The level-set flow of the topologist's sine curve is smooth

In this note we prove that the level-set flow of the topologist's sine curve is a smooth closed curve. In previous work it was shown by the second author that under level-set flow, a locally-connected set in the plane evolves to be smooth, either as a curve or as a positive area region bounded by smooth curves. Here we give the first example of a domain whose boundary is not locally-connected for which the level-set flow is instantaneously smooth. Our methods also produce an example of a non path-connected set that instantly evolves into a smooth closed curve.