Our main result includes the following, slightly surprising, fact: a 4-connected nonplanar graph *G* has crossing number at least 2 if and only if, for every pair of edges having no common incident vertex, there are vertex-disjoint cycles in *G* with one containing *e* and the other containing *f*.

A weighting of the edges of a hypergraph is called vertex-coloring if the weighted degrees of the vertices yield a proper coloring of the graph, i.e. every edge contains at least two vertices with different weighted degrees. In this article, we show that such a weighting is possible from the weight set for all hypergraphs with maximum edge size and not containing edges solely consisting of identical vertices. The number is best possible for this statement.

We generalize a parity result of Fleishner and Stiebitz that being combined with Alon–Tarsi polynomial method allowed them to prove that a 4-regular graph formed by a Hamiltonian cycle and several disjoint triangles is always 3-choosable. Also we show how a version of polynomial method gives slightly more combinatorial information about colorings than direct application of Alon's Combinatorial Nullstellensatz.

In this article, we prove three theorems. The first is that every connected graph of order *n* and size *m* has an induced forest of order at least with equality if and only if such a graph is obtained from a tree by expanding every vertex to a clique of order either 4 or 5. This improves the previous lower bound of Alon–Kahn–Seymour for , and implies that such a graph has an induced forest of order at least for . This latter result relates to the conjecture of Albertson and Berman that every planar graph of order *n* has an induced forest of order at least . The second is that every connected triangle-free graph of order *n* and size *m* has an induced forest of order at least . This bound is sharp by the cube and the Wagner graph. It also improves the previous lower bound of Alon–Mubayi–Thomas for , and implies that such a graph has an induced forest of order at least for . This latter result relates to the conjecture of Akiyama and Watanabe that every bipartite planar graph of order *n* has an induced forest of order at least . The third is that every connected planar graph of order *n* and size *m* with girth at least 5 has an induced forest of order at least with equality if and only if such a graph is obtained from a tree by expanding every vertex to one of five specific graphs. This implies that such a graph has an induced forest of order at least , where was conjectured to be the best lower bound by Kowalik, Lužar, and Škrekovski.

For graphs *G* and *H*, an *H*-coloring of *G* is a map from the vertices of *G* to the vertices of *H* that preserves edge adjacency. We consider the following extremal enumerative question: for a given *H*, which connected *n*-vertex graph with minimum degree δ maximizes the number of *H*-colorings? We show that for nonregular *H* and sufficiently large *n*, the complete bipartite graph is the unique maximizer. As a corollary, for nonregular *H* and sufficiently large *n* the graph is the unique *k*-connected graph that maximizes the number of *H*-colorings among all *k*-connected graphs. Finally, we show that this conclusion does not hold for all regular *H* by exhibiting a connected *n*-vertex graph with minimum degree δ that has more -colorings (for sufficiently large *q* and *n*) than .

A proper *k*-coloring of a graph is a function such that , for every . The chromatic number is the minimum *k* such that there exists a proper *k*-coloring of *G*. Given a spanning subgraph *H* of *G*, a *q*-backbone *k*-coloring of is a proper *k*-coloring *c* of such that , for every edge . The *q*-backbone chromatic number is the smallest *k* for which there exists a *q*-backbone *k*-coloring of . In this work, we show that every connected graph *G* has a spanning tree *T* such that , and that this value is the best possible. As a direct consequence, we get that every connected graph *G* has a spanning tree *T* for which , if , or , otherwise. Thus, by applying the Four Color Theorem, we have that every connected nonbipartite planar graph *G* has a spanning tree *T* such that . This settles a question by Wang, Bu, Montassier, and Raspaud (J Combin Optim 23(1) (2012), 79–93), and generalizes a number of previous partial results to their question.

Let *G* be a planar graph without 4-cycles and 5-cycles and with maximum degree . We prove that . For arbitrarily large maximum degree Δ, there exist planar graphs of girth 6 with . Thus, our bound is within 1 of being optimal. Further, our bound comes from coloring greedily in a good order, so the bound immediately extends to *online* list-coloring. In addition, we prove bounds for -labeling. Specifically, and, more generally, , for positive integers *p* and *q* with . Again, these bounds come from a greedy coloring, so they immediately extend to the list-coloring and online list-coloring variants of this problem.

For positive integers and *m*, let be the smallest integer such that for each graph *G* with *m* edges there exists a *k*-partition in which each contains at most edges. Bollobás and Scott showed that . Ma and Yu posed the following problem: is it true that the limsup of tends to infinity as *m* tends to infinity? They showed it holds when *k* is even, establishing a conjecture of Bollobás and Scott. In this article, we solve the problem completely. We also present a result by showing that every graph with a large *k*-cut has a *k*-partition in which each vertex class contains relatively few edges, which partly improves a result given by Bollobás and Scott.

