@article{MS2_1991__68_1_167146,
     author = {Mika Sepp\"al\"a and Tuomas Sorvali},
     title = {Traces of commutators of {M\"obius} transformations.},
     journal = {Mathematica Scandinavica},
     pages = {53--58},
     publisher = {mathdoc},
     volume = {68},
     number = {1},
     year = {1991},
     zbl = {0898.30037},
     url = {http://geodesic.mathdoc.fr/item/MS2_1991__68_1_167146/}
}