Self-Intersecting Periodic Curves in the Plane

Abstract

Suppose a smooth planar curve $\gamma$ is $2\pi$-periodic in the $x$ direction and the length of one period is $\ell$. It is shown that if $\gamma$ self-intersects, then it has a segment of length $\ell- 2\pi$ on which it self-intersects and somewhere its curvature is at least $2\pi/(\ell - 2\pi)$. The proof involves the projection $\Gamma$ of $\gamma$ onto a cylinder. (The complex relation between $\gamma$ and $\Gamma$ was recently observed analytically by T. M. Apostol and M. A. Mnatsakanian. When $\gamma$ is in general position there is a bijection between self-intersection points of $\gamma$ modulo the periodicity, and self-intersection points of $\Gamma$ with winding number 0 around the cylinder. However, our proof depends on the observation that a loop in $\Gamma$ with winding number 1 leads to a self-intersection point of $\gamma$.