1. Arnold, S.F., 1990. Mathematical Statistics. Prentice-Hall, Englewood Cliffs, NJ.
2. Barnes et al., 1995.
3. Social structure from multiple networks: II. Role structures;Boorman;Am. J. Sociol.,1976
4. Boyd, J.P., 1991. Social Semigroups. George Mason Univ. Press, Fairfax, VA.
5. Boyd, 1992.