CorollaryStatement: Literature-sourcedProof: AI-adaptedSession-authored (Fable 5 assisted)precheck passaudited 2026-08-13

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Every bipartite graph with at least one edge has Turán density zero

Statement

If $H$ is a finite bipartite graph with at least one edge, then

$π (H) = 0.$

Facts & Assumptions

Given: The hypotheses and notation of the statement above.

[F1]

The complete bipartite graph $K_{A, B}$ has exactly all edges joining a vertex of $A$ to a vertex of $B$ (Empty and complete graphs, complete bipartite graphs, and the convention that $P_{n}$ and $C_{n}$ have $n$ vertices).

[F2]

For $s, t \geq 1$ , the Kővári–Sós–Turán theorem gives $ex (N, K_{s, t}) = O_{s, t} (N^{2 - 1 / s})$ (Kővári–Sós–Turán: exact bipartite and ordinary-graph upper bounds for excluding $K_{s, t}$ ).

[F3]

For every finite graph $H$ with an edge, the normalized extremal numbers converge to $π (H)$ , their infimum over $n \geq 2$ (Every finite graph with an edge has a Turán density $π (H) = \lim_{n \to \infty} ex (n, H) / (\binom{n}{2})$ ).

Proof

technique · embed the forbidden graph in a complete bipartite graph

1.1

Choose a bipartition of $H$ and enlarge its two sides, including any isolated vertices, to positive sizes $s, t$ such that $H$ is an ordinary subgraph of $K_{s, t}$ . Every $H$ -free graph is then $K_{s, t}$ -free.

givenF1

2.1

Hence $0 \leq ex (n, H) \leq ex (n, K_{s, t}) = O_{s, t} (n^{2 - 1 / s})$ . Dividing by $(\binom{n}{2})$ makes the right side tend to $0$ . The existing limit $π (H)$ is therefore $0$ .

step 1.1givenF2F3

3.1

The at-least-one-edge hypothesis ensures both bipartition sides can be chosen positive and is exactly the scope in which the extremal density was defined.

step 1.1step 2.1∎

Depends on

Used by

The asymptotic extremal density is determined exactly by chromatic number: π(H)=1-1/(χ(H)-1) Corollary

Dependency tree · next 3 levels

Direct dependencies and their dependencies through the next three levels: 28 results over 9 levels. An arrow runs from a result to what uses it, and this result sits at the bottom with a heavier outline. Click the chart to enlarge it.

Sources

Yufei Zhao, Graph Theory and Additive Combinatorics (standard reference, not scraped)