@article{JRAM_1975__276_151603,
     author = {Harro Walk},
     title = {\"Uber die {Approximation} unbeschr\"ankter {Funktionen} durch lineare positive {Operatoren.}},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {83--94},
     publisher = {mathdoc},
     volume = {276},
     year = {1975},
     zbl = {0308.41016},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1975__276_151603/}
}