@article{GDZPPN002086581,
title = {Relations Between a Manifold and its Focal Set.},
journal = {Inventiones mathematicae},
volume = {4},
year = {1967},
pages = {300-307},
zbl = {0156.42301},
url = {http://dml.mathdoc.fr/item/GDZPPN002086581}
}
CARTER, S.; ROBERTSON, S.A. Relations Between a Manifold and its Focal Set.. Inventiones mathematicae, Tome 4 (1967) pp. 300-307. http://gdmltest.u-ga.fr/item/GDZPPN002086581/