English

Regularity and h-polynomials of binomial edge ideals

Commutative Algebra 2020-08-27 v2 Combinatorics

Abstract

Let GG be a finite simple graph on the vertex set [n]={1,,n}[n] = \{ 1, \ldots, n \} and K[X,Y]=K[x1,,xn,y1,,yn]K[X, Y] = K[x_1, \ldots, x_n, y_1, \ldots, y_n] the polynomial ring in 2n2n variables over a field KK with each degxi=degyj=1\mathrm{deg} x_i = \mathrm{deg} y_j = 1. The binomial edge ideal of GG is the binomial ideal JGK[X,Y]J_G \subset K[X, Y] which is generated by those binomials xiyjxjyix_iy_j - x_jy_i for which {i,j}\{i, j\} is an edge of GG. The Hilbert series HK[X,Y]/JG(λ)H_{K[X, Y]/J_G}(\lambda) of K[X,Y]/JGK[X, Y]/J_G is of the form HK[X,Y]/JG(λ)=hK[X,Y]/JG(λ)/(1λ)dH_{K[X, Y]/J_G}(\lambda) = h_{K[X, Y]/J_G}(\lambda)/(1 - \lambda)^d, where d=dimK[X,Y]/JGd = \mathrm{dim} K[X, Y]/J_G and where hK[X,Y]/JG(λ)=h0+h1λ+h2λ2++hsλsh_{K[X, Y]/J_G}(\lambda) = h_0 + h_1\lambda + h_2\lambda^2 + \cdots + h_s\lambda^s with each hiZh_i \in \mathbb{Z} and with hs0h_s \neq 0 is the hh-polynomial of K[X,Y]/JGK[X, Y]/J_G. It is known that, when K[X,Y]/JGK[X, Y]/J_G is Cohen-Macaulay, one has reg(K[X,Y]/JG)=deghK[X,Y]/JG(λ)\mathrm{reg}(K[X, Y]/J_G) = \mathrm{deg} h_{K[X, Y]/J_G}(\lambda), where reg(K[X,Y]/JG) \mathrm{reg}(K[X, Y]/J_G) is the (Castelnuovo-Mumford) regularity of K[X,Y]/JGK[X, Y]/J_G. In the present paper, given arbitrary integers rr and ss with 2rs2 \leq r \leq s, a finite simple graph GG for which reg(K[X,Y]/JG)=r\mathrm{reg}(K[X, Y]/J_G) = r and deghK[X,Y]/JG(λ)=s\mathrm{deg} h_{K[X, Y]/J_G}(\lambda) = s will be constructed.

Keywords

Cite

@article{arxiv.1808.06984,
  title  = {Regularity and h-polynomials of binomial edge ideals},
  author = {Takayuki Hibi and Kazunori Matsuda},
  journal= {arXiv preprint arXiv:1808.06984},
  year   = {2020}
}

Comments

6 pages. Conjecture 0.1 has been deleted. A counterexample of Conjecture 0.1 were given in arXiv:1906.05510

R2 v1 2026-06-23T03:39:43.286Z