Balanced supersaturation for some degenerate hypergraphs

A classical theorem of Simonovits from the 1980s asserts that every graph $G$ satisfying ${e(G) \gg v(G)^{1+1/k}}$ must contain $\gtrsim \left(\frac{e(G)}{v(G)}\right)^{2k}$ copies of $C_{2k}$. Recently, Morris and Saxton established a balanced version of Simonovits' theorem, showing that such $G$ has $\gtrsim \left(\frac{e(G)}{v(G)}\right)^{2k}$ copies of $C_{2k}$, which are `uniformly distributed' over the edges of $G$. Moreover, they used this result to obtain a sharp bound on the number of $C_{2k}$-free graphs via the container method. In this paper, we generalise Morris-Saxton's results for even cycles to $\Theta$-graphs. We also prove analogous results for complete $r$-partite $r$-graphs.Comment: Changed title, abstract and introduction were rewritte

Bounds for the number of meeting edges in graph partitioning

summary:Let $G$ be a weighted hypergraph with edges of size at most 2. Bollobás and Scott conjectured that $G$ admits a bipartition such that each vertex class meets edges of total weight at least $(w_1-\Delta _1)/2+2w_2/3$, where $w_i$ is the total weight of edges of size $i$ and $\Delta _1$ is the maximum weight of an edge of size 1. In this paper, for positive integer weighted hypergraph $G$ (i.e., multi-hypergraph), we show that there exists a bipartition of $G$ such that each vertex class meets edges of total weight at least $(w_0-1)/6+(w_1-\Delta _1)/3+2w_2/3$, where $w_0$ is the number of edges of size 1. This generalizes a result of Haslegrave. Based on this result, we show that every graph with $m$ edges, except for $K_2$ and $K_{1,3}$, admits a tripartition such that each vertex class meets at least $\lceil {2m}/{5}\rceil$ edges, which establishes a special case of a more general conjecture of Bollobás and Scott

On several partitioning problems of Bollobás and Scott

AbstractJudicious partitioning problems on graphs and hypergraphs ask for partitions that optimize several quantities simultaneously. Let G be a hypergraph with mi edges of size i for i=1,2. We show that for any integer k⩾1, V(G) admits a partition into k sets each containing at most m1/k+m2/k2+o(m2) edges, establishing a conjecture of Bollobás and Scott. We also prove that V(G) admits a partition into k⩾3 sets, each meeting at least m1/k+m2/(k−1)+o(m2) edges, which, for large graphs, implies a conjecture of Bollobás and Scott (the conjecture is for all graphs). For k=2, we prove that V(G) admits a partition into two sets each meeting at least m1/2+3m2/4+o(m2) edges, which solves a special case of a more general problem of Bollobás and Scott

A hierarchy of randomness for graphs

AbstractIn this paper we formulate four families of problems with which we aim at distinguishing different levels of randomness.The first one is completely non-random, being the ordinary Ramsey–Turán problem and in the subsequent three problems we formulate some randomized variations of it. As we will show, these four levels form a hierarchy. In a continuation of this paper we shall prove some further theorems and discuss some further, related problems

On Sharp Thresholds in Random Geometric Graphs

We give a characterization of vertex-monotone properties with sharp thresholds in a Poisson random geometric graph or hypergraph. As an application we show that a geometric model of random k-SAT exhibits a sharp threshold for satisfiability