@article{oai:tsukuba.repo.nii.ac.jp:00016242, author = {Innami, Nobuhiro}, issue = {1}, journal = {Tsukuba Journal of Mathematics}, month = {Jun}, pages = {35--39}, title = {A note on symmetry of perpendicularity in a G-space with nonpositive curvature}, volume = {6}, year = {1982} }