OFFSET
0,3
COMMENTS
An idempotent semigroup is one whose elements are all idempotents.
REFERENCES
R. J. Plemmons, There are 15973 semigroups of order 6, Math. Algor., 2 (1967), 2-17; 3 (1968), 23.
R. J. Plemmons, Construction and analysis of non-equivalent finite semigroups, pp. 223-228 of J. Leech, editor, Computational Problems in Abstract Algebra. Pergamon, Oxford, 1970.
S. Satoh, K. Yama and M. Tokizawa, Semigroups of order 8; Semigroup Forum 49, 1994.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
LINKS
Andreas Distler, Classification and Enumeration of Finite Semigroups, A Thesis Submitted for the Degree of PhD, University of St Andrews (2010).
R. J. Plemmons, There are 15973 semigroups of order 6 (annotated and scanned copy)
Eric Weisstein's World of Mathematics, Semigroup.
CROSSREFS
KEYWORD
nonn,nice,hard
AUTHOR
EXTENSIONS
Additional reference and comments from Michael Somos
a(7) term from Christian G. Bower, Feb 19 2001
a(8) (from the Satoh et al. reference) sent by Tom Kelsey (tom(AT)cs.st-and.ac.uk), Jun 17 2008
a(9)-a(10) from Andreas Distler, Jan 12 2011
STATUS
approved