Let *c* be a proper edge coloring of a graph with integers . Then , while Vizing's theorem guarantees that we can take . On the course of investigating irregularities in graphs, it has been conjectured that with only slightly larger *k*, that is, , we could enforce an additional strong feature of *c*, namely that it attributes distinct sums of incident colors to adjacent vertices in *G* if only this graph has no isolated edges and is not isomorphic to *C*_{5}. We prove the conjecture is valid for planar graphs of sufficiently large maximum degree. In fact an even stronger statement holds, as the necessary number of colors stemming from the result of Vizing is proved to be sufficient for this family of graphs. Specifically, our main result states that every planar graph *G* of maximum degree at least 28, which contains no isolated edges admits a proper edge coloring such that for every edge of *G*.

This article introduces a new variant of hypercubes . The *n*-dimensional twisted hypercube is obtained from two copies of the -dimensional twisted hypercube by adding a perfect matching between the vertices of these two copies of . We prove that the *n*-dimensional twisted hypercube has diameter . This improves on the previous known variants of hypercube of dimension *n* and is optimal up to an error of order . Another type of hypercube variant that has similar structure and properties as is also discussed in the last section.

The *k*-linkage problem is as follows: given a digraph and a collection of *k* terminal pairs such that all these vertices are distinct; decide whether *D* has a collection of vertex disjoint paths such that is from to for . A digraph is *k*-linked if it has a *k*-linkage for every choice of 2*k* distinct vertices and every choice of *k* pairs as above. The *k*-linkage problem is NP-complete already for [11] and there exists no function such that every -strong digraph has a *k*-linkage for every choice of 2*k* distinct vertices of *D* [17]. Recently, Chudnovsky et al. [9] gave a polynomial algorithm for the *k*-linkage problem for any fixed *k* in (a generalization of) semicomplete multipartite digraphs. In this article, we use their result as well as the classical polynomial algorithm for the case of acyclic digraphs by Fortune et al. [11] to develop polynomial algorithms for the *k*-linkage problem in locally semicomplete digraphs and several classes of decomposable digraphs, including quasi-transitive digraphs and directed cographs. We also prove that the necessary condition of being -strong is also sufficient for round-decomposable digraphs to be *k*-linked, obtaining thus a best possible bound that improves a previous one of . Finally we settle a conjecture from [3] by proving that every 5-strong locally semicomplete digraph is 2-linked. This bound is also best possible (already for tournaments) [1].

Recently, Borodin, Kostochka, and Yancey (Discrete Math 313(22) (2013), 2638–2649) showed that the vertices of each planar graph of girth at least 7 can be 2-colored so that each color class induces a subgraph of a matching. We prove that any planar graph of girth at least 6 admits a vertex coloring in colors such that each monochromatic component is a path of length at most 14. Moreover, we show a list version of this result. On the other hand, for each positive integer , we construct a planar graph of girth 4 such that in any coloring of vertices in colors there is a monochromatic path of length at least *t*. It remains open whether each planar graph of girth 5 admits a 2-coloring with no long monochromatic paths.

A graph is intrinsically knotted if every embedding contains a nontrivially knotted cycle. It is known that intrinsically knotted graphs have at least 21 edges and that the KS graphs, *K*_{7} and the 13 graphs obtained from *K*_{7} by moves, are the only minor minimal intrinsically knotted graphs with 21 edges [1, 9, 11, 12]. This set includes exactly one bipartite graph, the Heawood graph. In this article we classify the intrinsically knotted bipartite graphs with at most 22 edges. Previously known examples of intrinsically knotted graphs of size 22 were those with KS graph minor and the 168 graphs in the *K*_{3, 3, 1, 1} and families. Among these, the only bipartite example with no Heawood subgraph is Cousin 110 of the family. We show that, in fact, this is a complete listing. That is, there are exactly two graphs of size at most 22 that are minor minimal bipartite intrinsically knotted: the Heawood graph and Cousin 110.

Hedetniemi conjectured in 1966 that if *G* and *H* are finite graphs with chromatic number *n*, then the chromatic number of the direct product of *G* and *H* is also *n*. We mention two well-known results pertaining to this conjecture and offer an improvement of the one, which partially proves the other. The first of these two results is due to Burr et al. (Ars Combin 1 (1976), 167–190), who showed that when every vertex of a graph *G* with is contained in an *n*-clique, then whenever . The second, by Duffus et al. (J Graph Theory 9 (1985), 487–495), and, obtained independently by Welzl (J Combin Theory Ser B 37 (1984), 235–244), states that the same is true when *G* and *H* are connected graphs each with clique number *n*. Our main result reads as follows: If *G* is a graph with and has the property that the subgraph of *G* induced by those vertices of *G* that are not contained in an *n*-clique is homomorphic to an -critical graph *H*, then . This result is an improvement of the result by the first authors. In addition we will show that our main result implies a special case of the result by the second set of authors. Our approach will employ a construction of a graph *F*, with chromatic number , that is homomorphic to *G* and *H*.

Let and be the largest order of a Cayley graph and a Cayley graph based on an abelian group, respectively, of degree *d* and diameter *k*. When , it is well known that with equality if and only if the graph is a Moore graph. In the abelian case, we have . The best currently lower bound on is for all sufficiently large *d*. In this article, we consider the construction of large graphs of diameter 2 using generalized difference sets. We show that for sufficiently large *d* and if , and *m* is odd.

