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Graph isomorphism<br />

Definition<br />

Isomorphism of graphs G 1 (V 1 , E 1 ) and G 2 (V 2 , E 2 ) is a bijection<br />

between the vertex sets ϕ : V 1 → V 2 such that<br />

∀u, v ∈ V 1 (u, v) ∈ E 1 ⇔ (ϕ(u), ϕ(v)) ∈ E 2 .<br />

Facts<br />

• No algorithm, other than brute force, is known for testing<br />

whether two arbitrary graphs are isomorphic.<br />

• It is still an open question (!) whether graph isomorphism is<br />

NP complete.<br />

• Polynomial time isomorphism algorithms for various graph<br />

subclasses such as trees are known.<br />

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