26820 F. Bennett/L. Wu: On minimum matrix representations of closure operations. Discrete Appl. Math. 26 (1990), 25-40. 26819 F. Bennett/L. Wu: Minimum matrix representations of Sperner systems. Discrete Appl. Math. 8/1 (1998), 9-17. Konrad Engel: Sperner theory. Cambridge UP 1997, 420p. Pds. 50.