Bela Csaba
Let $q$ be a positve integer, and $G$ be a $q$-partite simple graph on $qn$ vertices, with $n$ vertices in each vertex class. Let $δ={k_q \over k_q+1}$, where $k_q=q+O(\log{q})$. If each vertex of $G$ is adjacent to at least $δn$ vertices in each of the other vertex classes, $q$ is bounded and $n$ is large enough, then $G$ has a $K_q$-factor.
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A Hajnal-Szemerédi Theorem for Skewed Colorings
本文推广Hajnal-Szemerédi定理至偏斜染色，在自然上界条件下保证任意指定大小颜色类的存在。
Hajnal and Szemeredi proved that every graph G with |G|=ks and minimum degree at least k(s-1) contains k vertex disjoint s-cliques; moreover this degree bound is optimal. We extend their theorem to directed graphs by showing that every directed graph D with |D|=ks and minimum (total) degree at least 2k(s-1)-1 contains k vertex disjoint transitive tournaments on s vertices. Our result implies the Hajnal-Szemeredi Theorem, and the degree bound is optimal. We also make some conjectures regarding even more general results for multigraphs and partitioning into other tournaments. One of these conjec…
Let $H$ be a triple system with maximum degree $d>1$ and let $r>10^7\sqrt{d}\log^{2}d$. Then $H$ has a proper vertex coloring with $r$ colors such that any two color classes differ in size by at most one. The bound on $r$ is sharp in order of magnitude apart from the logarithmic factors. Moreover, such an $r$-coloring can be found via a randomized algorithm whose expected running time is polynomial in the number of vertices of $\cH$. This is the first result in the direction of generalizing the Hajnal-Szemerédi theorem to hypergraphs.
We say that a graph $G$ has a perfect $H$-packing if there exists a set of vertex-disjoint copies of $H$ which cover all the vertices in $G$. The seminal Hajnal--Szemerédi theorem characterises the minimum degree that ensures a graph $G$ contains a perfect $K_r$-packing. Balogh, Kostochka and Treglown proposed a degree sequence version of the Hajnal--Szemerédi theorem which, if true, gives a strengthening of the Hajnal--Szemerédi theorem. In this paper we prove this conjecture asymptotically. Another fundamental result in the area is the Alon--Yuster theorem which gives a minimum degree condit…
The celebrated Hajnal-Szemerédi theorem gives the precise minimum degree threshold that forces a graph to contain a perfect K_k-packing. Fischer's conjecture states that the analogous result holds for all multipartite graphs except for those formed by a single construction. Recently, we deduced an approximate version of this conjecture from new results on perfect matchings in hypergraphs. In this paper, we apply a stability analysis to the extremal cases of this argument, thus showing that the exact conjecture holds for any sufficiently large graph.
A perfect $K_r$-tiling in a graph $G$ is a collection of vertex-disjoint copies of the clique $K_r$ in $G$ covering every vertex of $G$. The famous Hajnal--Szemerédi theorem determines the minimum degree threshold for forcing a perfect $K_r$-tiling in a graph $G$. The notion of discrepancy appears in many branches of mathematics. In the graph setting, one assigns the edges of a graph $G$ labels from $\{-1,1\}$, and one seeks substructures $F$ of $G$ that have `high' discrepancy (i.e. the sum of the labels of the edges in $F$ is far from $0$). In this paper we determine the minimum degree thres…
A perfect $K_t$-matching in a graph $G$ is a spanning subgraph consisting of vertex disjoint copies of $K_t$. A classic theorem of Hajnal and Szemerédi states that if $G$ is a graph of order $n$ with minimum degree $δ(G) \ge (t-1)n/t$ and $t| n$, then $G$ contains a perfect $K_t$-matching. Let $G$ be a $t$-partite graph with vertex classes $V_1$,..., $V_t$ each of size $n$. We show that if every vertex $x \in V_i$ is joined to at least $((t-1)/t + γ)n $ vertices of $V_j$ for $i \ne j$, then $G$ contains a perfect $K_t$-matching, thus verifying a conjecture of Fisher asymptotically. Furthermore…