Recently, Jones et al. (Electron J Comb 22(2) (2015), #P2.53) introduced the study of *u*-representable graphs, where *u* is a word over containing at least one 1. The notion of a *u*-representable graph is a far-reaching generalization of the notion of a word-representable graph studied in the literature in a series of papers. Jones et al. have shown that any graph is 11⋅⋅⋅1-representable assuming that the number of 1s is at least three, while the class of 12-representable graphs is properly contained in the class of comparability graphs, which, in turn, is properly contained in the class of word-representable graphs corresponding to 11-representable graphs. Further studies in this direction were conducted by Nabawanda (M.Sc. thesis, 2015), who has shown, in particular, that the class of 112-representable graphs is not included in the class of word-representable graphs. Jones et al. raised a question on classification of *u*-representable graphs at least for small values of *u*. In this article, we show that if *u* is of length at least 3 then any graph is *u*-representable. This rather unexpected result shows that from existence of representation point of view there are only two interesting nonequivalent cases in the theory of *u*-representable graphs, namely, those of and .

Let *F* be a graph that contains an edge whose deletion reduces its chromatic number. For such a graph *F*, a classical result of Simonovits from 1966 shows that every graph on vertices with more than edges contains a copy of *F*. In this article we derive a similar theorem for multipartite graphs. For a graph *H* and an integer , let be the minimum real number such that every ℓ-partite graph whose edge density between any two parts is greater than contains a copy of *H*. Our main contribution in this article is to show that for all sufficiently large if and only if *H* admits a vertex-coloring with colors such that all color classes but one are independent sets, and the exceptional class induces just a matching. When *H* is a complete graph, this recovers a result of Pfender (Combinatorica 32 (2012), 483–495). We also consider several extensions of Pfender's result.

Given a family and a host graph *H*, a graph is -*saturated relative to H* if no subgraph of *G* lies in but adding any edge from to *G* creates such a subgraph. In the -*saturation game* on *H*, players *Max* and *Min* alternately add edges of *H* to *G*, avoiding subgraphs in , until *G* becomes -saturated relative to *H*. They aim to maximize or minimize the length of the game, respectively; denotes the length under optimal play (when Max starts).

Let denote the family of odd cycles and the family of *n*-vertex trees, and write *F* for when . Our results include , for , for , and for . We also determine ; with , it is *n* when *n* is even, *m* when *n* is odd and *m* is even, and when is odd. Finally, we prove the lower bound . The results are very similar when Min plays first, except for the *P*_{4}-saturation game on .

Let *G* be a regular bipartite graph and . We show that there exist perfect matchings of *G* containing both, an odd and an even number of edges from *X* if and only if the signed graph , that is a graph *G* with exactly the edges from *X* being negative, is not equivalent to . In fact, we prove that for a given signed regular bipartite graph with minimum signature, it is possible to find perfect matchings that contain exactly no negative edges or an arbitrary one preselected negative edge. Moreover, if the underlying graph is cubic, there exists a perfect matching with exactly two preselected negative edges. As an application of our results we show that each signed regular bipartite graph that contains an unbalanced circuit has a 2-cycle-cover such that each cycle contains an odd number of negative edges.

Given a digraph *G*, we propose a new method to find the recurrence equation for the number of vertices of the *k*-iterated line digraph , for , where . We obtain this result by using the minimal polynomial of a quotient digraph of *G*.

We consider random-turn positional games, introduced by Peres, Schramm, Sheffield, and Wilson in 2007. A *p*-random-turn positional game is a two-player game, played the same as an ordinary positional game, except that instead of alternating turns, a coin is being tossed before each turn to decide the identity of the next player to move (the probability of Player I to move is *p*). We analyze the random-turn version of several classical Maker–Breaker games such as the game Box (introduced by Chvátal and Erdős in 1987), the Hamilton cycle game and the *k*-vertex-connectivity game (both played on the edge set of ). For each of these games we provide each of the players with a (randomized) efficient strategy that typically ensures his win in the asymptotic order of the minimum value of *p* for which he typically wins the game, assuming optimal strategies of both players.

A graph with a trivial automorphism group is said to be *rigid*. Wright proved (Acta Math 126(1) (1971), 1–9) that for a random graph is rigid whp (with high probability). It is not hard to see that this lower bound is sharp and for with positive probability is nontrivial. We show that in the sparser case , it holds whp that *G*'s 2-core is rigid. We conclude that for all *p*, a graph in is reconstructible whp. In addition this yields for a canonical labeling algorithm that almost surely runs in polynomial time with *o*(1) error rate. This extends the range for which such an algorithm is currently known (T. Czajka and G. Pandurangan, J Discrete Algorithms 6(1) (2008), 85–92).

