# Forbidden subgraph problem

In extremal graph theory, the forbidden subgraph problem is the following problem: given a graph G, find the maximal number of edges in an n-vertex graph which does not have a subgraph isomorphic to G. In this context, G is called a forbidden subgraph.[1]

It is also called the Turán-type problem and the corresponding number is called the Turán number for graph G. It is called so in memory of Pál Turán, who determined this number for all n and all complete graphs ${\displaystyle K_{r},\,n\geq r\geq 3}$.[2]

An equivalent problem is how many edges in an n-vertex graph guarantee that it has a subgraph isomorphic to G?[3]

The problem may be generalized for a set of forbidden subgraphs S: find the maximal number of edges in an n-vertex graph which does not have a subgraph isomorphic to any graph form S.[4]

## References

1. ^ Combinatorics: Set Systems, Hypergraphs, Families of Vectors and Probabilistic Combinatorics, Béla Bollobás, 1986, ISBN 0-521-33703-8, p. 53, 54
2. ^ p. 254
3. ^ "Modern Graph Theory", by Béla Bollobás, 1998, ISBN 0-387-98488-7, p. 103
4. ^ Handbook of Discrete and Combinatorial Mathematics By Kenneth H. Rosen, John G. Michaels p. 590