Cycle Double Cover Research Articles

Perfect Pseudo-Matchings in Cubic Graphs

AbstractA M in a cubic graph G is a spanning subgraph of G such that every component of M is isomorphic to $$K_2$$ K 2 or to $$K_{1,3}$$ K 1 , 3 . In view of snarks G with dominating cycle C, this is a natural generalization of perfect matchings since $$G \setminus E(C)$$ G \ E ( C ) is a perfect pseudo-matching. Of special interest are such M where G/M is planar because such G have a cycle double cover. We show that various well known classes of snarks contain planarizing perfect pseudo-matchings, and that there are at least as many snarks with planarizing perfect pseudo-matchings as there are cyclically $$5-$$ 5 - edge-connected snarks.

Counting circuit double covers

AbstractWe study a counting version of Cycle Double Cover Conjecture. We discuss why it is more interesting to count circuits (i.e., graphs isomorphic to for some ) instead of cycles (graphs with all degrees even). We give an almost‐exponential lower bound for graphs with a surface embedding of representativity at least 4. We also prove an exponential lower bound for planar graphs. We conjecture that any bridgeless cubic graph has at least circuit double covers and we show an infinite class of graphs for which this bound is tight.

Five‐cycle double cover and shortest cycle cover

AbstractThe 5‐even subgraph cycle double cover conjecture (5‐CDC conjecture) asserts that every bridgeless graph has a 5‐even subgraph double cover. A shortest even subgraph cover of a graph is a family of even subgraphs which cover all the edges of and the sum of their lengths is minimum. It is conjectured that every bridgeless graph has an even subgraph cover with total length at most . In this paper, we study those two conjectures for weak oddness 2 cubic graphs and present a sufficient condition for such graphs to have a 5‐CDC containing a member with many vertices. As a corollary, we show that for every oddness 2 cubic graph satisfying the sufficient condition has a 4‐even subgraph ‐cover with total length at most . We also show that every oddness 2 cubic graph with girth at least 30 has a 5‐CDC containing a member of length at least and thus it has a 4‐even subgraph ‐cover with total length at most .

Some snarks are worse than others

Many conjectures and open problems in graph theory can either be reduced to cubic graphs or are directly stated for them. Furthermore, it is known that for a lot of problems, a counterexample must be a snark, that is, a bridgeless cubic graph which is not 3-edge-colourable. In this paper we deal with the fact that the family of potential counterexamples to many interesting conjectures can be narrowed even further to the family S≥5 of bridgeless cubic graphs whose edge set cannot be covered with four perfect matchings. The Cycle Double Cover Conjecture, the Shortest Cycle Cover Conjecture and the Fan–Raspaud Conjecture are examples of statements for which S≥5 is crucial. In this paper we study parameters which have the potential to further refine S≥5 and thus enlarge the set of cubic graphs for which the mentioned conjectures can be verified. We show that S≥5 can be naturally decomposed into subsets with increasing complexity, thereby producing a natural scale for proving these conjectures. More precisely, we consider the following parameters and questions: given a bridgeless cubic graph, (i) how many perfect matchings need to be added, (ii) how many copies of the same perfect matching need to be added, and (iii) how many 2-factors need to be added so that the resulting regular graph is Class I? We present new results for these parameters and we also establish some strong relations between these problems and some long-standing conjectures.

Cubic graphs that cannot be covered with four perfect matchings

A conjecture of Berge suggests that every bridgeless cubic graph can have its edges covered with at most five perfect matchings. Since three perfect matchings suffice only when the graph in question is 3-edge-colourable, the rest of cubic graphs fall into two classes: those that can be covered with four perfect matchings, and those that need at least five. Cubic graphs that require more than four perfect matchings to cover their edges are particularly interesting as potential counterexamples to several profound and long-standing conjectures including the celebrated cycle double cover conjecture. However, so far they have been extremely difficult to find.In this paper we build a theory that describes coverings with four perfect matchings as flows whose flow values represent points and outflow patterns represent lines of a configuration of ten points and six lines spanned by four points of the 3-dimensional projective space P3(F2) in general position. This theory provides powerful tools for investigation of graphs that do not admit such a cover and offers a great variety of methods for their construction. As an illustrative example we produce a rich family of snarks (nontrivial cubic graphs with no 3-edge-colouring) that cannot be covered with four perfect matchings. The family contains all previously known graphs with this property.