A robust version of the multipartite Hajnal--Szemerédi theorem
本文强化多部 Hajnal–Szemerédi 定理，证明在满足最小度条件且边概率达到给定阈值时，随机扰动的均衡 r 部图以高概率含 K_r-因子，并推出横截版本。
Let $G$ be a $k$-partite graph with $n$ vertices in parts such that each vertex is adjacent to at least $δ^*(G)$ vertices in each of the other parts. Magyar and Martin \cite{MaMa} proved that for $k=3$, if $δ^*(G)\ge 2/3n $ and $n$ is sufficiently large, then $G$ contains a $K_3$-factor (a spanning subgraph consisting of $n$ vertex-disjoint copies of $K_3$) except that $G$ is one particular graph. Martin and Szemerédi \cite{MaSz} proved that $G$ contains a $K_4$-factor when $δ^*(G)\ge 3/4n$ and $n$ is sufficiently large. Both results were proved by the Regularity Lemma. In this paper we give a…
The Hajnal-Szemerédi theorem in digraphs revisited
本研究发现Aboulker等人隐含的简单归约可直接由原始图论Hajnal–Szemerédi定理证明Treglown猜想及Czygrinow等人的有向图相关结果，修改归约后还得到了此类有向图公平无环$k$着色的多项式时间算法。
Density Hajnal--Szemerédi theorem for cliques of size four
本文研究Hajnal–Szemerédi定理的边密度版本，首次渐近确定了$r=4$时保证图中存在$k$个顶点不相交$K_4$副本的五类极值构造，并为一般$r \ge 5$提出$r+1$类候选极值构造。
For a positive integer r>=2, a K_r-factor of a graph is a collection vertex-disjoint copies of K_r which covers all the vertices of the given graph. The celebrated theorem of Hajnal and Szemerédi asserts that every graph on n vertices with minimum degree at least (1-1/r)n contains a K_r-factor. In this note, we propose investigating the relation between minimum degree and existence of perfect K_r-packing for edge-weighted graphs. The main question we study is the following. Suppose that a positive integer r>=2 and a real t in [0,1] is given. What is the minimum weighted degree of K_n that guar…
We say that a (di)graph $G$ has a perfect $H$-packing if there exists a set of vertex-disjoint copies of $H$ which cover all the vertices in $G$. The seminal Hajnal--Szemerédi theorem characterises the minimum degree that ensures a graph $G$ contains a perfect $K_r$-packing. In this paper we prove the following analogue for directed graphs: Suppose that $T$ is a tournament on $r$ vertices and $G$ is a digraph of sufficiently large order $n$ where $r$ divides $n$. If $G$ has minimum in- and outdegree at least $ (1-1/r)n$ then $G$ contains a perfect $T$-packing. In the case when $T$ is a cyclic …
A perfect $K_r$-tiling in a graph $G$ is a collection of vertex-disjoint copies of $K_r$ that together cover all the vertices in $G$. In this paper we consider perfect $K_r$-tilings in the setting of randomly perturbed graphs; a model introduced by Bohman, Frieze and Martin where one starts with a dense graph and then adds $m$ random edges to it. Specifically, given any fixed $0< α<1-1/r$ we determine how many random edges one must add to an $n$-vertex graph $G$ of minimum degree $δ(G) \geq αn$ to ensure that, asymptotically almost surely, the resulting graph contains a perfect $K_r$-tiling. A…
A seminal result of Hajnal and Szemerédi states that if a graph $G$ with $n$ vertices has minimum degree $δ(G) \ge (r-1)n/r$ for some integer $r \ge 2$, then $G$ contains a $K_r$-factor, assuming $r$ divides $n$. Extremal examples which show optimality of the bound on $δ(G)$ are very structured and, in particular, contain large independent sets. In analogy to the Ramsey-Turán theory, Balogh, Molla, and Sharifzadeh initiated the study of how the absence of such large independent sets influences sufficient minimum degree. We show the following two related results: $\bullet$ For any $r > \ell \ge…
