@article{10_4153_CMB_1970_006_x,
     author = {Butler, G. J. and Erbe, L. H. and Mathsen, R. M.},
     title = {On the {Non-Existence} of {Conjugate} {Points}},
     journal = {Canadian mathematical bulletin},
     pages = {31--37},
     year = {1970},
     volume = {13},
     number = {1},
     doi = {10.4153/CMB-1970-006-x},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CMB-1970-006-x/}
}