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 A118581 Number of nonisomorphic semigroups of order <= n. 1
 1, 2, 7, 31, 219, 2134, 30768, 1658440, 3685688857, 105981863625149 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Semigroup analog of A063756 Number of groups of order <= n. a semigroup is an algebraic structure consisting of a set S closed under an associative binary operation (and thus is an associative groupoid). Some sources require that a semigroup have an identity element (in which case semigroups are identical to monoids). Not all sources agree that S should be nonempty. This sequence assumes that a semigroup may be empty and need not have an identity. LINKS FORMULA a(n) = Sum_{i=0..n} A027851(i). a(n) = Sum_{i=0..n} (2*A001423(i) - A029851(i)). EXAMPLE a(7) = 1658440 = 1 + 1 + 5 + 24 + 188 + 1915 + 28634 + 1627672. CROSSREFS Cf. A001329, A001423, A001426, A023814, A027851, A029851, A058108, A063756, A079173. Sequence in context: A306037 A046907 A091312 * A102159 A309164 A102163 Adjacent sequences:  A118578 A118579 A118580 * A118582 A118583 A118584 KEYWORD hard,nonn AUTHOR Jonathan Vos Post, May 07 2006 EXTENSIONS a(8)-a(9) (using A027851) from Giovanni Resta, Jun 16 2016 STATUS approved

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Last modified October 21 08:47 EDT 2019. Contains 328292 sequences. (Running on oeis4.)