A classical result by Hajnal and Szemerédi from 1970 determines the minimal degree conditions necessary to guarantee for a graph to contain a $K_r$-factor. Namely, any graph on $n$ vertices, with minimum degree $δ(G) \ge \left(1-\frac{1}{r}\right) n$ and $r$ dividing $n$ has a $K_r$-factor. This result is tight but the extremal examples are unique in that they all have a large independent set which is the bottleneck. Nenadov and Pehova showed that by requiring a sub-linear independence number the minimum degree condition in the Hajnal-Szemerédi theorem can be improved. We show that, with the s…
In this paper, we prove the asymptotic multipartite version of the Alon-Yuster theorem, which is a generalization of the Hajnal-Szemerédi theorem: If $k\geq 3$ is an integer, $H$ is a $k$-colorable graph and $γ>0$ is fixed, then, for every sufficiently large $n$, where $|V(H)|$ divides $n$, and for every balanced $k$-partite graph $G$ on $kn$ vertices with each of its corresponding $\binom{k}{2}$ bipartite subgraphs having minimum degree at least $(k-1)n/k+γn$, $G$ has a subgraph consisting of $kn/|V(H)|$ vertex-disjoint copies of $H$. The proof uses the Regularity method together with linear …
We present a unified approach to proving Ramsey-type theorems for graphs with a forbidden induced subgraph which can be used to extend and improve the earlier results of Rodl, Erdos-Hajnal, Promel-Rodl, Nikiforov, Chung-Graham, and Luczak-Rodl. The proofs are based on a simple lemma (generalizing one by Graham, Rodl, and Rucinski) that can be used as a replacement for Szemeredi's regularity lemma, thereby giving much better bounds. The same approach can be also used to show that pseudo-random graphs have strong induced Ramsey properties. This leads to explicit constructions for upper bounds on…
Turán number of four vertex-disjoint cliques
本文通过Hajnal-Szemerédi定理的证明思想和电荷法，确定了不含四个不相交完全子图的n顶点图的最大边数。
We solve four similar problems: For every fixed $s$ and large $n$, we describe all values of $n_1,\ldots,n_s$ such that for every $2$-edge-coloring of the complete $s$-partite graph $K_{n_1,\ldots,n_s}$ there exists a monochromatic (i) cycle $C_{2n}$ with $2n$ vertices, (ii) cycle $C_{\geq 2n}$ with at least $2n$ vertices, (iii) path $P_{2n}$ with $2n$ vertices, and (iv) path $P_{2n+1}$ with $2n+1$ vertices. This implies a generalization for large $n$ of the conjecture by Gyárfás, Ruszinkó, Sárkőzy and Szemerédi that for every $2$-edge-coloring of the complete $3$-partite graph $K_{n,n,n}$ the…
For graphs $G$ and $H$, let $G\to (H,H)$ signify that any red/blue edge coloring of $G$ contains a monochromatic $H$ as a subgraph, and $\mathcal{H}(Δ,n)=\{H:|V(H)|=n,Δ(H)\le Δ\}$. For fixed $Δ$ and $n$, we say that $G$ is a partition universal graph for $\mathcal{H}(Δ,n)$ if $G\to (H,H)$ for every $H\in\mathcal{H}(Δ,n)$. In 1983, Chvátal, Rödl, Szemerédi and Trotter proved that for any $Δ\ge2$ there exists a constant $B$ such that, for any $n$, if $N\ge Bn$ then $K_N$ is partition universal for $\mathcal{H}(Δ,n)$. Recently, Kohayakawa, Rödl, Schacht and Szemerédi proved that the complete grap…
Given a graph $G$ and an integer $\ell\ge 2$, we denote by $α_{\ell}(G)$ the maximum size of a $K_{\ell}$-free subset of vertices in $V(G)$. A recent question of Nenadov and Pehova asks for determining the best possible minimum degree conditions forcing clique-factors in $n$-vertex graphs $G$ with $α_{\ell}(G) = o(n)$, which can be seen as a Ramsey--Turán variant of the celebrated Hajnal--Szemerédi theorem. In this paper we find the asymptotical sharp minimum degree threshold for $K_r$-factors in $n$-vertex graphs $G$ with $α_\ell(G)=n^{1-o(1)}$ for all $r\ge \ell\ge 2$.
