You need to know: Graph, notation for complete graph on n vertices, notations
and
for the number of vertices and edges in graph H, respectively.
Background: For a graph with
and
, let
be the maximum value of
over all subgraphs H of F with
. We say that a graph is F-free if it does not contain a copy of a graph F as a subgraph. For graph G, we write
for the maximal number of edges that an F-free subgraph of G may have. Number
is known as Turán density of graph F. The Erdős-Renyi graph
is a random graph with n vertices, such that every possible edge is included independently with probability
.
The Theorem: On 16th October 2009, Mathias Schacht submitted to the Annals of Mathematics a paper in which he proved that for every graph with
and
and every
, there exists
such that for every sequence of probabilities
, we have
.
Short context: Function was first studied by Mantel, Erdős, and Turán. In particular, Turán determined in 1941
for all integers n and k. For general graph G, it is known that
. The Theorem proves that if G is a random graph, then the opposite inequality
holds with high probability. It confirms a conjecture of Kohayakawa, Łuczak, and Roedl made in 1997.
Links: Free arxiv version of the original paper is here, journal version is here.