@article{IM_1992__108_2_143986,
     author = {G\'abor Tardos},
     title = {On the intersection of subgroups of a free group.},
     journal = {Inventiones mathematicae},
     pages = {29--36},
     publisher = {mathdoc},
     volume = {108},
     number = {2},
     year = {1992},
     zbl = {0798.20015},
     url = {http://geodesic.mathdoc.fr/item/IM_1992__108_2_143986/}
}