Past exam of the mathematics course of the University of Cambridge / 2024 / iii / Paper 221 / 3 / ii

In an acyclic directed mixed graph, a fixable vertex $v$ is one whose bidirected district contains no proper directed descendant of $v$:Here $A$ and $Y$ lie in the same district because $A\leftrightarrow Y$, and $Y$ is a directed descendant of $A$ along $A\to M\to Y$. Thus $A$ is not fixable.