A118581 - OEIS

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A118581

Number of nonisomorphic semigroups of order <= n.

1, 2, 7, 31, 219, 2134, 30768, 1658440, 3685688857, 105981863625149

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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

Table of n, a(n) for n=0..9.

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: A365563 A365565 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