Projective monomial curves associated to numerical semigroups with multiplicity $e$, width $e-1$, and embedding dimension $e-2$

Abstract

Numerical semigroups with multiplicity $e$, width $e-1$, and embedding dimension $e-2$ are of the form $$S(e,m,n)=⟨\{e,e+1,\dots ,2e-1\}\setminus \{e+m,e+n\}⟩,$$ for some $1\le m<n\le e-2$. Inspired by the work of Sally, Herzog and Stamate studied the special case $S(e,2,3)$, which they called the ``Sally numerical semigroups''. Recently, Dubey et. al. computed a minimal generating set of the defining ideal of the numerical semigroups $S(e,m,n)$ for $m\ge 2$. In this article, we first obtain an analog for the numerical semigroups $S(e,1,n)$, and then shift our focus to the projective monomial curves in ${\mathbb{P}}^{e-2}$ defined by the semigroups $S(e,m,n)$. We obtain a Gröbner basis for the defining ideal of the projective monomial curves associated to the semigroups $S(e,m,n)$. Moreover, we provide characterizations of Cohen--Macaulay and Gorenstein properties of these curves. Specifically, we prove that these are Cohen--Macaulay if and only if $(m,n)\ne (e-4,e-3)$, and Gorenstein if and only if $(e,m,n)\in \{(4,1,2),(5,2,3)\}$. Furthermore, when these curves are Cohen--Macaulay, we compute the Castelnuovo--Mumford regularity of their coordinate ring.

are welcome. 16 pages