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 A027851 Number of nonisomorphic semigroups of order n. 20
 1, 1, 5, 24, 188, 1915, 28634, 1627672, 3684030417, 105978177936292 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 LINKS Peter Cameron's Blog, The combinatorial explosion, Posted 18/02/2016. Andreas Distler, Classification and Enumeration of Finite Semigroups, A Thesis Submitted for the Degree of PhD, University of St Andrews (2010). A. Distler and T. Kelsey, The semigroups of order 9 and their automorphism groups, arXiv preprint arXiv:1301.6023 [math.CO], 2013. C. Noebauer, Home page C. Noebauer, The Numbers of Small Rings C. Noebauer, Thesis on the enumeration of near-rings Eric Postpischil Posting to sci.math newsgroup, May 21 1990 Jeremy G. Sumner, Michael D. Woodhams, Lie-Markov models derived from finite semigroups, arXiv:1709.00520 [math.GR], 2017. Michael Torpey, Semigroup congruences: computational techniques and theoretical applications, Ph.D. Thesis, University of St. Andrews (Scotland, 2019). A. de Vries, Formal Languages: An Introduction, 2012. Eric Weisstein's World of Mathematics, Semigroup. FORMULA a(n) = A001423(n)*2 - A029851(n). a(n) + A079173(n) = A001329(n). CROSSREFS Cf. A001426, A023814, A058108. Cf. A001423, A029851, A079173, A001329. Sequence in context: A009601 A009676 A192995 * A120765 A259355 A297664 Adjacent sequences:  A027848 A027849 A027850 * A027852 A027853 A027854 KEYWORD nonn,hard,nice AUTHOR Christian G. Bower, Dec 13 1997, updated Feb 19 2001 EXTENSIONS a(8)-a(9) from Andreas Distler, Jan 13 2011 STATUS approved

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Last modified October 15 22:25 EDT 2019. Contains 328038 sequences. (Running on oeis4.)