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@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/} }
Halfar, Edwin. A note on point-set operators. Portugaliae mathematica, Tome 10 (1951) no. 2, pp. 103-104. http://geodesic.mathdoc.fr/item/PORMA_1951__10_2_114517/