We look at several saturation problems in complete balanced blow-ups of graphs. We let denote the blow-up of *H* onto parts of size *n* and refer to a copy of *H* in as *partite* if it has one vertex in each part of . We then ask how few edges a subgraph *G* of can have such that *G* has no partite copy of *H* but such that the addition of any new edge from creates a partite *H*. When *H* is a triangle this value was determined by Ferrara, Jacobson, Pfender, and Wenger in . Our main result is to calculate this value for when *n* is large. We also give exact results for paths and stars and show that for 2-connected graphs the answer is linear in *n* whilst for graphs that are not 2-connected the answer is quadratic in *n*. We also investigate a similar problem where *G* is permitted to contain partite copies of *H* but we require that the addition of any new edge from creates an extra partite copy of *H*. This problem turns out to be much simpler and we attain exact answers for all cliques and trees.

A graph *G* is called -choosable if for any list assignment *L* that assigns to each vertex *v* a set of *a* permissible colors, there is a *b*-tuple *L*-coloring of *G*. An (*a*, 1)-choosable graph is also called *a*-choosable. In the pioneering article on list coloring of graphs by Erdős et al. , 2-choosable graphs are characterized. Confirming a special case of a conjecture in , Tuza and Voigt proved that 2-choosable graphs are -choosable for any positive integer *m*. On the other hand, Voigt proved that if *m* is an odd integer, then these are the only -choosable graphs; however, when *m* is even, there are -choosable graphs that are not 2-choosable. A graph is called 3-choosable-critical if it is not 2-choosable, but all its proper subgraphs are 2-choosable. Voigt conjectured that for every positive integer *m*, all bipartite 3-choosable-critical graphs are -choosable. In this article, we determine which 3-choosable-critical graphs are (4, 2)-choosable, refuting Voigt's conjecture in the process. Nevertheless, a weaker version of the conjecture is true: we prove that there is an even integer *k* such that for any positive integer *m*, every bipartite 3-choosable-critical graph is -choosable. Moving beyond 3-choosable-critical graphs, we present an infinite family of non-3-choosable-critical graphs that have been shown by computer analysis to be (4, 2)-choosable. This shows that the family of all (4, 2)-choosable graphs has rich structure.

Given two graphs *G* and *H*, an *H*-*decomposition* of *G* is a partition of the edge set of *G* such that each part is either a single edge or forms a graph isomorphic to *H*. Let be the smallest number ϕ such that any graph *G* of order *n* admits an *H*-decomposition with at most ϕ parts. Pikhurko and Sousa conjectured that for and all sufficiently large *n*, where denotes the maximum number of edges in a graph on *n* vertices not containing *H* as a subgraph. Their conjecture has been verified by Özkahya and Person for all edge-critical graphs *H*. In this article, the conjecture is verified for the *k*-fan graph. The *k*-*fan graph*, denoted by , is the graph on vertices consisting of *k* triangles that intersect in exactly one common vertex called the *center* of the *k*-fan.

For graphs *F* and *H*, we say *F* is *Ramsey for H* if every 2-coloring of the edges of *F* contains a monochromatic copy of *H*. The graph *F* is *Ramsey H*-*minimal* if *F* is Ramsey for *H* and there is no proper subgraph of *F* so that is Ramsey for *H*. Burr et al. defined to be the minimum degree of *F* over all Ramsey *H*-minimal graphs *F*. Define to be a graph on vertices consisting of a complete graph on *t* vertices and one additional vertex of degree *d*. We show that for all values ; it was previously known that , so it is surprising that is much smaller. We also make some further progress on some sparser graphs. Fox and Lin observed that for all graphs *H*, where is the minimum degree of *H*; Szabó et al. investigated which graphs have this property and conjectured that all bipartite graphs *H* without isolated vertices satisfy . Fox et al. further conjectured that all connected triangle-free graphs with at least two vertices satisfy this property. We show that *d*-regular 3-connected triangle-free graphs *H*, with one extra technical constraint, satisfy ; the extra constraint is that *H* has a vertex *v* so that if one removes *v* and its neighborhood from *H*, the remainder is connected.

We give a complete description of the set of triples of real numbers with the following property. There exists a constant *K* such that is a lower bound for the matching number of every connected subcubic graph *G*, where denotes the number of vertices of degree *i* for each *i*.

Given a directed graph, an acyclic set is a set of vertices inducing a directed subgraph with no directed cycle. In this note, we show that for all integers , there exist oriented planar graphs of order *n* and digirth *g* for which the size of the maximum acyclic set is at most . When this result disproves a conjecture of Harutyunyan and shows that a question of Albertson is best possible.

Tutte's 5-flow conjecture from 1954 states that every bridgeless graph has a nowhere-zero 5-flow. It suffices to prove the conjecture for cyclically 6-edge-connected cubic graphs. We prove that every cyclically 6-edge-connected cubic graph with oddness at most 4 has a nowhere-zero 5-flow. This implies that every minimum counterexample to the 5-flow conjecture has oddness at least 6.

