Large induced trees in Kr -free graphs

  • Authors:
  • Jacob Fox;Po-Shen Loh;Benny Sudakov

  • Affiliations:
  • Department of Mathematics, Princeton University, Princeton, NJ 08544, United States;Department of Mathematics, Princeton University, Princeton, NJ 08544, United States;Department of Mathematics, UCLA, Los Angeles 90095, United States

  • Venue:
  • Journal of Combinatorial Theory Series B
  • Year:
  • 2009

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Abstract

For a graph G, let t(G) denote the maximum number of vertices in an induced subgraph of G that is a tree. In this paper, we study the problem of bounding t(G) for graphs which do not contain a complete graph K"r on r vertices. This problem was posed twenty years ago by Erdos, Saks, and Sos. Substantially improving earlier results of various researchers, we prove that every connected triangle-free graph on n vertices contains an induced tree of order n. When r=4, we also show that t(G)=logn4logr for every connected K"r-free graph G of order n. Both of these bounds are tight up to small multiplicative constants, and the first one disproves a recent conjecture of Matousek and Samal.