@article{GDZPPN002067927, title = {Homomorphisms of constant stretch between M\"obius groups.}, journal = {Commentarii mathematici Helvetici}, volume = {66}, year = {1991}, pages = {155-168}, zbl = {0719.30035}, url = {http://dml.mathdoc.fr/item/GDZPPN002067927} }
Tukia, Pekka. Homomorphisms of constant stretch between Möbius groups.. Commentarii mathematici Helvetici, Tome 66 (1991) pp. 155-168. http://gdmltest.u-ga.fr/item/GDZPPN002067927/