@article{MS2_1973__32_166299,
     author = {Melven Krom and Myren Krom},
     title = {Free {Subgroups} and {Folners} {Conditions.}},
     journal = {Mathematica Scandinavica},
     pages = {242--244},
     publisher = {mathdoc},
     volume = {32},
     year = {1973},
     zbl = {0276.20028},
     url = {http://geodesic.mathdoc.fr/item/MS2_1973__32_166299/}
}