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@article{MS2_1981__48_166736, author = {G\"unter K\"ohler}, title = {On {Hsiao's} conjecture on {Hecke} groups.}, journal = {Mathematica Scandinavica}, pages = {56--58}, publisher = {mathdoc}, volume = {48}, year = {1981}, zbl = {0463.10013}, url = {http://geodesic.mathdoc.fr/item/MS2_1981__48_166736/} }
Günter Köhler. On Hsiao's conjecture on Hecke groups.. Mathematica Scandinavica, Tome 48 (1981), pp. 56-58. http://geodesic.mathdoc.fr/item/MS2_1981__48_166736/