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%I M1679 N0661 #26 Dec 20 2021 20:12:15
%S 1,1,2,6,26,135,875,6749,60601,618111,7033090
%N Idempotent semigroups of order n, considered to be equivalent when they are isomorphic or anti-isomorphic (by reversal of the operator).
%C An idempotent semigroup is one whose elements are all idempotents.
%D R. J. Plemmons, There are 15973 semigroups of order 6, Math. Algor., 2 (1967), 2-17; 3 (1968), 23.
%D 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.
%D S. Satoh, K. Yama and M. Tokizawa, Semigroups of order 8; Semigroup Forum 49, 1994.
%D N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
%D N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
%H Andreas Distler, <a href="http://hdl.handle.net/10023/945">Classification and Enumeration of Finite Semigroups</a>, A Thesis Submitted for the Degree of PhD, University of St Andrews (2010).
%H R. J. Plemmons, <a href="/A001423/a001423_2.pdf">There are 15973 semigroups of order 6</a> (annotated and scanned copy)
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Semigroup.html">Semigroup.</a>
%H <a href="/index/Se#semigroups">Index entries for sequences related to semigroups</a>
%Y Cf. A001423. Main diagonal of A058123.
%K nonn,nice,hard
%O 0,3
%A _N. J. A. Sloane_
%E Additional reference and comments from Michael Somos
%E a(7) term from _Christian G. Bower_, Feb 19 2001
%E a(8) (from the Satoh et al. reference) sent by Tom Kelsey (tom(AT)cs.st-and.ac.uk), Jun 17 2008
%E a(9)-a(10) from _Andreas Distler_, Jan 12 2011
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