Perfect Matching Index versus Circular Flow Number of a Cubic Graph

The perfect matching index of a cubic graph $G$, denoted by $\pi(G)$, is the smallest number of perfect matchings that cover all the edges of $G$. According to the Berge--Fulkerson conjecture, $\pi(G)\le5$ for every bridgeless cubic graph $G$. The class of graphs with $\pi\ge 5$ is of particular interest as many conjectures and open problems, including the famous cycle double cover conjecture, can be reduced to it. Although nontrivial examples of such graphs are very difficult to find, a few infinite families are known, all with circular flow number $\Phi_c(G)=5$. It has been therefore suggested [Abreu et al., Electron. J. Combin., 23 (2016), P3.54] that $\pi(G)\ge 5$ might imply $\Phi_c(G)\ge 5$. In this article we dispel these hopes and present a family of cyclically 4-edge-connected cubic graphs of girth at least 5 with $\pi\ge 5$ and $\Phi_c\le 4+\frac23$.

Short Cycle Covers of Graphs with at Most 77% Vertices of Degree Two

Let $G$ be a bridgeless multigraph with $m$ edges and $n_2$ vertices of degree two and let $cc(G)$ be the length of its shortest cycle cover. It is known that if $cc(G) < 1.4m$ in bridgeless graphs with $n_2 \le m/10$, then the Cycle Double Cover Conjecture holds. Fan (2017) proved that if $n_2 = 0$, then $cc(G) < 1.6258m$ and $cc(G) < 1.6148m$ provided that $G$ is loopless; morever, if $n_2 \le m/30$, then $cc(G) < 1.6467m$. We show that for a bridgeless multigraph with $m$ edges and $n_2$ vertices of degree two, $cc(G) < 1.6148m + 0.0741n_2$. Therefore, if $n_2=0$, then $cc(G) < 1.6148m$ even if $G$ has loops; if $n_2 \le m/30$, then $cc(G) < 1.6173m$; and if $n_2 \le m/10$, then $cc(G) < 1.6223|E(G)|$. Our improvement is obtained by randomizing Fan's construction.

Homomorphisms of Cayley graphs and Cycle Double Covers

We study the following conjecture of Matt DeVos: If there is a graph homomorphism from a Cayley graph $\mathrm{Cay}(M, B)$ to another Cayley graph $\mathrm{Cay}(M', B')$ then every graph with an $(M,B)$-flow has an $(M',B')$-flow. This conjecture was originally motivated by the flow-tension duality. We show that a natural strengthening of this conjecture does not hold in all cases but we conjecture that it still holds for an interesting subclass of them and we prove a partial result in this direction. We also show that the original conjecture implies the existence of an oriented cycle double cover with a small number of cycles.

A model for finding transition-minors

The well known cycle double cover conjecture in graph theory is strongly related to the compatible circuit decomposition problem. A recent result by Fleischner et al. (2018) gives a sufficient condition for the existence of a compatible circuit decomposition in a transitioned 2-connected Eulerian graph, which is based on an extension of the definition of K5-minors to transitioned graphs. Graphs satisfying this condition are called SUD-K5-minor-free graphs. In this work we formulate a generalization of this property by replacing the K5 by a 4-regular transitioned graph H, which is part of the input. Furthermore, we consider the decision problem of checking for two given graphs if the extended property holds. We prove that this problem is NP-complete and fixed parameter tractable with the size of H as parameter. We then formulate an equivalent problem, present a mathematical model for it, and prove its correctness. This mathematical model is then translated into a mixed integer linear program (MIP) for solving it in practice. Computational results show that the MIP formulation can be solved for small instances in reasonable time. In our computations we found snarks with perfect matchings whose contraction leads to SUD-K5-minor-free graphs that contain K5-minors. Furthermore, we verified that there exists a perfect pseudo-matching whose contraction leads to a SUD-K5-minor-free graph for all snarks with up to 22 vertices.

