@article{MS2_1972__30_166243,
     author = {R.M. Brooks},
     title = {Boundaries for {Natural} {Systems} {II.}},
     journal = {Mathematica Scandinavica},
     pages = {281--289},
     publisher = {mathdoc},
     volume = {30},
     year = {1972},
     zbl = {0245.46079},
     url = {http://geodesic.mathdoc.fr/item/MS2_1972__30_166243/}
}