@article{JRAM_1968__232_150882,
     author = {Hans G\"unzler},
     title = {Means over countable semigroups and almost-periodicity in l1.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {194--206},
     publisher = {mathdoc},
     volume = {232},
     year = {1968},
     zbl = {0174.19201},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1968__232_150882/}
}