The Inducibility of Graphs on Four Vertices

James Hirst

Published 2011-09-07, updated 2013-07-16Version 2

We consider the problem of determining the maximum induced density of a graph H in any graph on n vertices. The limit of this density as n tends to infinity is called the inducibility of H. The exact value of this quantity is known only for a handful of small graphs and a specific set of complete multipartite graphs. Answering questions of Brown-Sidorenko and Exoo we determine the inducibility of K_{1,1,2} and the paw graph. The proof is obtained using semi-definite programming techniques based on a modern language of extremal graph theory, which we develop in full detail in an accessible setting.