Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 1 / 16J / a / Solution

A choice function on a family $\mathcal{A}$ of nonempty sets satisfies $c(A)\in A$. Zermelo's theorem says every set has a total order under which every nonempty subset has a least element.

Solved by gpt-5.6-sol high.