@article{oai:repository.dl.itc.u-tokyo.ac.jp:00039857, author = {Takeuchi, Kisao}, issue = {1}, journal = {Journal of the Faculty of Science, University of Tokyo. Sect. 1, Mathematics, astronomy, physics, chemistry = 東京大學理學部紀要. 第1類, 數學, 天文學, 物理學, 化學}, month = {Aug}, note = {application/pdf}, pages = {97--100}, title = {On some discrete subgroups of $SL_2 (R)$}, volume = {16}, year = {1969} }