Thermostats without conjugate points
This paper generalizes Hopf's theorem to thermostats by establishing that non-positive total curvature characterizes systems without conjugate points, proving the equivalence between transverse Green bundles and projective Anosov flows, and providing a counterexample to Hopf's rigidity on the 2-torus via the first known instance of a projectively Anosov thermostat that is not Anosov.