Komlos [Komlos: Tiling Turan Theorems, Combinatorica, 2000] determined the asymptotically optimal minimum-degree condition for covering a given proportion of vertices of a host graph by vertex-disjoint copies of a fixed graph H, thus essentially extending the Hajnal-Szemeredi theorem which deals with the case when H is a clique. We give a proof of a graphon version of Komlos's theorem. To prove this graphon version, and also to deduce from it the original statement about finite graphs, we use the machinery introduced in [Hladky, Hu, Piguet: Tilings in graphons, arXiv:1606.03113]. We further pr…
In 1975 Bollobás, Erd\H os, and Szemerédi asked the following question: given positive integers $n, t, r$ with $2\le t\le r-1$, what is the largest minimum degree $δ(G)$ among all $r$-partite graphs $G$ with parts of size $n$ and which do not contain a copy of $K_{t+1}$? The $r=t+1$ case has attracted a lot of attention and was fully resolved by Haxell and Szabó, and Szabó and Tardos in 2006. In this paper we investigate the $r>t+1$ case of the problem, which has remained dormant for over forty years. We resolve the problem exactly in the case when $r \equiv -1 \pmod{t}$, and up to an additive…
Given integers $ n \ge k >l \ge 1 $ and a $k$-graph $F$ with $|V(F)|$ divisible by $n$, define $t_l^k(n,F)$ to be the smallest integer $d$ such that every $k$-graph $H$ of order $n$ with minimum $l$-degree $δ_l(H) \ge d $ contains an $F$-factor. A classical theorem of Hajnal and Szemerédi implies that $t^2_1(n,K_t) = (1-1/t)n$ for integers $t$. For $k \ge 3$, $t^k_{k-1}(n,K_k^k)$ (the $δ_{k-1}(H)$ threshold for perfect matchings) has been determined by Kühn and Osthus (asymptotically) and Rödl, Ruciński and Szemerédi (exactly) for large $n$. In this paper, we generalise the absorption techniqu…
The Hajnal--Szemerédi theorem states that for any integer $r \ge 1$ and any multiple $n$ of $r$, if $G$ is a graph on $n$ vertices and $δ(G) \ge (1 - 1/r)n$, 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 integer $r \ge 4$ 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 $r$ eac…
Let r be an integer, f(n) a function, and H a graph. Introduced by Erdős, Hajnal, Sós, and Szemerédi, the r-Ramsey-Turán number of H, RT_r(n, H, f(n)), is defined to be the maximum number of edges in an n-vertex, H-free graph G with α_r(G) <= f(n) where α_r(G) denotes the K_r-independence number of G. In this note, using isoperimetric properties of the high dimensional unit sphere, we construct graphs providing lower bounds for RT_r(n,K_{r+s},o(n)) for every 2 <= s <= r. These constructions are sharp for an infinite family of pairs of r and s. The only previous sharp construction was by Bollob…
Equitable tree colouring of graphs
研究图的树着色问题，证明当顶点数足够大时，存在公平的树着色方案，验证了相关猜想。
Number of independent transversals in multipartite graphs
本文证明了多部图在特定最大度限制下存在大量独立横截，证实了Haxell和Szabó的猜想，并推广至每部分含s个顶点的独立集，部分回答了Erdős等人的问题。
Kim, Kühn, Osthus and Tyomkyn (Trans. Amer. Math. Soc. 371 (2019), 4655--4742) greatly extended the well-known blow-up lemma of Komlós, Sárközy and Szemerédi by proving a `blow-up lemma for approximate decompositions' which states that multipartite quasirandom graphs can be almost decomposed into any collection of bounded degree graphs with the same multipartite structure and slightly fewer edges. This result has already been used by Joos, Kim, Kühn and Osthus to prove the tree packing conjecture due to Gyárfás and Lehel from 1976 and Ringel's conjecture from 1963 for bounded degree trees as w…
On the boundedness of degenerate hypergraphs
本文研究退化超图 Turán 问题中高度数顶点的影响，证明偶环、多数完全二部图及其扩张等退化超图具有有界性，并通过解决一个 3-图 Zarankiewicz 型问题支持完全二部图扩张的有界性证明。
Spectral Extremal Graphs without a $K_k$-Factor
本文确定了当 $m\ge 2k-1$ 时不含 $K_k$-因子的 $n$ 顶点图的最大谱半径，并证明唯一极值图是 $H_{n,k}$，方法结合分解引理、Motzkin--Straus 不等式与谱估计。
Rainbow spanning structures in graph and hypergraph systems
该研究推广经典子图包含问题，提出图与超图系统的彩虹子图概念，建立了满足最小度条件的彩虹团因子和彩虹完美匹配的渐近最优结果，并给出通用求解框架。
A parameterized algorithm for $K_r$-factors in graphs of high minimum degree
该研究提出了一种参数化算法，用于在高最小度数图中寻找$K_r$-因子，并展示了其固定参数可解性。
Independent transversal blow-up of graphs
本文研究多部图中独立横截的存在性，提出紧的部大小条件并推广已有结果，同时给出最小度条件及Turán型结论，并探讨ITS计数。
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