Extremal Graph Theory and Forbidden Subgraphs

Core Idea

Extremal graph theory studies the maximum number of edges in graphs avoiding certain subgraphs. Given forbidden graphs H, ex(n, H) is the maximum edges in n-vertex graphs with no copy of H. Classical results include Turán (forbidding cliques), Kővári-Sós-Turán (forbidding complete bipartite graphs), and connections to combinatorial designs and incidence geometry.

Explainer

Extremal graph theory asks a precise optimization question: how many edges can a graph on n vertices have, given that it is *not allowed* to contain a particular subgraph? The function ex(n, H) — called the Turán number of H — captures the answer. The graph achieving this maximum (or asymptotically approaching it) is called an extremal graph for H.

From Turán's theorem, you already know one landmark result: ex(n, Kᵣ₊₁), the maximum edges avoiding a complete graph on r+1 vertices, is achieved by the Turán graph T(n, r) — a balanced complete r-partite graph. The key insight there was that forbidding cliques forces a multipartite structure, and balancing the parts maximizes edges within those constraints. Extremal graph theory generalizes this: for any forbidden graph H, what structure must an extremal graph have?

For complete bipartite graphs, the Kővári-Sós-Turán theorem gives an upper bound: any graph on n vertices with more than roughly (1/2)t^1/s · n^2−1/s edges must contain a copy of Kₛ,ₜ (s ≤ t). This matters for incidence geometry — if you have n points and n lines in the plane, asking how many incidences (point-on-line pairs) can occur is equivalent to a forbidden subgraph problem on a bipartite incidence graph, and KST bounds the answer.

The deepest results in the field come from the Zarankiewicz problem and connections to the Bondy-Simonovits theorem, which says that for any graph H that isn't bipartite, ex(n, H) grows like n^(1+1/(χ(H)−1)), where χ(H) is the chromatic number. This ties extremal density directly to coloring structure: the more chromatic the forbidden graph, the fewer edges you can have while avoiding it. For bipartite forbidden graphs, the problem is much harder and often open — the correct growth rate is not known even for simple cases like C₆. Extremal graph theory lives at the intersection of combinatorics, algebra, and geometry, and its techniques (probabilistic arguments, algebraic constructions, flag algebras) represent some of the deepest tools in modern combinatorics.