@article{10_1007_s10587_015_0195_0,
     author = {Gelbukh, Irina},
     title = {Co-rank and {Betti} number of a group},
     journal = {Czechoslovak Mathematical Journal},
     pages = {565--567},
     publisher = {mathdoc},
     volume = {65},
     number = {2},
     year = {2015},
     doi = {10.1007/s10587-015-0195-0},
     mrnumber = {3360446},
     zbl = {06486966},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.1007/s10587-015-0195-0/}
}