In the *graph sharing game*, two players share a connected graph *G* with nonnegative weights assigned to the vertices claiming and collecting the vertices of *G* one by one, while keeping the set of all claimed vertices connected through the whole game. Each player wants to maximize the total weight of the vertices they have gathered by the end of the game, when the whole *G* has been claimed. It is proved that for any class of graphs with an odd number of vertices and with forbidden subdivision of a fixed graph (e.g., for the class of planar graphs with an odd number of vertices), there is a constant such that the first player can secure at least the proportion of the total weight of *G* whenever . Known examples show that such a constant does no longer exist if any of the two conditions on the class (an odd number of vertices or a forbidden subdivision) is removed. The main ingredient in the proof is a new structural result on weighted graphs with a forbidden subdivision.

Li et al. (Discrete Math 310 (2010), 3579–3583) asked how long the longest monochromatic cycle in a 2-edge-colored graph *G* with minimum degree at least could be. In this article, an answer is given for all to an asymptotic form of their question.

A spanning subgraph *F* of a graph *G* is called *perfect* if *F* is a forest, the degree of each vertex *x* in *F* is odd, and each tree of *F* is an induced subgraph of *G*. Alex Scott (Graphs Combin 17 (2001), 539–553) proved that every connected graph *G* contains a perfect forest if and only if *G* has an even number of vertices. We consider four generalizations to directed graphs of the concept of a perfect forest. While the problem of existence of the most straightforward one is NP-hard, for the three others this problem is polynomial-time solvable. Moreover, every digraph with only one strong component contains a directed forest of each of these three generalization types. One of our results extends Scott's theorem to digraphs in a nontrivial way.

The article gives a thorough introduction to spectra of digraphs via its Hermitian adjacency matrix. This matrix is indexed by the vertices of the digraph, and the entry corresponding to an arc from *x* to *y* is equal to the complex unity *i* (and its symmetric entry is ) if the reverse arc is not present. We also allow arcs in both directions and unoriented edges, in which case we use 1 as the entry. This allows to use the definition also for mixed graphs. This matrix has many nice properties; it has real eigenvalues and the interlacing theorem holds for a digraph and its induced subdigraphs. Besides covering the basic properties, we discuss many differences from the properties of eigenvalues of undirected graphs and develop basic theory. The main novel results include the following. Several surprising facts are discovered about the spectral radius; some consequences of the interlacing property are obtained; operations that preserve the spectrum are discussed—they give rise to a large number of cospectral digraphs; for every , all digraphs whose spectrum is contained in the interval are determined.

We study the class of 1-perfectly orientable graphs, that is, graphs having an orientation in which every out-neighborhood induces a tournament. 1-perfectly orientable graphs form a common generalization of chordal graphs and circular arc graphs. Even though they can be recognized in polynomial time, little is known about their structure. In this article, we develop several results on 1-perfectly orientable graphs. In particular, we (i) give a characterization of 1-perfectly orientable graphs in terms of edge clique covers, (ii) identify several graph transformations preserving the class of 1-perfectly orientable graphs, (iii) exhibit an infinite family of minimal forbidden induced minors for the class of 1-perfectly orientable graphs, and (iv) characterize the class of 1-perfectly orientable graphs within the classes of cographs and of cobipartite graphs. The class of 1-perfectly orientable cobipartite graphs coincides with the class of cobipartite circular arc graphs.

We present two heuristics for finding a small power dominating set of cubic graphs. We analyze the performance of these heuristics on random cubic graphs using differential equations. In this way, we prove that the proportion of vertices in a minimum power dominating set of a random cubic graph is asymptotically almost surely at most 0.067801. We also provide a corresponding lower bound of using known results on bisection width.

In this article, we introduce and study the properties of some target graphs for 2-edge-colored homomorphism. Using these properties, we obtain in particular that the 2-edge-colored chromatic number of the class of triangle-free planar graphs is at most 50. We also show that it is at least 12.

The partition of graphs into “nice” subgraphs is a central algorithmic problem with strong ties to matching theory. We study the partitioning of undirected graphs into same-size stars, a problem known to be NP-complete even for the case of stars on three vertices. We perform a thorough computational complexity study of the problem on subclasses of perfect graphs and identify several polynomial-time solvable cases, for example, on interval graphs and bipartite permutation graphs, and also NP-complete cases, for example, on grid graphs and chordal graphs.

We prove that every normal plane map (NPM) has a path on three vertices (3-path) whose degree sequence is bounded from above by one of the following triplets: (3, 3, ∞), (3,15,3), (3,10,4), (3,8,5), (4,7,4), (5,5,7), (6,5,6), (3,4,11), (4,4,9), and (6,4,7). This description is tight in the sense that no its parameter can be improved and no term dropped. We also pose a problem of describing all tight descriptions of 3-paths in NPMs and make a modest contribution to it by showing that there exist precisely three one-term descriptions: (5, ∞, 6), (5, 10, ∞), and (10, 5, ∞).

Let *G* be a 5-connected triangulation of a surface Σ different from the sphere, and let be the Euler characteristic of Σ. Suppose that with even and *M* and *N* are two matchings in of sizes *m* and *n* respectively such that . It is shown that if the pairwise distance between any two elements of is at least five and the face-width of the embedding of *G* in Σ is at least , then there is a perfect matching *M*_{0} in containing *M* such that .

