scholarly journals Monochromatic Cycle Partitions of $2$-Coloured Graphs with Minimum Degree $3n/4$

10.37236/7239 ◽  
2019 ◽  
Vol 26 (1) ◽  
Author(s):  
Shoham Letzter
Keyword(s):  
Minimum Degree ◽  
Disjoint Cycles ◽  
Large N ◽  
Vertex Set ◽  
Vertex Disjoint ◽  
Edge Colouring

Balogh, Barát, Gerbner, Gyárfás, and Sárközy made the following conjecture. Let $G$ be a graph on $n$ vertices with minimum degree at least $3n/4$. Then for every $2$-edge-colouring of $G$, the vertex set $V(G)$ may be partitioned into two vertex-disjoint cycles, one of each colour. We prove this conjecture for large $n$, improving approximate results by the aforementioned authors and by DeBiasio and Nelsen.  

scholarly journals On Graphs with Cyclic Defect or Excess

10.37236/415 ◽  
2010 ◽  
Vol 17 (1) ◽  
Author(s):  
Charles Delorme ◽  
Guillermo Pineda-Villavicencio
Keyword(s):  
Upper Bound ◽  
Adjacency Matrix ◽  
Minimum Degree ◽  
Integer Coefficients ◽  
Disjoint Cycles ◽  
The Matrix ◽  
Odd Degree ◽  
Unexplored Area ◽  
Trivial Graph ◽  
Vertex Disjoint

The Moore bound constitutes both an upper bound on the order of a graph of maximum degree $d$ and diameter $D=k$ and a lower bound on the order of a graph of minimum degree $d$ and odd girth $g=2k+1$. Graphs missing or exceeding the Moore bound by $\epsilon$ are called graphs with defect or excess $\epsilon$, respectively. While Moore graphs (graphs with $\epsilon=0$) and graphs with defect or excess 1 have been characterized almost completely, graphs with defect or excess 2 represent a wide unexplored area. Graphs with defect (excess) 2 satisfy the equation $G_{d,k}(A) = J_n + B$ ($G_{d,k}(A) = J_n - B$), where $A$ denotes the adjacency matrix of the graph in question, $n$ its order, $J_n$ the $n\times n$ matrix whose entries are all 1's, $B$ the adjacency matrix of a union of vertex-disjoint cycles, and $G_{d,k}(x)$ a polynomial with integer coefficients such that the matrix $G_{d,k}(A)$ gives the number of paths of length at most $k$ joining each pair of vertices in the graph. In particular, if $B$ is the adjacency matrix of a cycle of order $n$ we call the corresponding graphs graphs with cyclic defect or excess; these graphs are the subject of our attention in this paper. We prove the non-existence of infinitely many such graphs. As the highlight of the paper we provide the asymptotic upper bound of $O(\frac{64}3d^{3/2})$ for the number of graphs of odd degree $d\ge3$ and cyclic defect or excess. This bound is in fact quite generous, and as a way of illustration, we show the non-existence of some families of graphs of odd degree $d\ge3$ and cyclic defect or excess. Actually, we conjecture that, apart from the Möbius ladder on 8 vertices, no non-trivial graph of any degree $\ge 3$ and cyclic defect or excess exists.

scholarly journals Disjoint Cycles of Different Lengths in Graphs and Digraphs

10.37236/6921 ◽  
2017 ◽  
Vol 24 (4) ◽  
Author(s):  
Julien Bensmail ◽  
Ararat Harutyunyan ◽  
Ngoc Khang Le ◽  
Binlong Li ◽  
Nicolas Lichiardopol
Keyword(s):  
Minimum Degree ◽  
Directed Graphs ◽  
Probabilistic Method ◽  
Undirected Graphs ◽  
Disjoint Cycles ◽  
Vertex Disjoint ◽  
Degree Bound ◽  
Directed Cycles

In this paper, we study the question of finding a set of $k$ vertex-disjoint cycles (resp. directed cycles) of distinct lengths in a given graph (resp. digraph). In the context of undirected graphs, we prove that, for every $k \geq 1$, every graph with minimum degree at least $\frac{k^2+5k-2}{2}$ has $k$ vertex-disjoint cycles of different lengths, where the degree bound is best possible. We also consider other cases such as when the graph is triangle-free, or the $k$ cycles are required to have different lengths modulo some value $r$. In the context of directed graphs, we consider a conjecture of Lichiardopol concerning the least minimum out-degree required for a digraph to have $k$ vertex-disjoint directed cycles of different lengths. We verify this conjecture for tournaments, and, by using the probabilistic method, for some regular digraphs and digraphs of small order.

