Generic Initial ideals of Singular Curves in Graded Lexicographic Order

Abstract

In this paper, we are interested in the generic initial ideals of \textit{singular} projective curves with respect to the graded lexicographic order. Let $C$ be a \textit{singular} irreducible projective curve of degree $d\ge 5$ with the arithmetic genus ${\rho }_a(C)$ in $\p^r$ where $r\ge 3$. If $M(I_C)$ is the regularity of the lexicographic generic initial ideal of $I_C$ in a polynomial ring $k[x_0,...,x_r]$ then we prove that $M(I_C)$ is $1+\left(\genfrac{}{}{0ex}{}{d-1}{2}\right)-{\rho }_a(C)$ which is obtained from the monomial $$x_{r-3}{x}_{r-1}{}^{\left(\genfrac{}{}{0ex}{}{d-1}{2}\right)-{\rho }_a(C)},$$ provided that $\dim\Tan_p(C)=2$ for every singular point $p\in C$. This number is equal to one plus the number of non-isomorphic points under a generic projection of $C$ into $\p^2$. %if $°(C)=3,4$ then $M(I_C)=°(C)$ by the direct computation. Our result generalizes the work of J. Ahn for \textit{smooth} projective curves and that of A. Conca and J. Sidman \cite{CS} for \textit{smooth} complete intersection curves in $\p^3$. The case of singular curves was motivated by \cite[Example 4.3]{CS} due to A. Conca and J. Sidman. We also provide some illuminating examples of our results via calculations done with {\it Macaulay 2} and \texttt {Singular} \cite{DGPS, GS}.

11 pages