Let S be an n-element set and F⊂(Sk) an intersecting family. Improving earlier results it is proved that for n>72k there is an element of S that is contained in all but (n−3k−2) members of F. One of the main ingredients of the proof is the following statement. If G⊂(Sk) is intersecting, |G|≥(n−2k−2) and n≥72k then there is an element of S that is contained in more than half of the members of G.

For integers k≥3 and 1≤ℓ≤k−1, we prove that for any α>0, there exist ϵ>0 and C>0 such that for sufficiently large n∈(k−ℓ)N, the union of a k-uniform hypergraph with minimum vertex degree αnk−1 and a binomial random k-uniform hypergraph G(k)(n,p) with p≥n−(k−ℓ)−ϵ for ℓ≥2 and p≥Cn−(k−1) for ℓ=1 on the same vertex set contains a Hamiltonian ℓ-cycle with high probability. Our result is best possible up to the values of ϵ and C and answers a question of Krivelevich, Kwan and Sudakov.

A recent conjecture of the author and Teng Fang states that there are only finitely many finite simple groups with no cubic graphical regular representation. In this paper, we make crucial progress towards this conjecture by giving an affirmative answer for groups of Lie type of large rank.

We prove that every strongly 1050t-connected tournament contains all possible 1-factors with at most t components and this is best possible up to constant. In addition, we can ensure that each cycle in the 1-factor contains a prescribed vertex. This answers a question by Kühn, Osthus, and Townsend. Indeed, we prove more results on partitioning tournaments. We prove that a strongly Ω(k4tq)-connected tournament admits a vertex partition into t strongly k-connected tournaments with prescribed sizes such that each tournament contains q prescribed vertices, provided that the prescribed sizes are Ω(n). This result improves the earlier result of Kühn, Osthus, and Townsend. We also prove that for a strongly Ω(t)-connected n-vertex tournament T and given 2t distinct vertices x1,…,xt,y1,…,yt of T, we can find t vertex disjoint paths P1,…,Pt such that each path Pi connecting xi and yi has the prescribed length, provided that the prescribed lengths are Ω(n). For both results, the condition of connectivity being linear in t is best possible, and the condition of prescribed sizes being Ω(n) is also best possible.

Given two k-graphs H and F, a perfect F-packing in H is a collection of vertex-disjoint copies of F in H which together cover all the vertices in H. In the case when F is a single edge, a perfect F-packing is simply a perfect matching. For a given fixed F, it is often the case that the decision problem whether an n-vertex k-graph H contains a perfect F-packing is NP-complete. Indeed, if k≥3, the corresponding problem for perfect matchings is NP-complete [17], [7] whilst if k=2 the problem is NP-complete in the case when F has a component consisting of at least 3 vertices [14]. In this paper we give a general tool which can be used to determine classes of (hyper)graphs for which the corresponding decision problem for perfect F-packings is polynomial time solvable. We then give three applications of this tool: (i) Given 1≤ℓ≤k−1, we give a minimum ℓ-degree condition for which it is polynomial time solvable to determine whether a k-graph satisfying this condition has a perfect matching; (ii) Given any graph F we give a minimum degree condition for which it is polynomial time solvable to determine whether a graph satisfying this condition has a perfect F-packing; (iii) We also prove a similar result for perfect K-packings in k-graphs where K is a k-partite k-graph. For a range of values of ℓ,k (i) resolves a conjecture of Keevash, Knox and Mycroft [20]; (ii) answers a question of Yuster [47] in the negative; whilst (iii) generalises a result of Keevash, Knox and Mycroft [20]. In many cases our results are best possible in the sense that lowering the minimum degree condition means that the corresponding decision problem becomes NP-complete.

We disprove a conjecture of Frank [3] stating that each weakly 2k-connected graph has a k-vertex-connected orientation. For k≥3, we also prove that the problem of deciding whether a graph has a k-vertex-connected orientation is NP-complete.

A graph G is k-ordered if for any distinct vertices v1,v2,…,vk∈V(G), it has a cycle through v1,v2,…,vk in order. Let f(k) denote the minimum integer so that every f(k)-connected graph is k-ordered. The first non-trivial case of determining f(k) is when k=4, where the previously best known bounds are 7≤f(4)≤40. We prove that in fact f(4)=7.

