectoids: convex, rigid objects, which roll without slip or spin on a flat plane along a prescribed periodic, unbounded planar path. A geometric construction method applicable to many paths was introduced, and the theory was experimentally verified using objects rolling downwards on slightly inclined planes. The construction method was applicable to many but not all curves. A possible extension of the method (referred to as period-n trajectoids) was also proposed, but the limits of applicability were not clarified. Here, a geometric proof is given for the existence of period-n trajectoids for any sufficiently smooth prescribed curve. A somewhat different proof was recently proposed by O. Muller independently from this work. We also highlight some related geometry problems.