Algebraic invariants of projective monomial curves associated to generalized arithmetic sequences

Abstract

Let $K$ be an infinite field and let $m_1,\ldots,m_n$ be a generalized arithmetic sequence of positive integers, i.e., there exist $h, d, m_1 \in\mathbb{Z}^+$ such that $m_i = h m_1 + (i-1)d$ for all $i \in \{2,\ldots,n\}$. We consider the projective monomial curve $\mathcal C\subset \mathbb{P}^{n}_{K}$ parametrically defined by $$x_1=s^{m_1}t^{m_n-m_1},\dots,x_{n-1}=s^{m_{n-1}}t^{m_n-m_{n-1}},x_n=s^{m_n},x_{n+1}=t^{m_n}.$$ In this work, we characterize the Cohen-Macaulay and Koszul properties of the homogeneous coordinate ring $K[\mathcal C]$ of $\mathcal C$. Whenever $K[\mathcal C]$ is Cohen-Macaulay we also obtain a formula for its Cohen-Macaulay type. Moreover, when $h$ divides $d$, we obtain a minimal Gr\"obner basis $\mathcal G$ of the vanishing ideal of $\mathcal C$ with respect to the degree reverse lexicographic order. From $\mathcal G$ we derive formulas for the Castelnuovo-Mumford regularity, the Hilbert series and the Hilbert function of $K[\mathcal C]$ in terms of the sequence.