On tori without conjugate points
The authors prove that a Riemannian metric on the \(n\)-torus is flat if it does not have conjugate points and the corresponding Heber foliation is Lipschitz. This is a special case of the so-called Hopf conjecture, which was proved recently by \textit{D. Burago} and \textit{S. Ivanov} [Geom. Funct. Anal. 4, No. 3, 259-269 (1994; Zbl 0808.53038)].
scientific article; zbMATH DE number 784001
scientific article