1. BRANDON, R. L., HARDY, D. W. and G. MARKOWSKY.The Schützenbeger group of an H-class in the semigroup of bid relations, omens, Semigroup Forum, 5 (1972, 45–53.
2. BUTLER, K. K.-H.Combinatorial properties of binary relations, to appear in the Periodica Mathematica Hungarica.
3. ——,The number of partially ordered sets, J. Comb. Theory, Series B, 13 (1972), 276–289.
4. BUTLER, K. K.-H. and G. MARKOWSKY.Enumeration of finite topologies, Proc. of the Fourth Southeastern on Combinatorics, Graph Theory, and Computing, Utilitas Mathematica Publishing Inc., Winnipeg, Canada, 1973, 169–184.
5. Math. Survey;A. H. Clifford,1961