scholarly journals Algebraic Properties of Edge Ideals via Combinatorial Topology

10.37236/68 ◽  
2009 ◽  
Vol 16 (2) ◽  
Author(s):  
Anton Dochtermann ◽  
Alexander Engström
Keyword(s):  
Generating Functions ◽  
Betti Numbers ◽  
Projective Dimension ◽  
Vertex Degree ◽  
Combinatorial Topology ◽  
Edge Ideals ◽  
Algebraic Properties ◽  
Ferrers Graphs ◽  
Recursive Relations

We apply some basic notions from combinatorial topology to establish various algebraic properties of edge ideals of graphs and more general Stanley-Reisner rings. In this way we provide new short proofs of some theorems from the literature regarding linearity, Betti numbers, and (sequentially) Cohen-Macaulay properties of edge ideals associated to chordal, complements of chordal, and Ferrers graphs, as well as trees and forests. Our approach unifies (and in many cases strengthens) these results and also provides combinatorial/enumerative interpretations of certain algebraic properties. We apply our setup to obtain new results regarding algebraic properties of edge ideals in the context of local changes to a graph (adding whiskers and ears) as well as bounded vertex degree. These methods also lead to recursive relations among certain generating functions of Betti numbers which we use to establish new formulas for the projective dimension of edge ideals. We use only well-known tools from combinatorial topology along the lines of independence complexes of graphs, (not necessarily pure) vertex decomposability, shellability, etc.

On the Betti numbers of edge ideals of skew Ferrers graphs

2019 ◽  
Vol 30 (01) ◽  
pp. 125-139
Author(s):  
Do Trong Hoang
Keyword(s):  
Explicit Formula ◽  
Betti Number ◽  
Betti Numbers ◽  
Edge Ideal ◽  
Edge Ideals ◽  
Ferrers Graph ◽  
Ferrers Graphs ◽  
Extremal Betti Number

We prove that [Formula: see text] for any staircase skew Ferrers graph [Formula: see text], where [Formula: see text] and [Formula: see text]. As a consequence, Ene et al. conjecture is confirmed to hold true for the Betti numbers in the last column of the Betti table in a particular case. An explicit formula for the unique extremal Betti number of the binomial edge ideal of some closed graphs is also given.

scholarly journals Certain homological invariants of bipartite kneser graphs

2021 ◽  
pp. 2250206
Author(s):  
Ajay Kumar ◽  
Pavinder Singh ◽  
Rohit Verma
Keyword(s):  
Lower Bound ◽  
Betti Numbers ◽  
Projective Dimension ◽  
Upper Bounds ◽  
Combinatorial Formula ◽  
Lower And Upper Bounds ◽  
Edge Ideals ◽  
Homological Invariants ◽  
Kneser Graphs ◽  
Combinatorial Data

In this paper, we obtain a combinatorial formula for computing the Betti numbers in the linear strand of edge ideals of bipartite Kneser graphs. We deduce lower and upper bounds for regularity of powers of edge ideals of these graphs in terms of associated combinatorial data and show that the lower bound is attained in some cases. Also, we obtain bounds on the projective dimension of edge ideals of these graphs in terms of combinatorial data.

scholarly journals Minimal Cellular Resolutions of the Edge Ideals of Forests

10.37236/8810 ◽  
2020 ◽  
Vol 27 (2) ◽  
Author(s):  
Margherita Barile ◽  
Antonio Macchia
Keyword(s):  
Morse Theory ◽  
Betti Numbers ◽  
Projective Dimension ◽  
Explicit Construction ◽  
Discrete Morse Theory ◽  
Edge Ideals ◽  
Cellular Resolutions ◽  
Graded Betti Numbers ◽  
Free Modules

We present an explicit construction of minimal cellular resolutions for the edge ideals of forests, based on discrete Morse theory. In particular, the generators of the free modules are subsets of the generators of the modules in the Lyubeznik resolution. This procedure allows us to ease the computation of the graded Betti numbers and the projective dimension.

Algebraic Properties of Edge Ideals of Some Vertex-Weighted Oriented Cyclic Graphs

2021 ◽  
Vol 28 (02) ◽  
pp. 253-268
Author(s):  
Hong Wang ◽  
Guangjun Zhu ◽  
Li Xu ◽  
Jiaqi Zhang
Keyword(s):  
Projective Dimension ◽  
Common Vertex ◽  
Edge Ideals ◽  
Algebraic Properties ◽  
Cyclic Graphs

We provide some exact formulas for the projective dimension and regularity of edge ideals associated to some vertex-weighted oriented cyclic graphs with a common vertex or edge. These formulas are functions in the weight of the vertices, and the numbers of edges and cycles. Some examples show that these formulas are related to direction selection and the assumption that [Formula: see text] for any vertex [Formula: see text] cannot be dropped.