New bounds on the decycling number of generalized de Bruijn digraphs

2017 ◽  
Vol 09 (05) ◽  
pp. 1750062
Author(s):  
Jyhmin Kuo ◽  
Hung-Lin Fu
Keyword(s):  
Feedback Vertex Set ◽  
Systematic Method ◽  
Minimum Cardinality ◽  
Disjoint Cycles ◽  
Decycling Number ◽  
Vertex Set ◽  
De Bruijn ◽  
De Bruijn Digraphs ◽  
Vertex Disjoint ◽  
De Bruijn Digraph

A set of vertices of a graph whose removal leaves an acyclic graph is referred as a decycling set, or a feedback vertex set, of the graph. The minimum cardinality of a decycling set of a graph [Formula: see text] is referred to as the decycling number of [Formula: see text]. For [Formula: see text], the generalized de Bruijn digraph [Formula: see text] is defined by congruence equations as follows: [Formula: see text] and [Formula: see text]. In this paper, we give a systematic method to find a decycling set of [Formula: see text] and give a new upper bound that improve the best known results. By counting the number of vertex-disjoint cycles with the idea of constrained necklaces, we obtain new lower bounds on the decycling number of generalized de Bruijn digraphs.

A Note on Vertex-Disjoint Cycles

2002 ◽  
Vol 11 (1) ◽  
pp. 97-102 ◽  
Author(s):  
JACQUES VERSTRAËTE
Keyword(s):  
Minimum Degree ◽  
Quadratic Function ◽  
Average Degree ◽  
Disjoint Cycles ◽  
Vertex Disjoint

Häggkvist and Scott asked whether one can find a quadratic function q(k) such that, if G is a graph of minimum degree at least q(k), then G contains vertex-disjoint cycles of k consecutive even lengths. In this paper, it is shown that if G is a graph of average degree at least k2+19k+10 with sufficiently many vertices, then G contains vertex-disjoint cycles of k consecutive even lengths, answering the above question in the affirmative. The coefficient of k2 cannot be decreased and, in this sense, this result is best possible.

scholarly journals Edge-Maximal Graphs Containing No $r$ Vertex-Disjoint Triangles

2017 ◽  
Vol 10 (1) ◽  
pp. 110
Author(s):  
Mohammad Hailat
Keyword(s):  
Graph Theory ◽  
Upper Bound ◽  
Tripartite Graph ◽  
Disjoint Cycles ◽  
Large N ◽  
Vertex Disjoint

An important problem in graph theory is that of determining the maximum number of edges in a given graph $G$ that contains no specific subgraphs. This problem has attracted the attention of many researchers. An example of such a problem is the determination of an upper bound on the number of edges of a graph that has no triangles. In this paper, we let $\mathcal{G}(n,V_{r,3})$ denote the class of graphs on $n$ vertices containing no $r$-vertex-disjoint cycles of length $3$. We show that for large $n$, $\mathcal{E}(G)\les \lfloor \frac{(n-r+1)^2}{4} \rfloor +(r-1)(n-r+1)$ for every $G\in\mathcal{G}(n,V_{r,3})$. Furthermore, equality holds if and only if $G=\Omega(n,r)=K_{r-1,\lfloor \frac{n-r+1}2\rfloor,\lceil \frac{n-r+1}2\rceil}$ where $\Omega(n,r)$ is a tripartite graph on $n$ vertices.

scholarly journals Covering a Graph with Cycles of Length at Least 4

10.37236/4099 ◽  
2018 ◽  
Vol 25 (1) ◽  
Author(s):  
Hong Wang
Keyword(s):  
Positive Integer ◽  
Minimum Degree ◽  
Disjoint Cycles ◽  
Vertex Disjoint

Let $G$ be a graph of order $n\geq 4k$, where $k$ is a positive integer. Suppose that the minimum degree of $G$ is at least $\lceil n/2\rceil$. We show that $G$ contains $k$ vertex-disjoint cycles covering all the vertices of $G$ such that $k-1$ of them are quadrilaterals.

