References
- D. Burago and S. Ivanov, Riemannian tori without conjugate points are flat, Geom. Funct. Anal. 4 (1994), no. 3, 259–269.
- H. Busemann, The Geometry of Geodesics, Acad. Press, New York, 1955.
- K. Grove and K. Shiohama, A generalized sphere theorem, Annals of Math. 106, (1977), 201–211.
- E. Hopf, Closed surfaces without conjugate points, Proc. Nat. Acad. of Sci. 34 (1948), 47–51.