1. G.F. Clements, A generalization of Sperner's theorem on subsets of finite sets, Preprint, Univ. of Colorado, Boulder, CO.
2. Maximale h-familien in endlichen ordnungen;Engel,1981
3. Proof techniques in the theory of finite sets;Greene,1978
4. Collections of subsets with the Sperner property;Griggs,1980