Twenty years ago Bondy and Vince conjectured that for any nonnegative integer k, except finitely many counterexamples, every graph with k vertices of degree less than three contains two cycles whose lengths differ by one or two. The case k≤2 was proved by Bondy and Vince, which resolved an earlier conjecture of Erdős et al. In this paper we confirm this conjecture for all k.

Assume k is a positive integer, λ={k1,k2,…,kq} is a partition of k and G is a graph. A λ-assignment of G is a k-assignment L of G such that the colour set ⋃v∈V(G)L(v) can be partitioned into q subsets C1∪C2…∪Cq and for each vertex v of G, |L(v)∩Ci|=ki. We say G is λ-choosable if for each λ-assignment L of G, G is L-colourable. It follows from the definition that if λ={k}, then λ-choosable is the same as k-choosable, if λ={1,1,…,1}, then λ-choosable is equivalent to k-colourable. For the other partitions of k sandwiched between {k} and {1,1,…,1} in terms of refinements, λ-choosability reveals a complex hierarchy of colourability of graphs. We prove that for two partitions λ,λ′ of k, every λ-choosable graph is λ′-choosable if and only if λ′ is a refinement of λ. Then we study λ-choosability of special families of graphs. The Four Colour Theorem says that every planar graph is {1,1,1,1}-choosable. A very recent result of Kemnitz and Voigt implies that for any partition λ of 4 other than {1,1,1,1}, there is a planar graph which is not λ-choosable. We observe that, in contrast to the fact that there are non-4-choosable 3-chromatic planar graphs, every 3-chromatic planar graph is {1,3}-choosable, and that if G is a planar graph whose dual G⁎ has a connected spanning Eulerian subgraph, then G is {2,2}-choosable. We prove that if n is a positive even integer, λ is a partition of n−1 in which each part is at most 3, then Kn is edge λ-choosable. Finally we study relations between λ-choosability of graphs and colouring of signed graphs and generalized signed graphs. A conjecture of Máčajová, Raspaud and Škoviera that every planar graph is signed 4-colcourable is recently disproved by Kardoš and Narboni. We prove that every signed 4-colourable graph is weakly 4-choosable, and every signed Z4-colourable graph is {1,1,2}-choosable. The later result combined with the above result of Kemnitz and Voigt disproves a conjecture of Kang and Steffen that every planar graph is signed Z4-colourable. We shall show that a graph constructed by Wegner in 1973 is also a counterexample to Kang and Steffen's conjecture, and present a new construction of a non-{1,3}-choosable planar graphs.

A key theorem in algorithmic graph-minor theory is a min-max relation between the treewidth of a graph (i.e., the minimum width of a tree-decomposition) and the maximum size of a grid minor. This min-max relation is a keystone of the graph minor theory of Robertson and Seymour, which ultimately proves Wagner's Conjecture about properties of minor-closed graphs. Demaine and Hajiaghayi proved a remarkable linear min-max relation for graphs excluding any fixed minor H: every H-minor-free graph of treewidth at least cHr has an r×r-grid minor for some constant cH. However, as they pointed out, a major issue with this theorem is that their proof heavily depends on the graph minor theory, most of which lacks explicit bounds and is believed to have very large bounds. Motivated by this problem, we give another (relatively short and simple) proof of this result without using the machinery of the graph minor theory. Hence we give an explicit bound for cH, which is an exponential function of a polynomial in |H|. Furthermore, our result gives a constant w=2O(r2log⁡r) such that every graph of treewidth at least w has an r×r-grid minor.

