Year: 2020
Type: preprint
Abstract: Let $G$ be a simple graph and $I(X_G)=φ^{-1}(x_i^2-x_j^2 : i,j\in V_G)$, where $φ\colon K[E_G]\to K[V_G]$ is the homomorphism that sends an edge to the product of its vertices. The ideal $I(X_G)$ is Cohen--Macaulay, one-dimensional and binomial. If $G$ is bipartite, it is known that the Castelnuovo--Mumford regularity of $I(X_G)$ is equal to the maximum cardinality of a set of edges having no more than half of the edges of any Eulerian subgraph of $G$. Here, with respect to the grevlex order associated to an ordering of the edge set of $G$, we describe a Gröbner basis for $I(X_G)$, and we characterize the standard monomials of the ideal $(I(X_G),t_e)$ in terms of even sets of vertices marked with a parity. Using these results, we give a combinatorial interpretation of the degree of $I(X_G)$, via the set of even sets of vertices of $G$; and we show that the Castelnuovo--Mumford regularity of $I(X_G)$, for any graph, is the maximum cardinality of a set of edges having no more than half of the edges of any \emph{even} Eulerian subgraph of $G$ or, equivalently, the maximum cardinality of a minimum fixed parity $T$-join.
Source: arXiv (Cornell University)
Author: Neves, Jorge
Language: English
Subfield: Algebra and Number Theory
Field: Mathematics
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W4295137649