Given two graphs, a mapping between their edge-sets is *cycle-continuous*, if the preimage of every cycle is a cycle. The motivation for this definition is Jaeger's conjecture that for every bridgeless graph there is a cycle-continuous mapping to the Petersen graph, which, if solved positively, would imply several other important conjectures (e.g., the Cycle double cover conjecture). Answering a question of DeVos, Nešetřil, and Raspaud, we prove that there exists an infinite set of graphs with no cycle-continuous mapping between them. Further extending this result, we show that every countable poset can be represented by graphs and the existence of cycle-continuous mappings between them.

The complete graph on *n* vertices can be quadrangularly embedded on an orientable (resp. nonorientable) closed surface *F*^{2} with Euler characteristic if and only if (resp. and ). In this article, we shall show that if quadrangulates a closed surface *F*^{2}, then has a quadrangular embedding on *F*^{2} so that the length of each closed walk in the embedding has the parity specified by any given homomorphism , called the cycle parity.

The Erdős–Lovász Tihany conjecture asserts that every graph *G* with ) contains two vertex disjoint subgraphs *G*_{1} and *G*_{2} such that and . Under the same assumption on *G*, we show that there are two vertex disjoint subgraphs *G*_{1} and *G*_{2} of *G* such that (a) and or (b) and . Here, is the chromatic number of is the clique number of *G*, and col(*G*) is the coloring number of *G*.

For a graph *G*, let denote the largest *k* such that *G* has *k* pairwise disjoint pairwise adjacent connected nonempty subgraphs, and let denote the largest *k* such that *G* has *k* pairwise disjoint pairwise adjacent connected subgraphs of size 1 or 2. Hadwiger's conjecture states that , where is the chromatic number of *G*. Seymour conjectured for all graphs without antitriangles, that is, three pairwise nonadjacent vertices. Here we concentrate on graphs *G* with exactly one -coloring. We prove generalizations of the following statements: (i) if and *G* has exactly one -coloring then , where the proof does *not* use the four-color-theorem, and (ii) if *G* has no antitriangles and *G* has exactly one -coloring then .

Two Hamilton paths in are separated by a cycle of length *k* if their union contains such a cycle. For we bound the asymptotics of the maximum cardinality of a family of Hamilton paths in such that any pair of paths in the family is separated by a cycle of length *k*. We also deal with related problems, including directed Hamilton paths.

The choosability of a graph *G* is the minimum *k* such that having *k* colors available at each vertex guarantees a proper coloring. Given a toroidal graph *G*, it is known that , and if and only if *G* contains *K*_{7}. Cai et al. (J Graph Theory 65(1) (2010), 1–15) proved that a toroidal graph *G* without 7-cycles is 6-choosable, and if and only if *G* contains *K*_{6}. They also proved that a toroidal graph *G* without 6-cycles is 5-choosable, and conjectured that if and only if *G* contains *K*_{5}. We disprove this conjecture by constructing an infinite family of non-4-colorable toroidal graphs with neither *K*_{5} nor cycles of length at least 6; moreover, this family of graphs is embeddable on every surface except the plane and the projective plane. Instead, we prove the following slightly weaker statement suggested by Zhu: toroidal graphs containing neither (a *K*_{5} missing one edge) nor 6-cycles are 4-choosable. This is sharp in the sense that forbidding only one of the two structures does not ensure that the graph is 4-choosable.

For any graph *G*, let be the number of spanning trees of *G*, be the line graph of *G*, and for any nonnegative integer *r*, be the graph obtained from *G* by replacing each edge *e* by a path of length connecting the two ends of *e*. In this article, we obtain an expression for in terms of spanning trees of *G* by a combinatorial approach. This result generalizes some known results on the relation between and and gives an explicit expression if *G* is of order and size in which *s* vertices are of degree 1 and the others are of degree *k*. Thus we prove a conjecture on for such a graph *G*.

Given a graph *F*, a graph *G* is *uniquely F*-*saturated* if *F* is not a subgraph of *G* and adding any edge of the complement to *G* completes exactly one copy of *F*. In this article, we study uniquely -saturated graphs. We prove the following: (1) a graph is uniquely *C*_{5}-saturated if and only if it is a friendship graph. (2) There are no uniquely *C*_{6}-saturated graphs or uniquely *C*_{7}-saturated graphs. (3) For , there are only finitely many uniquely -saturated graphs (we conjecture that in fact there are none). Additionally, our results show that there are finitely many *k*-friendship graphs (as defined by Kotzig) for .

We prove that for every fixed *k*, the number of occurrences of the transitive tournament Tr_{k} of order *k* in a tournament on *n* vertices is asymptotically minimized when is random. In the opposite direction, we show that any sequence of tournaments achieving this minimum for any fixed is necessarily quasirandom. We present several other characterizations of quasirandom tournaments nicely complementing previously known results and relatively easily following from our proof techniques.

