@article{10_4153_CJM_1968_097_7,
     author = {Lowenthal, Franklin},
     title = {On {Subsemigroups} of the {Projective} {Group} on the {Line}},
     journal = {Canadian journal of mathematics},
     pages = {1001--1011},
     year = {1968},
     volume = {20},
     number = {1},
     doi = {10.4153/CJM-1968-097-7},
     url = {http://geodesic.mathdoc.fr/articles/10.4153/CJM-1968-097-7/}
}