@article{MAN_1980__247_163350,
     author = {Vlastimil Dlab and Claus Michael Ringel},
     title = {Perfect {Elements} in the {Free} {Modular} {Lattices.}},
     journal = {Mathematische Annalen},
     pages = {95--100},
     publisher = {mathdoc},
     volume = {247},
     year = {1980},
     zbl = {0443.06005},
     url = {http://geodesic.mathdoc.fr/item/MAN_1980__247_163350/}
}