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TheoremStatement: Literature-sourcedProof: AI-generatedprecheck passjudge pass (gpt-5.6-terra)audited 2026-08-31
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A cycle of length at least five and a forest complement have the Erdős-Hajnal property

Statement

Let C be a cycle of length at least 5 and let F be a forest. Then the pair {C,F} has the Erdős-Hajnal property.

Facts & Assumptions

Given: A cycle C of length 5 and a forest F.

[L1]

For every forest H, the pair {H,H} has the Erdős-Hajnal property (A forest complement and its star-expansion have the Erdős-Hajnal property).

[L2]

If a forest contains a path of length 4, then its star-expansion contains an induced -cycle (A star-expansion of a forest containing a long path contains the corresponding cycle).

[L3]

The Erdős-Hajnal property passes to hereditary subclasses (The Erdős–Hajnal property and each of its constants pass to hereditary subclasses).

Proof

technique · direct
1.1

Choose a forest H that contains F as an induced subgraph and also contains an induced path on 3 vertices. For instance, take the disjoint union of F with a path long enough to realize that length. By [L2], the star-expansion H contains an induced copy of C.

L2choose
2.1

If a graph G is C-free and F-free, then it is also H-free and H-free. Indeed, an induced copy of H would contain the induced cycle C from step 1.1, and an induced copy of H would contain F because F is an induced subgraph of H. Thus the class forbidding {C,F} is a hereditary subclass of the class forbidding {H,H}.

step 1.1given
3.1

By [L1], the larger class from step 2.1 has the Erdős-Hajnal property, so [L3] passes that property to the subclass forbidding {C,F}.

step 2.1L1L3

Depends on

Used by

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Sources