Two prisoners are chained to the walls of a triangular cell. The base is covered with a mirror and a perpendicular wall is separating them. They can each see the opposite corner through a hole. Proof they can see each other through another hole at the base of the perpendicular.

Proposed by Marshall W. Buck. Note that this problem is known as the Blanchet Theorem.

Solution

Two prisoners are chained to the walls of a triangular cell. The base is covered with a mirror and a perpendicular wall is separating them. They can each see the opposite corner through a hole. Proof they can see each other through another hole at the base of the perpendicular. pic.twitter.com/u2JDzKa43F

— Mirangu (@Mirangu1) April 21, 2023