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We prove two results regarding cycles in properly edge-colored graphs. First, we make a small improvement to the recent breakthrough work of Alon, Pokrovskiy and Sudakov who showed that every properly edge-colored complete graph $G$ on $n$ vertices has a rainbow cycle on at least $n - O(n^{3/4})$ vertices, by showing that $G$ has a rainbow cycle on at least $n - O(\log n \sqrt{n})$ vertices. Second, by modifying the argument of Hatami and Shor which gives a lower bound for the length of a partial transversal in a Latin Square, we prove that every properly colored complete graph has a Hamilton cycle in which at least $n - O((\log n)^2)$ different colors appear. For large $n$, this is an improvement of the previous best known lower bound of $n - \sqrt{2n}$ of Andersen., Comment: 12 pages

The leaf distance of a tree is the maximum d such that the distance between any pair of leaves in the tree is at least d. Kaneko provided sufficient conditions to force the existence of a spanning tree with leaf distance at least d=3 and conjectured that similar conditions suffice for larger d. The case when d=4 was later proved by Kaneko, Kano, and Suzuki. In this paper, we show that when d4, a stronger form of this conjecture holds for graphs with independence number at most five. As an immediate corollary, we obtain that when dn3, this stronger version holds for all n-vertex graphs, consequently proving Kanekos conjecture for dn3.

In this paper, we prove an analogue of Corradi and Hajnal's classical theorem. There exists n0n0 such that for every n∈3Zn∈3Z when n≥n0n≥n0 the following holds. If G is an oriented graph on n vertices and every vertex has both indegree and outdegree at least 7n/187n/18, then G contains a perfect transitive triangle tiling, which is a collection of vertex-disjoint transitive triangles covering every vertex of G . This result is best possible, as, for every n∈3Zn∈3Z, there exists an oriented graph G on n vertices without a perfect transitive triangle tiling in which every vertex has both indegree and outdegree at least ⌈7n/18⌉−1⌈7n/18⌉−1.

Let $G = (V, E)$ be an $n$-vertex edge-colored graph. In 2013, H. Li proved that if every vertex $v \in V$ is incident to at least $(n+1)/2$ distinctly colored edges, then $G$ admits a rainbow triangle. We prove that the same hypothesis ensures a rainbow $\ell$-cycle $C_{\ell}$ whenever $n \ge 432 \ell$. This result is sharp for all odd integers $\ell \geq 3$, and extends earlier work of the authors for when $\ell$ is even., 10 pages

Corradi and Hajnal (Acta Math Acad Sci Hung 14:423–439, 1963) proved that for all $$k\ge 1$$ and $$n\ge 3k$$ , every (simple) graph G on n vertices with minimum degree $$\delta (G)\ge 2k$$ contains k disjoint cycles. The degree bound is sharp. Enomoto and Wang proved the following Ore-type refinement of the Corradi–Hajnal theorem: For all $$k\ge 1$$ and $$n\ge 3k$$ , every graph G on n vertices contains k disjoint cycles, provided that $$d(x)+d(y)\ge 4k-1$$ for all distinct nonadjacent vertices x, y. Very recently, it was refined for $$k\ge 3$$ and $$n\ge 3k+1$$ : If G is a graph on n vertices such that $$d(x)+d(y)\ge 4k-3$$ for all distinct nonadjacent vertices x, y, then G has k vertex-disjoint cycles if and only if the independence number $$\alpha (G)\le n-2k$$ and G is not one of two small exceptions in the case $$k=3$$ . But the most difficult case, $$n=3k$$ , was not handled. In this case, there are more exceptional graphs, the statement is more sophisticated, and some of the proofs do not work. In this paper we resolve this difficult case and obtain the full picture of extremal graphs for the Ore-type version of the Corradi–Hajnal theorem. Since any k disjoint cycles in a 3k-vertex graph G must be 3-cycles, the existence of such k cycles is equivalent to the existence of an equitable k-coloring of the complement of G. Our proof uses the language of equitable colorings, and our result can be also considered as an Ore-type version of a partial case of the Chen–Lih–Wu Conjecture on equitable colorings.

