From Ramsey’s Theorem to the Erdös–Hajnal Conjecture

Patrice Ossona de Mendez

Abstract: Ordnung muss sein! (Order is inevitable.) This saying , attributed to Theodore Motzkin, aptly summarizes Ramsey theory, which aims to solve problems of the form: “From what size on can a structure no longer avoid having a certain property ?”

Translated by R. Matveev in 2026 from the French original published in 2017 at Images des mathématiques

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