@article{IM_1967__3_141877,
     author = {S. CARTER and S.A. ROBERTSON},
     title = {Relations {Between} a {Manifold} and its {Focal} {Set.}},
     journal = {Inventiones mathematicae},
     pages = {300--307},
     publisher = {mathdoc},
     volume = {3},
     year = {1967},
     zbl = {0156.42301},
     url = {http://geodesic.mathdoc.fr/item/IM_1967__3_141877/}
}