@article{MZ_1928__27_167999,
     author = {W? Blaschke},
     title = {Eine {Verallgemeinerung} der {Theorie} der konfokalen {F2}},
     journal = {Mathematische Zeitschrift},
     pages = {653--668},
     publisher = {mathdoc},
     volume = {27},
     year = {1928},
     zbl = {54.0790.04},
     url = {http://geodesic.mathdoc.fr/item/MZ_1928__27_167999/}
}