Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 109 / 3 / ii / Solution

No example exists for $n=2$, because the unique point in $A_1\cap A_2$ would also lie in the total intersection; the one-member case is excluded by nontriviality. For every $n\ge 3$, take ground set $[n]$ and define the near-pencilTwo small sets meet in $n$, each small set meets $A_0$ in its other point, and the total intersection is empty. Thus the required values are exactly $n\ge 3$.