Splits with forbidden subgraphs


Abstract in English

In this note, we fix a graph $H$ and ask into how many vertices can each vertex of a clique of size $n$ can be split such that the resulting graph is $H$-free. Formally: A graph is an $(n,k)$-graph if its vertex sets is a pairwise disjoint union of $n$ parts of size at most $k$ each such that there is an edge between any two distinct parts. Let $$ f(n,H) = min {k in mathbb N : mbox{there is an $(n,k)$-graph $G$ such that $H otsubseteq G$}} . $$ Barbanera and Ueckerdt observed that $f(n, H)=2$ for any graph $H$ that is not bipartite. If a graph $H$ is bipartite and has a well-defined Turan exponent, i.e., ${rm ex}(n, H) = Theta(n^r)$ for some $r$, we show that $Omega (n^{2/r -1}) = f(n, H) = O (n^{2/r-1} log ^{1/r} n)$. We extend this result to all bipartite graphs for which an upper and a lower Turan exponents do not differ by much. In addition, we prove that $f(n, K_{2,t}) =Theta(n^{1/3})$ for any fixed $t$.

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