The classical Corr adi-Hajnal theorem claims that every n-vertex graph G with (G) 2n=3 contains a triangle factor, when 3jn. In this paper we asymptotically determine the minimum degree condition necessary to guarantee a triangle factor in graphs with sublinear independence number. In particular, we show that if G is an n-vertex graph with (G) = o(n) and (G) (1=2 + o(1))n, then G has a triangle factor and this is asymptotically best possible. Furthermore, it is shown for every r that if every linear size vertex set of a graph G spans quadratic many edges, and (G) (1=2 +o(1))n, then G has a Kr-factor for n suciently large. We also propose many related open problems whose solutions could show a relationship with RamseyTur an theory. Additionally, we also consider a fractional variant of the Corr adi-Hajnal Theorem, settling a conjecture of Balogh-Kemkes-Lee-Young. Let t2 (0; 1) and w : E(Kn)! [0; 1]. We call a triangle in Kn heavy if the sum of the weights on its edges is more than 3t. We prove that if 3jn and w is such that for every vertex v the sum of w(e)

The Hajnal–Szemerédi theorem states that for any positive integer $r$ and any multiple $n$ of $r$, if $G$ is a graph on $n$ vertices and $\unicode[STIX]{x1D6FF}(G)\geqslant (1-1/r)n$, then $G$ can be partitioned into $n/r$ vertex-disjoint copies of the complete graph on $r$ vertices. We prove a very general analogue of this result for directed graphs: for any positive integer $r$ with $r\neq 3$ and any sufficiently large multiple $n$ of $r$, if $G$ is a directed graph on $n$ vertices and every vertex is incident to at least $2(1-1/r)n-1$ directed edges, then $G$ can be partitioned into $n/r$ vertex-disjoint subgraphs of size $r$ each of which contain every tournament on $r$ vertices (the case $r=3$ is different and was handled previously). In fact, this result is a consequence of a tiling result for standard multigraphs (that is multigraphs where there are at most two edges between any pair of vertices). A related Turán-type result is also proven.

An edge-ordering of a graph $G=(V,E)$ is a bijection $\phi:E\to\{1,2,\ldots,|E|\}$. Given an edge-ordering, a sequence of edges $P=e_1,e_2,\ldots,e_k$ is an increasing path if it is a path in $G$ which satisfies $\phi(e_i)

We study the minimum degree necessary to guarantee the existence of perfect and almost-perfect triangle-tilings in an $n$-vertex graph $G$ with sublinear independence number. In this setting, we show that if $\delta(G) \ge n/3 + o(n)$ then $G$ has a triangle-tiling covering all but at most four vertices. Also, for every $r \ge 5$, we asymptotically determine the minimum degree threshold for a perfect triangle-tiling under the additional assumptions that $G$ is $K_r$-free and $n$ is divisible by $3$., Comment: 24 pages

In 1963, Corradi and Hajnal settled a conjecture of Erd?s by proving that, for all $$k \ge 1$$k?1, any graph G with $$|G| \ge 3k$$|G|?3k and minimum degree at least 2k contains k vertex-disjoint cycles. In 2008, Finkel proved that for all $$k \ge 1$$k?1, any graph G with $$|G| \ge 4k$$|G|?4k and minimum degree at least 3k contains k vertex-disjoint chorded cycles. Finkel's result was strengthened by Chiba, Fujita, Gao, and Li in 2010, who showed, among other results, that for all $$k \ge 1$$k?1, any graph G with $$|G| \ge 4k$$|G|?4k and minimum Ore-degree at least $$6k-1$$6k-1 contains k vertex-disjoint chorded cycles. We refine this result, characterizing the graphs G with $$|G| \ge 4k$$|G|?4k and minimum Ore-degree at least $$6k-2$$6k-2 that do not have k vertex-disjoint chorded cycles.

Let $n$ be sufficiently large and suppose that $G$ is a digraph on $n$ vertices where every vertex has in- and outdegree at least $n/2$. We show that $G$ contains every orientation of a Hamilton cycle except, possibly, the antidirected one. The antidirected case was settled by DeBiasio and Molla, where the threshold is $n/2+1$. Our result is best possible and improves on an approximate result by H\"aggkvist and Thomason., Comment: Final version, to appear in SIAM Journal Discrete Mathematics (SIDMA)