Extremal results for berge hypergraphs

Let E(G) and V (G) denote the edge set and vertex set of a (hyper)graph G. Let G be a graph and H be a hypergraph. We say that a hypergraph H is a Berge-G if there is a bijection f : E(G) → E(H) such that for each e ϵ E(G) we have e ? f(e). This generalizes the established definitions of "Berge path" and "Berge cycle" to general graphs. For a fixed graph G we examine the maximum possible size of a hypergraph with no Berge-G as a subhypergraph. In the present paper we prove general bounds for this maximum when G is an arbitrary graph. We also consider the specific case when G is a complete bipartite graph and prove an analogue of the K?ovári-Sós-Turán theorem. In case G is C4, we improve the bounds given by Gy?ori and Lemons [Discrete Math., 312, (2012), pp. 1518-1520]. © 2017 Society for Industrial and Applied Mathematics

Exponential lower bound for Berge-Ramsey problems

We give an exponential lower bound for Berge-Ramsey problems

Partitioning the power set of [n][n] into CkC_k-free parts

We show that for $n \geq 3, n\ne 5$, in any partition of $\mathcal{P}(n)$, the set of all subsets of $[n]=\{1,2,\dots,n\}$, into $2^{n-2}-1$ parts, some part must contain a triangle --- three different subsets $A,B,C\subseteq [n]$ such that $A\cap B$, $A\cap C$, and $B\cap C$ have distinct representatives. This is sharp, since by placing two complementary pairs of sets into each partition class, we have a partition into $2^{n-2}$ triangle-free parts. We also address a more general Ramsey-type problem: for a given graph $G$, find (estimate) $f(n,G)$, the smallest number of colors needed for a coloring of $\mathcal{P}(n)$, such that no color class contains a Berge-$G$ subhypergraph. We give an upper bound for $f(n,G)$ for any connected graph $G$ which is asymptotically sharp (for fixed $k$) when $G=C_k, P_k, S_k$, a cycle, path, or star with $k$ edges. Additional bounds are given for $G=C_4$ and $G=S_3$.Comment: 12 page