Self-intersection numbers of length-equivalent curves on surfaces

Abstract

Two free homotopy classes of closed curves in an orientable surface with negative Euler characteristic are said to be length equivalent if for any hyperbolic structure on the surface, the length of the geodesic in one class is equal to the length of the geodesic in the other class. We show that there are elements in the free group of two generators that are length equivalent and have different self-intersection numbers as elements in the fundamental group of the punctured torus and as elements in the pair of pants. This result answers open questions about length equivalence classes and raises new ones.