@article{SLC_1989_21_a15,
     author = {Hanno Lefmann},
     title = {Extremalprobleme in {Verb\"anden}},
     journal = {S\'eminaire lotharingien de combinatoire},
     publisher = {mathdoc},
     volume = {21},
     year = {1989},
     url = {http://geodesic.mathdoc.fr/item/SLC_1989_21_a15/}
}