@article{CMH_1994__69_4_140325,
     author = {Holger Meinert},
     title = {The geometric invariants of direct products of virtually free groups.},
     journal = {Commentarii mathematici Helvetici},
     pages = {39--48},
     publisher = {mathdoc},
     volume = {69},
     number = {4},
     year = {1994},
     zbl = {0834.20039},
     url = {http://geodesic.mathdoc.fr/item/CMH_1994__69_4_140325/}
}