@article{MM2_1977__20_154447,
     author = {W.Wilson Stothers},
     title = {Subgroups of the (2,3,7) {Triangle} {Group.}},
     journal = {Manuscripta mathematica},
     pages = {323--334},
     year = {1977},
     volume = {20},
     zbl = {0355.20046},
     url = {http://geodesic.mathdoc.fr/item/MM2_1977__20_154447/}
}