In 1998, Reed conjectured that every graph G satisfies χ(G)≤⌈12(Δ(G)+1+ω(G))⌉, where χ(G) is the chromatic number of G, Δ(G) is the maximum degree of G, and ω(G) is the clique number of G. As evidence for his conjecture, he proved an “epsilon version” of it, i.e. that there exists some ε>0 such that χ(G)≤(1−ε)(Δ(G)+1)+εω(G). It is natural to ask if Reed's conjecture or an epsilon version of it is true for the list-chromatic number. In this paper we consider a “local version” of the list-coloring version of Reed's conjecture. Namely, we conjecture that if G is a graph with list-assignment L such that for each vertex v of G, |L(v)|≥⌈12(d(v)+1+ω(v))⌉, where d(v) is the degree of v and ω(v) is the size of the largest clique containing v, then G is L-colorable. Our main result is that an “epsilon version” of this conjecture is true, under some mild assumptions. Using this result, we also prove a significantly improved lower bound on the density of k-critical graphs with clique number less than k/2, as follows. For every α>0, if ε≤α21350, then if G is an L-critical graph for some k-list-assignment L such that ω(G)<(12−α)k and k is sufficiently large, then G has average degree at least (1+ε)k. This implies that for every α>0, there exists ε>0 such that if G is a graph with ω(G)≤(12−α)mad(G), where mad(G) is the maximum average degree of G, then χℓ(G)≤⌈(1−ε)(mad(G)+1)+εω(G)⌉. It also yields an improvement on the best known upper bound for the chromatic number of Kt-minor free graphs for large t, by a factor of .99982.

In 1968, Ringel and Youngs confirmed the last open case of the Heawood Conjecture by determining the genus of every complete graph Kn. In this paper, we investigate the minimum genus embeddings of the complete 3-uniform hypergraphs Kn3. Embeddings of a hypergraph H are defined as the embeddings of its associated Levi graph LH with vertex set V(H)⊔E(H), in which v∈V(H) and e∈E(H) are adjacent if and only if v and e are incident in H. We determine both the orientable and the non-orientable genus of Kn3 when n is even. Moreover, it is shown that the number of non-isomorphic minimum genus embeddings of Kn3 is at least 214n2log⁡n(1−o(1)). The construction in the proof may be of independent interest as a design-type problem.

We revisit the classical question of the relationship between the diameter of a graph and its expansion properties. One direction is well understood: expander graphs exhibit essentially the lowest possible diameter. We focus on the reverse direction, showing that “sufficiently large” graphs of fixed diameter and degree must be “good” expanders. We prove this statement for various definitions of “sufficiently large” (multiplicative/additive factor from the largest possible size), for different forms of expansion (edge, vertex, and spectral expansion), and for both directed and undirected graphs. A recurring theme is that the lower the diameter of the graph and (more importantly) the larger its size, the better the expansion guarantees. Aside from inherent theoretical interest, our motivation stems from the domain of network design. Both low-diameter networks and expanders are prominent approaches to designing high-performance networks in parallel computing, HPC, datacenter networking, and beyond. Our results establish that these two approaches are, in fact, inextricably intertwined. We leave the reader with many intriguing questions for future research.

In this series of two papers we examine the classical problem of ranking a set of players on the basis of a set of pairwise comparisons arising from a sports tournament, with the objective of minimizing the total number of upsets, where an upset occurs if a higher ranked player was actually defeated by a lower ranked player. This problem can be rephrased as the so-called minimum feedback arc set problem on tournaments, which arises in a rich variety of applications and has been a subject of extensive research. In this series we study this NP-hard problem using structure-driven and linear programming approaches. Let T=(V,A) be a tournament with a nonnegative integral weight w(e) on each arc e. A subset F of arcs is called a feedback arc set if T\F contains no cycles (directed). A collection C of cycles (with repetition allowed) is called a cycle packing if each arc e is used at most w(e) times by members of C. We call T cycle Mengerian (CM) if, for every nonnegative integral function w defined on A, the minimum total weight of a feedback arc set is equal to the maximum size of a cycle packing. The purpose of these two papers is to show that a tournament is CM iff it contains none of four Möbius ladders as a subgraph; such a tournament is referred to as Möbius-free. In this first paper we present a structural description of all Möbius-free tournaments, which relies heavily on a chain theorem concerning internally 2-strong tournaments.

Let M be a 3-connected matroid, and let N be a 3-connected minor of M. We say that a pair {x1,x2}⊆E(M) is N-detachable if one of the matroids M/x1/x2 or M\x1\x2 is both 3-connected and has an N-minor. This is the first in a series of three papers where we describe the structures that arise when M has no N-detachable pairs. In this paper, we prove that if no N-detachable pair can be found in M, then either M has a 3-separating set, which we call X, with certain strong structural properties, or M has one of three particular 3-separators that can appear in a matroid with no N-detachable pairs.

