Past exam of the mathematics course of the University of Cambridge / 2015 / iii / Paper 24 / 1 / c / Solution

On $[\kappa {]}^{\omega }$, declare $A\sim B$ when their symmetric difference is finite. Use the well-ordering theorem to choose one representative $R_{[A]}$ from each equivalence class, and color byRemoving one point from a countably infinite set does not change its equivalence class and reverses this parity. For any $H\subseteq \kappa$ of order type $\omega$ and $x\in H$, both $H$ and $H\setminus \{x\}$ belong to $[H{]}^{\omega }$ and receive different colors. There is no homogeneous $H$ of order type $\omega$. This proves the negative infinite-arity partition relationThe slashed arrow in the PDF is essential.