A topological proof of the Arnold four cusps theorem
Victor Anatolievich Vassiliev · Bulletin of the London Mathematical Society · 2011
The Arnold theorem (generalizing a consideration by Jacobi) states that on a generic Riemannian surface, which is sufficiently close to a sphere, the kth caustic of a generic point has at least four semi-cubical vertices. We prove this fact by the methods of the Morse theory; in particular, we replace the previous analytical condition of the ‘sufficient closeness to the sphere’ by a geometric one, which probably is considerably less restrictive.