Every 9-regular graph (possibly with multiple edges) with odd edge-connectivity >5 can be edge-decomposed into three 3-factors. If Tutte's 3-flow conjecture is true, it also holds for all 9-regular graphs with odd edge-connectivity 5, but not with odd edge-connectivity 3. It holds for all planar 2-edge-connected 9-regular graphs, an equivalent version of the 4-color theorem for planar graphs. We address the more general question: If G is an r-regular graph, and r=kq where k,q are natural numbers >1, can G be edge-decomposed into k q-factors? If q is even, then the decomposition exists trivially. If k,q are both odd, then we prove that the decomposition exists if G has odd edge-connectivity (size of smallest odd edge-cut) at least 3k−2, which is satisfied if the odd edge-connectivity is at least r−2. If q is odd and k is even, then we must require that G has an even number of vertices just to guarantee that G has a q-factor. If we want a decomposition into q-factors, then we also need the condition that, for any partition of the vertex set of G into two odd parts, there must be at least k edges between the parts. We prove that the edge-decomposition into q-factors is always possible if G has an even number of vertices and the edge-connectivity of G is at least 2k2+k.

An edge coloring of the n-vertex complete graph, Kn, is a Gallai coloring if it does not contain any rainbow triangle, that is, a triangle whose edges are colored with three distinct colors. We prove that for n large and every k with k≤2n/4300, the number of Gallai colorings of Kn that use at most k given colors is ((k2)+on(1))2(n2). Our result is asymptotically best possible and implies that, for those k, almost all Gallai k-colorings use only two colors. However, this is not true for k≥2n/2.

The number of cycles in a graph containing any fixed edge and also containing all vertices of odd degree is odd if and only if all vertices have even degree. If all vertices have even degree this is a theorem of Shunichi Toida. If all vertices have odd degree it is Andrew Thomason's extension of Smith's theorem.

A well known theorem of Kuratowski in 1932 states that a graph is planar if, and only if, it does not contain a subdivision of K5 or K3,3. Wagner proved in 1937 that if a graph other than K5 does not contain any subdivision of K3,3 then it is planar or it admits a cut of size at most 2. Kelmans and, independently, Seymour conjectured in the 1970s that if a graph does not contain any subdivision of K5 then it is planar or it admits a cut of size at most 4. In this paper, we give a proof of the Kelmans-Seymour conjecture. We also discuss several related results and problems.

The parity of the length of paths and cycles is a classical and well-studied topic in graph theory and theoretical computer science. The parity constraints can be extended to label constraints in a group-labeled graph, which is a directed graph with each arc labeled by an element of a group. Recently, paths and cycles in group-labeled graphs have been investigated, such as packing non-zero paths and cycles, where “non-zero” means that the identity element is a unique forbidden label. In this paper, we present a solution to finding an s–t path with two labels forbidden in a group-labeled graph. This also leads to an elementary solution to finding a zero s–t path in a Z3-labeled graph, which is the first nontrivial case of finding a zero path. This situation in fact generalizes the 2-disjoint paths problem in undirected graphs, which also motivates us to consider that setting. More precisely, we provide a polynomial-time algorithm for testing whether there are at most two possible labels of s–t paths in a group-labeled graph or not, and finding s–t paths attaining at least three distinct labels if exist. The algorithm is based on a necessary and sufficient condition for a group-labeled graph to have exactly two possible labels of s–t paths, which is the main technical contribution of this paper.

Seymour and, independently, Kelmans conjectured in the 1970s that every 5-connected nonplanar graph contains a subdivision of K5. This conjecture was proved by Ma and Yu for graphs containing K4−, and an important step in their proof is to deal with a 5-separation in the graph with a planar side. In order to establish the Kelmans-Seymour conjecture for all graphs, we need to consider 5-separations and 6-separations with less restrictive structures. The goal of this paper is to deal with special 5-separations and 6-separations, including those with an apex side. Results will be used in subsequent papers to prove the Kelmans-Seymour conjecture.

