_Jonathan Vos Post (jvospost3(AT)gmail.com), _, Feb 13 2011
_Jonathan Vos Post (jvospost3(AT)gmail.com), _, Feb 13 2011
editing
approved
Eric W. Weisstein, <a href="http://mathworld.wolfram.com/FiniteGroup.html">Finite Group.</a> From MathWorld--A Wolfram Web Resource.
Eric W. Weisstein, <a href="http://mathworld.wolfram.com/Semigroup.html">Semigroup.</a> From MathWorld--A Wolfram Web Resource
approved
editing
reviewed
approved
proposed
reviewed
Number of nonisomorphic semigroups of order n - minus number of groups of order n.
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.
nonn,hard,less,changed
allocated for Jonathan Vos Post Number of nonisomorphic semigroups of order n - number of groups of order n.
0, 4, 23, 186, 1914, 28632, 1627671, 3684030412, 105978177936290
1,2
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.
Eric W. Weisstein, <a href="http://mathworld.wolfram.com/FiniteGroup.html">Finite Group.</a> From MathWorld--A Wolfram Web Resource.
Eric W. Weisstein, <a href="http://mathworld.wolfram.com/Semigroup.html">Semigroup.</a> From MathWorld--A Wolfram Web Resource
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.
allocated
nonn,hard
Jonathan Vos Post (jvospost3(AT)gmail.com), Feb 13 2011
approved
proposed
allocated for Jonathan Vos Post
allocated
approved