OFFSET
1,2
COMMENTS
In a sense, this measures the increase in combinatorial structures available by dropping the requirement of inverses, and an identity element, in moving from the group axioms to the semigroup axioms. A semigroup is mathematical object defined for a set and a binary operator in which the multiplication operation is associative. No other restrictions are placed on a semigroup; thus a semigroup need not have an identity element and its elements need not have inverses within the semigroup. Other sequences may be derived by considering commutative semigroups and commutative groups, self-converse semigroup, counting idempotents, and the like.
LINKS
EXAMPLE
a(1) = 0 because there are unique groups and semigroups of order 1, so 1 - 1 = 0.
a(2) = 4 because there are 5 semigroups of order 2 groups and a unique group of order 2, so 5 - 1 = 4.
CROSSREFS
KEYWORD
nonn,hard,less
AUTHOR
Jonathan Vos Post, Feb 13 2011
STATUS
approved