We use K4− to denote the graph obtained from K4 by removing an edge, and use TK5 to denote a subdivision of K5. Let G be a 5-connected nonplanar graph and {x1,x2,y1,y2}⊆V(G) such that G[{x1,x2,y1,y2}]≅K4− with y1y2∉E(G). Let w1,w2,w3∈N(y2)−{x1,x2} be distinct. We show that G contains a TK5 in which y2 is not a branch vertex, or G−y2 contains K4−, or G has a special 5-separation, or G−{y2v:v∉{w1,w2,w3,x1,x2}} contains TK5. This result will be used to prove the Kelmans-Seymour conjecture that every 5-connected nonplanar graph contains TK5.

Let G be a 5-connected nonplanar graph and let x1,x2,y1,y2∈V(G) be distinct, such that G[{x1,x2,y1,y2}]≅K4− and y1y2∉E(G). We show that one of the following holds: G−x1 contains K4−, or G contains a K4− in which x1 is of degree 2, or G contains a TK5 in which x1 is not a branch vertex, or {x2,y1,y2} may be chosen so that for any distinct z0,z1∈N(x1)−{x2,y1,y2}, G−{x1v:v∉{z0,z1,x2,y1,y2}} contains TK5.

We show that each real-representable matroid is a minor of a complex-representable excluded minor for real-representability. More generally, for an infinite field F1 and a field extension F2, if F1-representability is not equivalent to F2-representability, then each F1-representable matroid is a minor of a F2-representable excluded minor for F1-representability.

We consider the Turán-type problem of bounding the size of a set M⊆F2n that does not contain a linear copy of a given fixed set N⊆F2k, where n is large compared to k. An Erdős-Stone type theorem [5] in this setting gives a bound that is tight up to a o(2n) error term; our first main result gives a stability version of this theorem, showing that such an M that is close in size to the upper bound in [5] is close to the obvious extremal example in the sense of symmetric difference. Our second result shows that the error term in [5] is exactly controlled by the solution to one of a class of ‘sparse’ extremal problems, and gives some examples where the error term can be eliminated completely to give a sharp upper bound on |M|.

We introduce a class of plane graphs called weak near-triangulations, and prove that this class is closed under certain graph operations. Then we use the properties of weak near-triangulations to prove that every plane triangulation on n>6 vertices has a dominating set of size at most 17n/53. This improves the bound n/3 obtained by Matheson and Tarjan.

We show that for every η>0 there exists n0 such that for every odd n≥n0 each 3-colouring of edges of a graph G with (4+η)n and minimum degree larger than (7/2+2η)n leads to a monochromatic cycle of length n. This result is, up to η terms, best possible.

Let G(n,r,s) denote a uniformly random r-regular s-uniform hypergraph on n vertices, where s is a fixed constant and r=r(n) may grow with n. An ℓ-overlapping Hamilton cycle is a Hamilton cycle in which successive edges overlap in precisely ℓ vertices, and 1-overlapping Hamilton cycles are called loose Hamilton cycles. When r,s≥3 are fixed integers, we establish a threshold result for the property of containing a loose Hamilton cycle. This partially verifies a conjecture of Dudek, Frieze, Ruciński and Šileikis (2015). In this setting, we also find the asymptotic distribution of the number of loose Hamilton cycles in G(n,r,s). Finally we prove that for ℓ=2,…,s−1 and for r growing moderately as n→∞, the probability that G(n,r,s) has a ℓ-overlapping Hamilton cycle tends to zero.

We give a complete answer to the GFR conjecture, proposed by Conder, Doyle, Tucker and Watkins: “All but finitely many Frobenius groups F=N⋊H with a given complement H have a GFR, with the exception when |H| is odd and N is Abelian but not an elementary 2-group”. Actually, we prove something stronger, we enumerate asymptotically GFRs; we show that, besides the exceptions listed above, as |N| tends to infinity, the proportion of GFRs among all Cayley graphs over N containing F in their automorphism group tends to 1.

