A002787 - OEIS

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A002787

Number of semigroups of order n with 2 idempotents, considered to be equivalent when they are isomorphic or anti-isomorphic (by reversal of the operator).
(Formerly M1802 N0712)

2, 7, 37, 216, 1780, 32652, 4665709, 12710266442 (list; graph; refs; listen; history; internal format)

	OFFSET	`2,1`
	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	`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, Link to a section of The World of Mathematics.` `Index entries for sequences related to semigroups`
	CROSSREFS	`Column 2 of A058123.` `Sequence in context: A041051 A175487 A047066 * A062394 A063766 A020040` `Adjacent sequences: A002784 A002785 A002786 * A002788 A002789 A002790`
	KEYWORD	`nonn`
	AUTHOR	`N. J. A. Sloane (njas(AT)research.att.com).`
	EXTENSIONS	`a(8)-a(9) from Andreas Distler (andreas(AT)mcs.st-andrews.ac.uk), Jan 13 2011`

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Last modified February 17 17:51 EST 2012. Contains 206061 sequences.