Cycle decompositions of pathwidth‐6 graphs

AbstractHajós' conjecture asserts that a simple Eulerian graph on vertices can be decomposed into at most cycles. The conjecture is only proved for graph classes in which every element contains vertices of degree 2 or 4. We develop new techniques to construct cycle decompositions. They work on the common neighborhood of two degree‐6 vertices. With these techniques, we find structures that cannot occur in a minimal counterexample to Hajós' conjecture and verify the conjecture for Eulerian graphs of pathwidth at most 6. This implies that these graphs satisfy the small cycle double cover conjecture.

Normal edge‐colorings of cubic graphs

AbstractA normal ‐edge‐coloring of a cubic graph is a proper edge‐coloring with colors having the additional property that when looking at the set of colors assigned to any edge and the four edges adjacent to it, we have either exactly five distinct colors or exactly three distinct colors. We denote by the smallest , for which admits a normal ‐edge‐coloring. Normal ‐edge‐colorings were introduced by Jaeger to study his well‐known Petersen Coloring Conjecture. More precisely, it is known that proving for every bridgeless cubic graph is equivalent to proving the Petersen Coloring Conjecture and then it implies, among others, Cycle Double Cover Conjecture and Berge‐Fulkerson Conjecture. Considering the larger class of all simple cubic graphs (not necessarily bridgeless), some interesting questions naturally arise. For instance, there exist simple cubic graphs, not bridgeless, with . In contrast, the known best general upper bound for was . Here, we improve it by proving that for any simple cubic graph , which is best possible. We obtain this result by proving the existence of specific nowhere zero ‐flows in ‐edge‐connected graphs.

Normal 6-edge-colorings of some bridgeless cubic graphs

In an edge-coloring (proper) of a cubic graph, an edge is poor or rich, if the set of colors assigned to the edge and the four edges adjacent it, has exactly three or exactly five distinct colors, respectively. An edge is normal in an edge-coloring if it is rich or poor in this coloring. A normal k-edge-coloring of a cubic graph is an edge-coloring with k colors such that each edge of the graph is normal. We denote by χN′(G) the smallest k, for which G admits a normal k-edge-coloring. Normal edge-colorings were introduced by Jaeger in order to study his well-known Petersen Coloring Conjecture. It is known that proving χN′(G)≤5 for every bridgeless cubic graph is equivalent to proving Petersen Coloring Conjecture. Moreover, Jaeger was able to show that it implies classical conjectures like Cycle Double Cover Conjecture and Berge–Fulkerson Conjecture. Recently, two of the authors were able to show that any simple cubic graph admits a normal 7-edge-coloring, and this result is best possible. In the present paper, we show that any claw-free bridgeless cubic graph, permutation snark, tree-like snark admits a normal 6-edge-coloring. Finally, we show that any bridgeless cubic graph G admits a 6-edge-coloring such that at least 79⋅|E| edges of G are normal.

Cycle double covers and non-separating cycles

Which 2-regular subgraph R of a cubic graph G can be extended to a cycle double cover of G? We provide a condition which ensures that every R satisfying this condition is part of a cycle double cover of G. As one consequence, we prove that every 2-connected cubic graph which has a decomposition into a spanning tree and a 2-regular subgraph C consisting of k circuits with k≤3, has a cycle double cover containing C.

