@article{CMH_1991__66_4_140228,
     author = {Pekka Tukia},
     title = {Homomorphisms of constant stretch between {M\"obius} groups.},
     journal = {Commentarii mathematici Helvetici},
     pages = {155--168},
     publisher = {mathdoc},
     volume = {66},
     number = {4},
     year = {1991},
     zbl = {0719.30035},
     url = {http://geodesic.mathdoc.fr/item/CMH_1991__66_4_140228/}
}