@article{SLC_1988_20_a9,
     author = {Pierre Duchet},
     title = {nn-2},
     journal = {S\'eminaire lotharingien de combinatoire},
     publisher = {mathdoc},
     volume = {20},
     year = {1988},
     url = {http://geodesic.mathdoc.fr/item/SLC_1988_20_a9/}
}