@article{MS2_1993__72_1_167243,
     author = {William L. Paschke},
     title = {A numerical invariant for finitely generated groups via actions on graphs.},
     journal = {Mathematica Scandinavica},
     pages = {148--160},
     publisher = {mathdoc},
     volume = {72},
     number = {1},
     year = {1993},
     zbl = {0812.20020},
     url = {http://geodesic.mathdoc.fr/item/MS2_1993__72_1_167243/}
}