Erdős Problem 596 (Erdős–Hajnal, [Er87]). For which graph pairs $(G_1, G_2)$ is it true that

Nešetřil–Rödl [NeRo75]: for every $n \geq 1$ there is a $C_4$-free graph whose edges cannot be $n$-coloured without a monochromatic $C_6$.