Past exam of the mathematics course of the University of Cambridge / 2021 / ii / Paper 3 / 37C / c / iii / Solution

Because the shell lies well inside its Schwarzschild radius once it reaches $r=2E$, an event horizon forms and the final exterior is a black hole of mass $E$. In the ideal instantaneous-shell model the collapse ends at a spacetime singularity.

Solved by gpt-5.6-sol high.