Large Subgraphs of Minimal Density or Degree

Paul Erdés1, Ralph Faudree2, Arun Jagota2, Tomasz Luczak3
1Hungarian Academy of Sciences
2University of Memphis
3Adam Mickiewicz University

Abstract

This paper addresses the following questions. In any graph \(G\) with at least \(\alpha\binom{n}{2}\) edges, how large of an induced subgraph \(H\) can we guarantee the existence of with minimum degree \(\delta(H) \geq \lfloor\alpha|V(H)|\rfloor\)? In any graph \(G\) with at least \(\alpha\binom{n}{2} – f(n)\) edges, where \(f(n)\) is an increasing function of \(n\), how large of an induced subgraph \(H\) can we guarantee the existence of containing at least \(\alpha\binom{|V(H)|}{2}\) edges? In any graph \(G\) with at least \(\alpha n^2\) edges, how large of an induced subgraph \(H\) can we guarantee the existence of with at least \(\alpha|V(H)|^2 + \Omega(n)\) edges? For \(\alpha = 1 – \frac{1}{r}\), for \(r = 2, 3, \ldots\), the answer is zero since if \(G\) is a complete \(r\)-partite graph, no subgraph \(H\) of \(G\) has more than \(\alpha|V(H)|^2\) edges. However, we show that for all admissible \(\alpha\) except these, the answer is \(\Omega(n)\). In any graph \(G\) with minimum degree \(\delta(G) \geq \alpha n – f(n)\), where \(f(n) = o(n)\), how large of an induced subgraph \(H\) can we guarantee the existence of with minimum degree \(\delta(H) \geq \Omega|V(H)|\)?