%I A002788 M1679 N0661
%S A002788 1,1,2,6,26,135,875,6749,60601
%N A002788 Idempotent semigroups of order n, considered to be equivalent when they 
               are isomorphic or anti-isomorphic (by reversal of the operator).
%C A002788 An idempotent semigroup is one whose elements are all idempotents.
%D A002788 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A002788 N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 
               (includes this sequence).
%D A002788 R. J. Plemmons, There are 15973 semigroups of order 6, Math. Algor., 
               2 (1967), 2-17; 3 (1968), 23.
%D A002788 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 A002788 S. Satoh, K. Yama and M. Tokizawa, Semigroups of order 8; Semigroup Forum 
               49, 1994.
%H A002788 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               Semigroup.html">Link to a section of The World of Mathematics.</a>
%H A002788 <a href="Sindx_Se.html#semigroups">Index entries for sequences related 
               to semigroups</a>
%Y A002788 Cf. A001423. Main diagonal of A058123.
%Y A002788 Sequence in context: A159667 A030957 A030898 this_sequence A134094 A009575 
               A127116
%Y A002788 Adjacent sequences: A002785 A002786 A002787 this_sequence A002789 A002790 
               A002791
%K A002788 nonn,nice,hard
%O A002788 0,3
%A A002788 N. J. A. Sloane (njas(AT)research.att.com).
%E A002788 Additional reference and comments from Michael Somos.
%E A002788 a(7) term from Christian G. Bower (bowerc(AT)usa.net), Feb 19 2001
%E A002788 a(8) (from the Satoh et al. reference) sent by Tom Kelsey (tom(AT)cs.st-and.ac.uk), 
               Jun 17 2008