In his monograph (D. V. Anosov: Geodesic flows on closed Riemann manifolds with negative curvature, Proceedings of the Steklov Institute 90 (1967) AMS 1969), Anosov writes: "Every five years or so, if not more often, someone 'discovers' the theorem of Hadamard and Perron proving it either by Hadamard's method or Perron's. I myself have been guilty of this."