We construct for all a *k*-edge-connected digraph *D* with such that there are no edge-disjoint and paths. We use in our construction “self-similar” graphs which technique could be useful in other problems as well.

In this article we prove a new result about partitioning colored complete graphs and use it to determine certain Ramsey numbers exactly. The partitioning theorem we prove is that for , in every edge coloring of with the colors red and blue, it is possible to cover all the vertices with *k* disjoint red paths and a disjoint blue balanced complete -partite graph. When the coloring of is connected in red, we prove a stronger result—that it is possible to cover all the vertices with *k* red paths and a blue balanced complete -partite graph. Using these results we determine the Ramsey number of an *n*-vertex path, , versus a balanced complete *t*-partite graph on vertices, , whenever . We show that in this case , generalizing a result of Erdős who proved the case of this result. We also determine the Ramsey number of a path versus the power of a path . We show that , solving a conjecture of Allen, Brightwell, and Skokan.

A drawing of a graph is *pseudolinear* if there is a pseudoline arrangement such that each pseudoline contains exactly one edge of the drawing. The *pseudolinear crossing number* of a graph *G* is the minimum number of pairwise crossings of edges in a pseudolinear drawing of *G*. We establish several facts on the pseudolinear crossing number, including its computational complexity and its relationship to the usual crossing number and to the rectilinear crossing number. This investigation was motivated by open questions and issues raised by Marcus Schaefer in his comprehensive survey of the many variants of the crossing number of a graph.

A graph is called hypohamiltonian if it is not hamiltonian but becomes hamiltonian if any vertex is removed. Many hypohamiltonian planar cubic graphs have been found, starting with constructions of Thomassen in 1981. However, all the examples found until now had 4-cycles. In this note we present the first examples of hypohamiltonian planar cubic graphs with cyclic connectivity 5, and thus girth 5. We show by computer search that the smallest members of this class are three graphs with 76 vertices.

We study the existence and the number of *k*-dominating independent sets in certain graph families. While the case namely the case of maximal independent sets—which is originated from Erdős and Moser—is widely investigated, much less is known in general. In this paper we settle the question for trees and prove that the maximum number of *k*-dominating independent sets in *n*-vertex graphs is between and if , moreover the maximum number of 2-dominating independent sets in *n*-vertex graphs is between and . Graph constructions containing a large number of *k*-dominating independent sets are coming from product graphs, complete bipartite graphs, and finite geometries. The product graph construction is associated with the number of certain Maximum Distance Separable (MDS) codes.

A natural topic of algebraic graph theory is the study of vertex transitive graphs. In the present article, we investigate locally 3-transitive graphs of girth 4. Taking our former results on locally symmetric graphs of girth 4 as a starting point, we show what properties are retained if we weaken the requirement of local symmetry to local 3-transitivity.

Treewidth is a graph parameter of fundamental importance to algorithmic and structural graph theory. This article surveys several graph parameters tied to treewidth, including separation number, tangle number, well-linked number, and Cartesian tree product number. We review many results in the literature showing these parameters are tied to treewidth. In a number of cases we also improve known bounds, provide simpler proofs, and show that the inequalities presented are tight.

Given graphs *H* and *F*, a subgraph is an *F*-*saturated subgraph* of *H* if , but for all . The *saturation number of F in H*, denoted , is the minimum number of edges in an *F*-saturated subgraph of *H*. In this article, we study saturation numbers of tripartite graphs in tripartite graphs. For and *n*_{1}, *n*_{2}, and *n*_{3} sufficiently large, we determine and exactly and within an additive constant. We also include general constructions of -saturated subgraphs of with few edges for .

The class of cographs is known to have unbounded linear clique-width. We prove that a hereditary class of cographs has bounded linear clique-width if and only if it does not contain all quasi-threshold graphs or their complements. The proof borrows ideas from the enumeration of permutation classes.

The crossing number cr(*G*) of a graph *G* is the minimum number of crossings in a drawing of *G* in the plane with no more than two edges intersecting at any point that is not a vertex. The rectilinear crossing number of *G* is the minimum number of crossings in a such drawing of *G* with edges as straight line segments. Zarankiewicz proved in 1952 that . We generalize the upper bound to

and prove . We also show that for *n* large enough, and , with the tighter rectilinear lower bound established through the use of flag algebras. A complete multipartite graph is balanced if the partite sets all have the same cardinality. We study asymptotic behavior of the crossing number of the balanced complete *r*-partite graph. Richter and Thomassen proved in 1997 that the limit as of over the maximum number of crossings in a drawing of exists and is at most . We define and show that for a fixed *r* and the balanced complete *r*-partite graph, is an upper bound to the limit superior of the crossing number divided by the maximum number of crossings in a drawing.

