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The crossing lemma: every graph with n vertices and m ≥ 4n edges has crossing number cr(G) = Ω(m³/n²). Covers the lemma's statement, proofs, constants and improvements (e.g. Pach–Tóth), sharpness constructions, and direct applications such as Székely-type bounds. Claims about named problems the lemma is applied to keep their own tags; general graph theory stays under Graph theory.
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