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Search: MSC category 06F05 ( Ordered semigroups and monoids [See also 20Mxx] )

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1. CMB Online first

Bhuniya, Anjan Kumar; Hansda, Kalyan
On radicals of Green's relations in ordered semigroups
In this paper, we give a new definition of radicals of Green's relations in an ordered semigroup and characterize left regular (right regular), intra regular ordered semigroups by radicals of Green's relations. Also we characterize the ordered semigroups which are unions and complete semilattices of t-simple ordered semigroups.

Keywords:radical of Green's relation, intra regular ordered semigroup, left regular, t-simple ordered semigroup
Category:06F05

2. CMB 2009 (vol 52 pp. 598)

Moreno, M. A.; Nicola, J.; Pardo, E.; Thomas, H.
Numerical Semigroups That Are Not Intersections of $d$-Squashed Semigroups
We say that a numerical semigroup is \emph{$d$-squashed} if it can be written in the form $$ S=\frac 1 N \langle a_1,\dots,a_d \rangle \cap \mathbb{Z}$$ for $N,a_1,\dots,a_d$ positive integers with $\gcd(a_1,\dots, a_d)=1$. Rosales and Urbano have shown that a numerical semigroup is 2-squashed if and only if it is proportionally modular. Recent works by Rosales \emph{et al.} give a concrete example of a numerical semigroup that cannot be written as an intersection of $2$-squashed semigroups. We will show the existence of infinitely many numerical semigroups that cannot be written as an intersection of $2$-squashed semigroups. We also will prove the same result for $3$-squashed semigroups. We conjecture that there are numerical semigroups that cannot be written as the intersection of $d$-squashed semigroups for any fixed $d$, and we prove some partial results towards this conjecture.

Keywords:numerical semigroup, squashed semigroup, proportionally modular semigroup
Categories:20M14, 06F05, 46L80

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