On unboundedness of some invariants of $\mathcal{C}$-semigroups

Abstract

In this article, we consider $\mathcal{C}$-semigroups in $\mathbb{N}^d$. We start with symmetric and almost symmetric $\mathcal{C}$-semigroups and prove that these notions are independent of term orders. We further investigate the conductor and the Ap\'ery set of a $\mathcal{C}$-semigroup with respect to a minimal extremal ray. Building upon this, we extend the notion of reduced type to $\mathcal{C}$-semigroups and study its extremal behavior. For all $d$ and fixed $e \geq 2d$, we give a class of $\mathcal{C}$-semigroups of embedding dimension $e$ such that both the type and the reduced type do not have any upper bound in terms of the embedding dimension. We further explore irreducible decompositions of a $\mathcal{C}$-semigroup and give a lower bound on the irreducible components in an irreducible decomposition. Consequently, we deduce that for each positive integer $k$, there exists a $\mathcal{C}$-semigroup $S$ such that the number of irreducible components of $S$ is at least $k$. A $\mathcal{C}$-semigroup is known as a generalized numerical semigroup when the rational cone spanned by the semigroup is full. We classify all the symmetric generalized numerical semigroups of embedding dimension $2d+1$. Consequently, when $d>1$, we deduce that a generalized numerical semigroup of embedding dimension $2d+1$ is almost symmetric if and only if it is symmetric.