@article{IM_1986__84_143335,
     author = {K. Vogtmann and M. Culler},
     title = {Moduli of graphs and automorphisms of free groups.},
     journal = {Inventiones mathematicae},
     pages = {91--120},
     publisher = {mathdoc},
     volume = {84},
     year = {1986},
     zbl = {0589.20022},
     url = {http://geodesic.mathdoc.fr/item/IM_1986__84_143335/}
}