The Frobenius problem for numerical semigroups

In this paper, we characterize those numerical semigroups containing 〈n₁,n₂〉. From this characterization, we give formulas for the genus and the Frobenius number of a numerical semigroup. These results can be used to give a method for computing the genus and the Frobenius number of a numerical semigroup with embedding dimension three in terms of its minimal system of generators.     

The Frobenius problem for repunit numerical semigroups

A repunit is a number consisting of copies of the single digit 1. The set of repunits in base b is \(\big \{\frac{b^n-1}{b-1} ~|~ n\in {\mathbb N}\backslash \{0\}\big \}\). A numerical semigroup S is a repunit numerical semigroup if there exist integers \(b\in {\mathbb N}\backslash \left\{ 0,1\right\} \) and \(n\in {\mathbb N}\backslash \left\{ 0\right\} \) such that \(S=\big \langle \big \{\frac{b^{n+i}-1}{b-1} ~|~ i\in {\mathbb N}\big \}\big \rangle \). In this work, we give formulas for the embedding dimension, the Frobenius number, the type and the genus for a repunit numerical semigroup.     

Irreducible numerical semigroups with multiplicity three and four

In this paper we analyze the irreducibility of numerical semigroups with multiplicity up to four. Our approach uses the notion of Kunz-coordinates vector of a numerical semigroup recently introduced in Blanco and Puerto (SIAM J. Discrete Math., 26(3):1210–1237, 2012). With this tool we also completely describe the whole family of minimal decompositions into irreducible numerical semigroups with the same multiplicity for this set of numerical semigroups. We give detailed examples to show the applicability of the methodology and conditions for the irreducibility of well-known families of numerical semigroups such as those that are generated by a generalized arithmetic progression.     

Frobenius pseudo-varieties in numerical semigroups

Annali di Matematica Pura ed Applicata (1923 -) - The common behavior of several families of numerical semigroups led up to defining the Frobenius varieties. However, some interesting families were...     

Symmetric numerical semigroups with arbitrary multiplicity and embedding dimension

We construct symmetric numerical semigroups for every minimal number of generators and multiplicity , . Furthermore we show that the set of their defining congruence is minimally generated by elements.

     

Atoms of the set of numerical semigroups with fixed Frobenius number

Given a positive integer g, we denote by F(g) the set of all numerical semigroups with Frobenius number g. The set (F(g),∩) is a semigroup. In this paper we study the generators of this semigroup.     

The Frobenius problem for the shuffle operation

Given a set S of words, let \(S^\dagger \) denote the iterated shuffle of S. We characterize the finite sets S for which \(S^\dagger \) is co-finite, and we give some bounds on the length of a longest word not in \(S^\dagger \).     

The loop problem for Rees matrix semigroups

We study the relationship between the loop problem of a semigroup, and that of a Rees matrix construction (with or without zero) over the semigroup. This allows us to characterize exactly those completely zero-simple semigroups for which the loop problem is context-free. We also establish some results concerning loop problems for subsemigroups and Rees quotients.     

Dense numerical semigroups

Semigroup Forum - A numerical semigroup S is dense if for all $$sin Sbackslash {0}$$ we have $$left{ s-1,s+1right} cap Sne emptyset $$ . We give algorithms to compute the whole set of...     

Homogeneous numerical semigroups

We introduce the concept of homogeneous numerical semigroups and show that all homogeneous numerical semigroups with Cohen–Macaulay tangent cones are of homogeneous type. In embedding dimension three, we classify all numerical semigroups of homogeneous type into numerical semigroups with complete intersection tangent cones and the homogeneous ones which are not symmetric with Cohen–Macaulay tangent cones. We also study the behavior of the homogeneous property by gluing and shiftings to construct large families of homogeneous numerical semigroups with Cohen–Macaulay tangent cones. In particular we show that these properties fulfill asymptotically in the shifting classes. Several explicit examples are provided along the paper to illustrate the property.     

Balanced numerical semigroups

We investigate the Frobenius number, genus, type, and minimal presentation of a class of numerical semigroups of embedding dimension 4 of the form \(S = \langle a_1, a_2, a_3, a_4 \rangle \) such that \(a_1 + a_4 = a_2 + a_3\). The investigation focuses on determining the Apery set of S with respect to the multiplicity.