*Equistable graphs* are graphs admitting positive weights on vertices such that a subset of vertices is a maximal stable set if and only if it is of total weight 1. *Strongly equistable graphs* are graphs such that for every and every nonempty subset *T* of vertices that is not a maximal stable set, there exist positive vertex weights assigning weight 1 to every maximal stable set such that the total weight of *T* does not equal *c*. *General partition graphs* are the intersection graphs of set systems over a finite ground set *U* such that every maximal stable set of the graph corresponds to a partition of *U*. General partition graphs are exactly the graphs every edge of which is contained in a strong clique. In 1994, Mahadev, Peled, and Sun proved that every strongly equistable graph is equistable, and conjectured that the converse holds as well. In 2009, Orlin proved that every general partition graph is equistable, and conjectured that the converse holds as well. Orlin's conjecture, if true, would imply the conjecture due to Mahadev, Peled, and Sun. An “intermediate” conjecture, posed by Miklavič and Milanič in 2011, states that every equistable graph has a strong clique. The above conjectures have been verified for several graph classes. We introduce the notion of equistarable graphs and based on it construct counterexamples to all three conjectures within the class of complements of line graphs of triangle-free graphs. We also show that not all strongly equistable graphs are general partition.

A graph is -colorable if its vertex set can be partitioned into *r* sets so that the maximum degree of the graph induced by is at most for each . For a given pair , the question of determining the minimum such that planar graphs with girth at least *g* are -colorable has attracted much interest. The finiteness of was known for all cases except when . Montassier and Ochem explicitly asked if *d*_{2}(5, 1) is finite. We answer this question in the affirmative with ; namely, we prove that all planar graphs with girth at least five are (1, 10)-colorable. Moreover, our proof extends to the statement that for any surface *S* of Euler genus γ, there exists a where graphs with girth at least five that are embeddable on *S* are (1, *K*)-colorable. On the other hand, there is no finite *k* where planar graphs (and thus embeddable on any surface) with girth at least five are (0, *k*)-colorable.

Hadwiger's conjecture asserts that every graph with chromatic number *t* contains a complete minor of order *t*. Given integers , the Kneser graph is the graph with vertices the *k*-subsets of an *n*-set such that two vertices are adjacent if and only if the corresponding *k*-subsets are disjoint. We prove that Hadwiger's conjecture is true for the complements of Kneser graphs.

Let be an integer and let *D* be a simple digraph on vertices. We prove that If

then *D* must have a nontrivial subdigraph *H* such that the strong arc connectivity of *H* is at least . We also show that this bound is best possible and present a constructive characterization for the extremal graphs.

The celebrated grid exclusion theorem states that for every *h*-vertex planar graph *H*, there is a constant such that if a graph *G* does not contain *H* as a minor then *G* has treewidth at most . We are looking for patterns of *H* where this bound can become a low degree polynomial. We provide such bounds for the following parameterized graphs: the wheel , the double wheel , any graph of pathwidth at most 2 , and the yurt graph .

For a graph , let denote the minimum number of pairwise edge disjoint complete bipartite subgraphs of *G* so that each edge of *G* belongs to exactly one of them. It is easy to see that for every graph *G*, , where is the maximum size of an independent set of *G*. Erdős conjectured in the 80s that for almost every graph *G* equality holds, that is that for the random graph , with high probability, that is with probability that tends to 1 as *n* tends to infinity. The first author showed that this is slightly false, proving that for most values of *n* tending to infinity and for , with high probability. We prove a stronger bound: there exists an absolute constant so that with high probability.

We prove that the adaptable chromatic number of a graph is at least asymptotic to the square root of the chromatic number. This is best possible.

A class of graphs is *hereditary* if it is closed under isomorphism and induced subgraphs. A class of graphs is χ*-bounded* if there exists a function such that for all graphs , and all induced subgraphs *H* of *G*, we have that . We prove that proper homogeneous sets, clique-cutsets, and amalgams together preserve χ-boundedness. More precisely, we show that if and are hereditary classes of graphs such that is χ-bounded, and such that every graph in either belongs to or admits a proper homogeneous set, a clique-cutset, or an amalgam, then the class is χ-bounded. This generalizes a result of [*J Combin Theory Ser B* 103(5) (2013), 567–586], which states that proper homogeneous sets and clique-cutsets together preserve χ-boundedness, as well as a result of [*European J Combin* 33(4) (2012), 679–683], which states that 1-joins preserve χ-boundedness. The *house* is the complement of the four-edge path. As an application of our result and of the decomposition theorem for “cap-free” graphs from [*J Graph Theory* 30(4) (1999), 289–308], we obtain that if *G* is a graph that does not contain any subdivision of the house as an induced subgraph, then .

Král' and Sgall (J Graph Theory 49(3) (2005), 177–186) introduced a refinement of list coloring where every color list must be subset to one predetermined palette of colors. We call this -*choosability* when the palette is of size at most ℓ and the lists must be of size at least *k*. They showed that, for any integer , there is an integer , satisfying as , such that, if a graph is -choosable, then it is *C*-choosable, and asked if *C* is required to be exponential in *k*. We demonstrate it must satisfy . For an integer , if is the least integer such that a graph is -choosable if it is -choosable, then we more generally supply a lower bound on , one that is super-polynomial in *k* if , by relation to an extremal set theoretic property. By the use of containers, we also give upper bounds on that improve on earlier bounds if .