A118100 Number of commutative semigroups of order <= n.

1, 2, 5, 17, 75, 400, 2543, 19834, 241639, 11787482, 3530717819

Offset: 0

Author

Jonathan Vos Post, May 11 2006

Keywords

• nonn
• more

Comments

A001426(n) is the number of commutative semigroups of order n. A001426(n) + A079193(n) + A079196(n) + A079199(n) = A001329(n). 2, 5, 17, 2543 and 241639 are primes.

Examples

			a(8) = 1 + 1 + 3 + 12 + 58 + 325 + 2143 + 17291 + 221805 = 241639.
		

Links

• Index entries for sequences related to semigroups.

Crossrefs

Cf. A001423, A001426, A023815, A027851, A058105, A058116.

Formula

a(n) = Sum_{i=1..n} A001426(i).

Extensions

a(9)-a(10) added using the terms in A001426 by Miles Englezou, May 27 2025