scholarly journals Homological invariants of the Stanley–Reisner ring of a k-decomposable simplicial complex

2017 ◽  
Vol 10 (03) ◽  
pp. 1750061
Author(s):  
Somayeh Moradi
Keyword(s):  
Simplicial Complex ◽  
Upper Bound ◽  
Betti Numbers ◽  
Recursive Formula ◽  
Projective Dimension ◽  
Simplicial Complexes ◽  
Monomial Ideals ◽  
Edge Ideals ◽  
Graded Betti Numbers ◽  
Shellable Simplicial Complex

In this paper, we study the regularity and the projective dimension of the Stanley–Reisner ring of a [Formula: see text]-decomposable simplicial complex and explain these invariants with a recursive formula. To this aim, the graded Betti numbers of decomposable monomial ideals which is the dual concept for [Formula: see text]-decomposable simplicial complexes are studied and an inductive formula for the Betti numbers is given. As a corollary, for a shellable simplicial complex [Formula: see text], a formula for the regularity of the Stanley–Reisner ring of [Formula: see text] is presented. Finally, for a chordal clutter [Formula: see text], an upper bound for [Formula: see text] is given in terms of the regularities of edge ideals of some chordal clutters which are minors of [Formula: see text].

scholarly journals Betti Numbers of Weighted Oriented Graphs

10.37236/9887 ◽  
2021 ◽  
Vol 28 (2) ◽  
Author(s):  
Beata Casiday ◽  
Selvi Kara
Keyword(s):  
Betti Number ◽  
Betti Numbers ◽  
Oriented Graph ◽  
Projective Dimension ◽  
Edge Ideal ◽  
Oriented Graphs ◽  
Edge Ideals ◽  
Mapping Cone ◽  
Recursive Formulas ◽  
Extremal Betti Number

Let $\mathcal{D}$ be a weighted oriented graph and $I(\mathcal{D})$ be its edge ideal. In this paper, we investigate the Betti numbers of $I(\mathcal{D})$ via upper-Koszul simplicial complexes, Betti splittings and the mapping cone construction. In particular, we provide recursive formulas for the Betti numbers of edge ideals of several classes of weighted oriented graphs. We also identify classes of weighted oriented graphs whose edge ideals have a unique extremal Betti number which allows us to compute the regularity and projective dimension for the identified classes. Furthermore, we characterize the structure of a weighted oriented graph $\mathcal{D}$ on $n$ vertices such that $\textrm{pdim } (R/I(\mathcal{D}))=n$ where $R=k[x_1,\ldots, x_n]$.

scholarly journals Betti Numbers of Monomial Ideals and Shifted Skew Shapes

10.37236/69 ◽  
2009 ◽  
Vol 16 (2) ◽  
Author(s):  
Uwe Nagel ◽  
Victor Reiner
Keyword(s):  
Lower Bound ◽  
Betti Numbers ◽  
Free Resolution ◽  
Monomial Ideals ◽  
Fixed Degree ◽  
Minimal Free Resolution ◽  
Edge Ideals ◽  
Combinatorial Interpretations ◽  
Ferrers Graphs ◽  
Strongly Stable Ideals

We present two new problems on lower bounds for Betti numbers of the minimal free resolution for monomial ideals generated in a fixed degree. The first concerns any such ideal and bounds the total Betti numbers, while the second concerns ideals that are quadratic and bihomogeneous with respect to two variable sets, but gives a more finely graded lower bound. These problems are solved for certain classes of ideals that generalize (in two different directions) the edge ideals of threshold graphs and Ferrers graphs. In the process, we produce particularly simple cellular linear resolutions for strongly stable and squarefree strongly stable ideals generated in a fixed degree, and combinatorial interpretations for the Betti numbers of other classes of ideals, all of which are independent of the coefficient field.

scholarly journals Projective dimension and regularity of the path ideal of the line graph

2018 ◽  
Vol 17 (04) ◽  
pp. 1850068 ◽  
Author(s):  
Guangjun Zhu
Keyword(s):  
Line Graph ◽  
Quotient Ring ◽  
Projective Dimension ◽  
Algebraic Properties

By generalizing the notion of the path ideal of a graph, we study some algebraic properties of some path ideals associated to a line graph. We show that the quotient ring of these ideals are always sequentially Cohen–Macaulay and also provide some exact formulas for the projective dimension and the regularity of these ideals. As some consequences, we give some exact formulas for the depth of these ideals.

On Betti numbers of edge ideals of crown graphs

2018 ◽  
Vol 60 (1) ◽  
pp. 123-136 ◽  
Author(s):  
Shahnawaz Ahmad Rather ◽  
Pavinder Singh
Keyword(s):  
Betti Numbers ◽  
Edge Ideals
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