@article{10_4171_ggd_82,
     author = {Robert Bieri and Ross Geoghegan},
     title = {Sigma invariants of direct products of groups},
     journal = {Groups, geometry, and dynamics},
     pages = {251--261},
     year = {2010},
     volume = {4},
     number = {2},
     doi = {10.4171/ggd/82},
     url = {http://geodesic.mathdoc.fr/articles/10.4171/ggd/82/}
}