The inverse eigenvalue problem of a given graph G is to determine all possible spectra of real symmetric matrices whose off-diagonal entries are governed by the adjacencies in G. Barrett et al. introduced the Strong Spectral Property (SSP) and the Strong Multiplicity Property (SMP) in [Generalizations of the Strong Arnold Property and the minimum number of distinct eigenvalues of a graph. Electron. J. Combin., 2017]. In that paper it was shown that if a graph has a matrix with the SSP (or the SMP) then a supergraph has a matrix with the same spectrum (or ordered multiplicity list) augmented with simple eigenvalues if necessary, that is, subgraph monotonicity. In this paper we extend this to a form of minor monotonicity, with restrictions on where the new eigenvalues appear. These ideas are applied to solve the inverse eigenvalue problem for all graphs of order five, and to characterize forbidden minors of graphs having at most one multiple eigenvalue.

The τ=2 Conjecture, the Replication Conjecture and the f-Flowing Conjecture, and the classification of binary matroids with the sums of circuits property are foundational to Clutter Theory and have far-reaching consequences in Combinatorial Optimization, Matroid Theory and Graph Theory. We prove that these conjectures and result can equivalently be formulated in terms of cuboids, which form a special class of clutters. Cuboids are used as means to (a) manifest the geometry behind primal integrality and dual integrality of set covering linear programs, and (b) reveal a geometric rift between these two properties, in turn explaining why primal integrality does not imply dual integrality for set covering linear programs. Along the way, we see that the geometry supports the τ=2 Conjecture. Studying the geometry also leads to over 700 new ideal minimally non-packing clutters over at most 14 elements, a surprising revelation as there was once thought to be only one such clutter.

A tournament T=(V,A) is called cycle Mengerian (CM) if it satisfies the minimax relation on packing and covering cycles, for every nonnegative integral weight function defined on A. The purpose of this series of two papers is to show that a tournament is CM iff it contains none of four Möbius ladders as a subgraph; such a tournament is referred to as Möbius-free. In the first paper we have given a structural description of all Möbius-free tournaments, and have proved that every CM tournament is Möbius-free. In this second paper we establish the converse by using our structural theorems and linear programming approach.

We prove that Gn,p=c/n w.h.p. has a k-regular subgraph if c is at least e−Θ(k) above the threshold for the appearance of a subgraph with minimum degree at least k; i.e. a non-empty k-core. In particular, this pins down the threshold for the appearance of a k-regular subgraph to a window of size e−Θ(k).

We relate two conjectures that play a central role in the reported proof of Rota's Conjecture. Let F be a finite field. The first conjecture states that: the branch-width of any F-representable N-fragile matroid is bounded by a function depending only upon F and N. The second conjecture states that: if a matroid M2 is obtained from a matroid M1 by relaxing a circuit-hyperplane and both M1 and M2 are F-representable, then the branch-width of M1 is bounded by a function depending only upon F. Our main result is that the second conjecture implies the first.

Given an edge colouring of a graph with a set of m colours, we say that the graph is m-coloured if each of the m colours is used. For an m-colouring Δ of N(2), the complete graph on N, we denote by FΔ the set all values γ for which there exists an infinite subset X⊂N such that X(2) is γ-coloured. Properties of this set were first studied by Erickson in 1994. Here, we are interested in estimating the minimum size of FΔ over all m-colourings Δ of N(2). Indeed, we shall prove the following result. There exists an absolute constant α>0 such that for any positive integer m≠{(n2)+1,(n2)+2:n≥2}, |FΔ|≥(1+α)2m, for any m-colouring Δ of N(2). This proves a conjecture of Narayanan. We remark the result is tight up to the value of α.

A simple topological graph is k-quasiplanar (k≥2) if it contains no k pairwise crossing edges, and k-planar if no edge is crossed more than k times. In this paper, we explore the relationship between k-planarity and k-quasiplanarity to show that, for k≥2, every k-planar simple topological graph can be transformed into a (k+1)-quasiplanar simple topological graph.