Smallest snarks with oddness 4 and cyclic connectivity 4 have order 44

The family of snarks -- connected bridgeless cubic graphs that cannot be 3-edge-coloured -- is well-known as a potential source of counterexamples to several important and long-standing conjectures in graph theory. These include the cycle double cover conjecture, Tutte's 5-flow conjecture, Fulkerson's conjecture, and several others. One way of approaching these conjectures is through the study of structural properties of snarks and construction of small examples with given properties. In this paper we deal with the problem of determining the smallest order of a nontrivial snark (that is, one which is cyclically 4-edge-connected and has girth at least 5) of oddness at least 4. Using a combination of structural analysis with extensive computations we prove that the smallest order of a snark with oddness at least 4 and cyclic connectivity 4 is 44. Formerly it was known that such a snark must have at least 38 vertices [J. Combin. Theory Ser. B 103 (2013), 468--488] and one such snark on 44 vertices was constructed by Lukot'ka et al. [Electron. J. Combin. 22 (2015), #P1.51]. The proof requires determining all cyclically 4-edge-connected snarks on 36 vertices, which extends the previously compiled list of all such snarks up to 34 vertices [J. Combin. Theory Ser. B, loc. cit.]. As a by-product, we use this new list to test the validity of several conjectures where snarks can be smallest counterexamples.

Measures of Edge-Uncolorability of Cubic Graphs

There are many hard conjectures in graph theory, like Tutte's 5-flow conjecture, and the $5$-cycle double cover conjecture, which would be true in general if they would be true for cubic graphs. Since most of them are trivially true for $3$-edge-colorable cubic graphs, cubic graphs which are not $3$-edge-colorable, often called snarks, play a key role in this context. Here, we survey parameters measuring how far apart a non $3$-edge-colorable graph is from being $3$-edge-colorable. We study their interrelation and prove some new results. Besides getting new insight into the structure of snarks, we show that such measures give partial results with respect to these important conjectures. The paper closes with a list of open problems and conjectures.

Signed Cycle Double Covers

The cycle double cover conjecture states that every bridgeless graph has a collection of cycles which together cover every edge of the graph exactly twice. A signed graph is a graph with each edge assigned by a positive or a negative sign. In this article, we prove a weak version of this conjecture that is the existence of a signed cycle double cover for all bridgeless graphs. We also show the relationships of the signed cycle double cover and other famous conjectures such as the Tutte flow conjectures and the shortest cycle cover conjecture etc.

Cycle double covers via Kotzig graphs

We show that every 2-connected cubic graph G has a cycle double cover if G has a spanning subgraph F such that (i) every component of F has an even number of vertices (ii) every component of F is either a cycle or a subdivision of a Kotzig graph and (iii) the components of F are connected to each other in a certain general manner.

Homomorphisms of Cayley graphs and Cycle Double Covers

We study the following conjecture of Matt DeVos: If there is a graph homomorphism from Cayley graph Cay(M, B) to another Cayley graph Cay(M′, B′) then every graph with (M, B)-flow has (M′, B′)-flow. This conjecture was originally motivated by the flow-tension duality. We show that a natural strengthening of this conjecture does not hold in all cases but we conjecture that it still holds for an interesting subclass of them and we prove a partial result in this direction. We also show that the original conjecture implies the existence of oriented cycle double cover with a small number of cycles.

Cycle double covers and long circuits of graphs

Cycle double covers and long circuits of graphs

Integer 4-Flows and Cycle Covers

Let G be a bridgeless graph and denote by cc(G) the shortest length of a cycle cover of G. Let V2(G) be the set of vertices of degree 2 in G. It is known that if cc(G)≤1.4|E(G)| for every bridgeless graph G with |V2(G)|≤ $$\frac{1}{10}$$ |E(G)|, then the Cycle Double Cover Conjecture is true. The best known result cc(G)≤ $$\frac{5}{3}$$ |E(G)| (≈1.6667|E(G)|) was established over 30 years ago. Recently, it was proved that cc(G) ≤ $$\frac{44}{27}$$ |E(G)| (≈ 1.6296|E(G)|) for loopless graphs with minimum degree at least 3. In this paper, we obtain results on integer 4-flows, which are used to find bounds for cc(G). We prove that if G has minimum degree at least 3 (loops being allowed), then cc(G)<1.6258|E(G)|. As a corollary, adding loops to vertices of degree 2, we obtain that cc(G)<1.6466|E(G)| for every bridgeless graph G with |V2(G)|≤ $$\frac{1}{30}$$ |E(G)|.