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A002786 Semigroups of order n with 1 idempotent, considered to be equivalent when they are isomorphic or anti-isomorphic (by reversal of the operator).
(Formerly M1522 N0596)
0
1, 2, 5, 19, 132, 3107, 623615, 1834861133, 52976551026562 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

H. Juergensen and P. Wick, Die Halbgruppen von Ordnungen <= 7, Semigroup Forum, 14 (1977), 69-79.

R. J. Plemmons, There are 15973 semigroups of order 6, Math. Algor., 2 (1967), 2-17; 3 (1968), 23.

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

Table of n, a(n) for n=1..9.

Andreas Distler, Classification and Enumeration of Finite Semigroups, A Thesis Submitted for the Degree of PhD, University of St Andrews (2010).

Eric Weisstein's World of Mathematics, Semigroup.

Index entries for sequences related to semigroups

CROSSREFS

Column 1 of A058123.

Sequence in context: A113346 A198945 A054926 * A039719 A198203 A014466

Adjacent sequences:  A002783 A002784 A002785 * A002787 A002788 A002789

KEYWORD

nonn,nice,hard

AUTHOR

N. J. A. Sloane.

EXTENSIONS

a(8)-a(9) from Andreas Distler, Jan 13 2011

STATUS

approved

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Last modified April 17 20:01 EDT 2014. Contains 240655 sequences.