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.
Note that this problem is known as the Blanchet Theorem.
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Solution
![](https://mirangu.com/wp-content/uploads/2023/04/PrisonersDilemmaSolution1.jpg)
![](https://mirangu.com/wp-content/uploads/2023/04/PrisonersDilemmaSolution2.jpg)