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.
EXAMPLE
a(7) = 1658440 = 1 + 1 + 5 + 24 + 188 + 1915 + 28634 + 1627672.
CROSSREFS
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