@article{RCM_2015_49_2_a3,
     author = {Christian Pommerenke and Margarita Toro},
     title = {Free subgroups of the parametrized modular group},
     journal = {Revista colombiana de matematicas},
     pages = {269--279},
     year = {2015},
     volume = {49},
     number = {2},
     url = {http://geodesic.mathdoc.fr/item/RCM_2015_49_2_a3/}
}