Numerical semigroup

In mathematics, a numerical semigroup is a special kind of a semigroup. Its underlying set is the set of all nonnegative integers except a finite number and the binary operation is the operation of addition of integers. Also, the integer 0 must be an element of the semigroup. For example, while the set {0, 2, 3, 4, 5, 6, ...} is a numerical semigroup, the set {0, 1, 3, 5, 6, ...} is not because 1 is in the set and 1 + 1 = 2 is not in the set. Numerical semigroups are commutative monoids and are also known as numerical monoids. 
The definition of numerical semigroup is intimately related to the problem of determining nonnegative integers that can be expressed in the form "x"1"n"1 + "x"2 "n"2 + ... + "x""r" "n""r" for a given set {"n"1, "n"2, ..., "n""r"} of positive integers and for arbitrary nonnegative integers "x"1, "x"2, ..., "x""r". This problem had been considered by several mathematicians like Frobenius (1849 – 1917) and Sylvester (1814 – 1897) at the end of the 19th century. During the second half of the twentieth century, interest in the study of numerical semigroups resurfaced because of their applications in algebraic geometry.
Definition and examples.
Definition.
Let "N" be the set of nonnegative integers. A subset "S" of "N" is called a numerical semigroup if the following conditions are satisfied.
There is a simple method to construct numerical semigroups. Let "A" = {"n"1, "n"2, ..., "n""r"} be a nonempty set of positive integers. The set of all integers of the form "x"1 "n"1 + "x"2 "n"2 + ... + "x""r" "n""r" is the subset of "N" generated by "A" and is denoted by 〈 "A" 〉. The following theorem fully characterizes numerical semigroups.
Theorem.
Let "S" be the subsemigroup of "N" generated by "A". Then "S" is a numerical semigroup if and only if gcd ("A") = 1. Moreover, every numerical semigroup arises in this way.
Examples.
The following subsets of "N" are numerical semigroups.
Embedding dimension, multiplicity.
The set "A" is a set of generators of the numerical semigroup 〈 "A" 〉. A set of generators of a numerical semigroup is a minimal system
of generators if none of its proper subsets generates the numerical semigroup. It is known that 
every numerical semigroup "S" has a unique minimal system of generators and also that this minimal system of generators is finite. The cardinality of the minimal set of generators is called the "embedding dimension" of the numerical semigroup "S" and is denoted by "e"("S"). The smallest member in the minimal system of generators is called the "multiplicity" of the numerical semigroup "S" and is denoted by "m"("S").
Frobenius number and genus.
There are several notable numbers associated with a numerical semigroup "S".
Examples.
Let "S" = 〈 5, 7, 9 〉. Then we have:
Numerical semigroups with small Frobenius number or genus
Computation of Frobenius number.
Numerical semigroups with embedding dimension two.
The following general results were known to Sylvester. Let "a" and "b" be positive integers such that gcd ("a", "b") = 1. Then 
Numerical semigroups with embedding dimension three.
There is no known general formula to compute the Frobenius number of numerical semigroups having embedding dimension three or more. No polynomial formula can be found to compute the Frobenius number or genus of a numerical semigroup with embedding dimension three. Every positive integer is the Frobenius number of some numerical semigroup with embedding dimension three.
Rödseth's algorithm.
The following algorithm, known as Rödseth's algorithm,
can be used to compute the Frobenius number of a numerical semigroup "S" generated by {"a"1, "a"2, "a"3} where "a"1 < "a"2 < "a"3 and gcd ( "a"1, "a"2, "a"3) = 1. Its worst-case complexity is not as good as Greenberg's algorithm
but it is much simpler to describe.
Special classes of numerical semigroups.
An "irreducible numerical semigroup" is a numerical semigroup such that it cannot be written as the intersection of two numerical semigroups properly containing it. A numerical semigroup "S" is irreducible if and only if "S" is maximal, with respect to set inclusion, in the collection of all numerical semigroups with Frobenius number "F"("S"). 
A numerical semigroup "S " is "symmetric" if it is irreducible and its Frobenius number "F"("S") is odd. We say that "S" is "pseudo-symmetric" provided that "S" is irreducible and F(S) is even. Such numerical semigroups have simple characterizations in terms of Frobenius number and genus: