@article{PORMA_1951__10_2_114517,
     author = {Halfar, Edwin},
     title = {A note on point-set operators},
     journal = {Portugaliae mathematica},
     pages = {103--104},
     publisher = {mathdoc},
     volume = {10},
     number = {2},
     year = {1951},
     mrnumber = {0045191},
     zbl = {0043.32801},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/PORMA_1951__10_2_114517/}
}