@article{oai:tsukuba.repo.nii.ac.jp:00015978, author = {Sakane, Yusuke and Naitoh, Hiroo}, issue = {1}, journal = {Tsukuba Journal of Mathematics}, month = {Jun}, pages = {143--152}, title = {On conjugate points of a nilpotent Lie group}, volume = {5}, year = {1981} }