@article{PORMA_1952__11_3_114707,
     author = {Bourgin, D.G.},
     title = {Sets of visibility},
     journal = {Portugaliae mathematica},
     pages = {137--140},
     publisher = {mathdoc},
     volume = {11},
     number = {3},
     year = {1952},
     mrnumber = {0051503},
     zbl = {0047.28901},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1952__11_3_114707/}
}