ON DISJOINT CYCLES

1994 ◽  
Vol 05 (01) ◽  
pp. 59-68 ◽  
Author(s):  
HANS L. BODLAENDER
Keyword(s):  
Feedback Vertex Set ◽  
Edge Disjoint ◽  
Disjoint Cycles ◽  
Vertex Set ◽  
Test Algorithm ◽  
Vertex Disjoint

It is shown, that for each constant k≥1, the following problems can be solved in [Formula: see text] time: given a graph G, determine whether G has k vertex disjoint cycles, determine whether G has k edge disjoint cycles, determine whether G has a feedback vertex set of size ≤k. Also, every class [Formula: see text], that is closed under minor taking, taking, and that does not contain the graph consisting of k disjoint copies of K3, has an [Formula: see text] membership test algorithm.

scholarly journals Remarks on the Local Irregularity Conjecture

Mathematics ◽  
2021 ◽  
Vol 9 (24) ◽  
pp. 3209
Author(s):  
Jelena Sedlar ◽  
Riste Škrekovski
Keyword(s):  
Minimum Degree ◽  
Edge Coloring ◽  
Chromatic Index ◽  
Sparse Graphs ◽  
Unicyclic Graphs ◽  
Disjoint Cycles ◽  
Cactus Graph ◽  
Colorable Graph ◽  
Vertex Disjoint ◽  
Local Irregularity

A locally irregular graph is a graph in which the end vertices of every edge have distinct degrees. A locally irregular edge coloring of a graph G is any edge coloring of G such that each of the colors induces a locally irregular subgraph of G. A graph G is colorable if it allows a locally irregular edge coloring. The locally irregular chromatic index of a colorable graph G, denoted by χirr′(G), is the smallest number of colors used by a locally irregular edge coloring of G. The local irregularity conjecture claims that all graphs, except odd-length paths, odd-length cycles and a certain class of cacti are colorable by three colors. As the conjecture is valid for graphs with a large minimum degree and all non-colorable graphs are vertex disjoint cacti, we study rather sparse graphs. In this paper, we give a cactus graph B which contradicts this conjecture, i.e., χirr′(B)=4. Nevertheless, we show that the conjecture holds for unicyclic graphs and cacti with vertex disjoint cycles.

scholarly journals A fast $$(2 + \frac{2}{7})$$-approximation algorithm for capacitated cycle covering

2021 ◽  
Author(s):  
Vera Traub ◽  
Thorben Tröbst
Keyword(s):  
Vehicle Routing Problem ◽  
Covering Problem ◽  
Post Processing ◽  
Routing Problem ◽  
Cycle Cover ◽  
Disjoint Cycles ◽  
Processing Step ◽  
Number Of Cycles ◽  
Vertex Disjoint ◽  
Capacitated Vehicle

AbstractWe consider the capacitated cycle covering problem: given an undirected, complete graph G with metric edge lengths and demands on the vertices, we want to cover the vertices with vertex-disjoint cycles, each serving a demand of at most one. The objective is to minimize a linear combination of the total length and the number of cycles. This problem is closely related to the capacitated vehicle routing problem (CVRP) and other cycle cover problems such as min-max cycle cover and bounded cycle cover. We show that a greedy algorithm followed by a post-processing step yields a $$(2 + \frac{2}{7})$$ ( 2 + 2 7 ) -approximation for this problem by comparing the solution to a polymatroid relaxation. We also show that the analysis of our algorithm is tight and provide a $$2 + \epsilon $$ 2 + ϵ lower bound for the relaxation.

scholarly journals Cycle partitions of regular graphs

2020 ◽  
pp. 1-24
Author(s):  
Vytautas Gruslys ◽  
Shoham Letzter
Keyword(s):  
Regular Graph ◽  
Bipartite Graphs ◽  
Disjoint Paths ◽  
Regular Graphs ◽  
Simple Construction ◽  
Vertex Set ◽  
Almost All ◽  
Vertex Disjoint

Abstract Magnant and Martin conjectured that the vertex set of any d-regular graph G on n vertices can be partitioned into $n / (d+1)$ paths (there exists a simple construction showing that this bound would be best possible). We prove this conjecture when $d = \Omega(n)$ , improving a result of Han, who showed that in this range almost all vertices of G can be covered by $n / (d+1) + 1$ vertex-disjoint paths. In fact our proof gives a partition of V(G) into cycles. We also show that, if $d = \Omega(n)$ and G is bipartite, then V(G) can be partitioned into n/(2d) paths (this bound is tight for bipartite graphs).

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