1. Coddington, E.A., Levinson, N.: Theory of Ordinary Differential Equations. McGraw-Hill Book Company Inc, New York (1955)
2. Eberlein, P.: When is a geodesic flow of Anosov type? I. J. Differ. Geom. 8, 437–463 (1973)
3. Graham, C.R., Guillarmou, C., Stefanov, P., Uhlmann, G.: X-ray transform and boundary rigidity for asymptotically hyperbolic manifolds. Ann. Inst. Fourier (Grenoble) 69(7), 2857–2919 (2019)
4. Guillarmou, C., Lassas, M., Tzou, L.: X-ray transform in asymptotically conic spaces. arXiv:1910.09631
5. Gulliver, R.: On the variety of manifolds without conjugate points. Trans. Am. Math. Soc. 210, 185–201 (1975)