@article{MZ_1955__63_169520,
     author = {Eberhard Hopf},
     title = {Die {Harnacksche} {Ungleichung} f\"ur positive harmonische {Funktionen.}},
     journal = {Mathematische Zeitschrift},
     pages = {156--157},
     year = {1955},
     volume = {63},
     zbl = {0065.09001},
     url = {http://geodesic.mathdoc.fr/item/MZ_1955__63_169520/}
}