@article{RO_1983__17_4_379_0,
     author = {Abel, Volker},
     title = {Conditions for lumping the grades of a hierarchical manpower system},
     journal = {RAIRO - Operations Research - Recherche Op\'erationnelle},
     pages = {379--383},
     publisher = {EDP-Sciences},
     volume = {17},
     number = {4},
     year = {1983},
     zbl = {0528.90056},
     language = {en},
     url = {http://www.numdam.org/item/RO_1983__17_4_379_0/}
}