@article{IM_1981__63_142788,
     author = {R.P. Osborne and H. Zieschang},
     title = {Primitives in the {Free} {Group} on {Two} {Generators.}},
     journal = {Inventiones mathematicae},
     pages = {17--24},
     publisher = {mathdoc},
     volume = {63},
     year = {1981},
     zbl = {0438.20017},
     url = {http://geodesic.mathdoc.fr/item/IM_1981__63_142788/}
}