The linear arboricity of a graph G, denoted by la(G), is the minimum number of edge-disjoint linear forests (i.e. forests in which every connected component is a path) in G whose union covers all the edges of G. A famous conjecture due to Akiyama, Exoo, and Harary from 1981 asserts that la(G)≤⌈(Δ(G)+1)/2⌉, where Δ(G) denotes the maximum degree of G. This conjectured upper bound would be best possible, as is easily seen by taking G to be a regular graph. In this paper, we show that for every graph G, la(G)≤Δ2+O(Δ2/3−α) for some α>0, thereby improving the previously best known bound due to Alon and Spencer from 1992. For graphs which are sufficiently good spectral expanders, we give even better bounds. Our proofs of these results further give probabilistic polynomial time algorithms for finding such decompositions into linear forests.

A class of graphs is χ-bounded if there is a function f such that χ(G)≤f(ω(G)) for every induced subgraph G of every graph in the class, where χ,ω denote the chromatic number and clique number of G respectively. In 1987, Gyárfás conjectured that for every c, if C is a class of graphs such that χ(G)≤ω(G)+c for every induced subgraph G of every graph in the class, then the class of complements of members of C is χ-bounded. We prove this conjecture. Indeed, more generally, a class of graphs is χ-bounded if it has the property that no graph in the class has c+1 odd holes, pairwise disjoint and with no edges between them. The main tool is a lemma that if C is a shortest odd hole in a graph, and X is the set of vertices with at least five neighbours in V(C), then there is a three-vertex set that dominates X.

A hole in a graph is a chordless cycle of length at least 4. A theta is a graph formed by three paths between the same pair of distinct vertices so that the union of any two of the paths induces a hole. A wheel is a graph formed by a hole and a node that has at least 3 neighbors in the hole. In this paper we obtain a decomposition theorem for the class of graphs that do not contain an induced subgraph isomorphic to a theta or a wheel, i.e. the class of (theta, wheel)-free graphs. The decomposition theorem uses clique cutsets and 2-joins. Clique cutsets are vertex cutsets that work really well in decomposition based algorithms, but are unfortunately not general enough to decompose more complex hereditary graph classes. A 2-join is an edge cutset that appeared in decomposition theorems of several complex classes, such as perfect graphs, even-hole-free graphs and others. In these decomposition theorems 2-joins are used together with vertex cutsets that are more general than clique cutsets, such as star cutsets and their generalizations (which are much harder to use in algorithms). This is a first example of a decomposition theorem that uses just the combination of clique cutsets and 2-joins. This has several consequences. First, we can easily transform our decomposition theorem into a complete structure theorem for (theta, wheel)-free graphs, i.e. we show how every (theta, wheel)-free graph can be built starting from basic graphs that can be explicitly constructed, and gluing them together by prescribed composition operations; and all graphs built this way are (theta, wheel)-free. Such structure theorems are very rare for hereditary graph classes, only a few examples are known. Secondly, we obtain an O(n4m)-time decomposition based recognition algorithm for (theta, wheel)-free graphs. Finally, in Parts III and IV of this series, we give further applications of our decomposition theorem.

A hole in a graph is a chordless cycle of length at least 4. A theta is a graph formed by three paths between the same pair of distinct vertices so that the union of any two of the paths induces a hole. A wheel is a graph formed by a hole and a vertex that has at least 3 neighbors in the hole. In this series of papers we study the class of graphs that do not contain as an induced subgraph a theta nor a wheel. In Part II of the series we prove a decomposition theorem for this class, that uses clique cutsets and 2-joins, and consequently obtain a polynomial time recognition algorithm for the class. In this paper we further use this decomposition theorem to obtain polynomial time algorithms for maximum weight clique, maximum weight stable set and coloring problems. We also show that for a graph G in the class, if its maximum clique size is ω, then its chromatic number is bounded by max⁡{ω,3}, and that the class is 3-clique-colorable.

Truemper configurations are four types of graphs (namely thetas, wheels, prisms and pyramids) that play an important role in the proof of several decomposition theorems for hereditary graph classes. In this paper, we prove two structure theorems: one for graphs with no thetas, wheels and prisms as induced subgraphs, and one for graphs with no thetas, wheels and pyramids as induced subgraphs. A consequence is a polynomial time recognition algorithms for these two classes. In Part II of this series we generalize these results to graphs with no thetas and wheels as induced subgraphs, and in Parts III and IV, using the obtained structure, we solve several optimization problems for these graphs.