Answer to the Puzzle :
The knowledge of each islander consists of:
– the color of the eyes of every other islander.
– any past pronouncement from the guru.
– the history of who left the island on previous days (including their eye color), which provides knowledge about other’s knowledge (that either they did or did not know their own eye color on previous days).
Blue eyed people leave on the 100th night.
If you (the person) have blue eyes then you can see 99 blue eyed and 100 brown eyed people (and one green eyed, the Guru). If 99 blue eyed people don’t leave on the 99th night then you know you have blue eyes and you will leave on the 100th night knowing so.
Proof:
Imagine a simpler version of the puzzle in which, on day #1 the guru announces that she can see at least 1 blue-eyed person, on day #2 she announces that she can see at least 2 blue eyed people, and so on until the blue-eyed people leave.
So long as the guru’s count of blue-eyed people doesn’t exceed your own, then her announcement won’t prompt you to leave. But as soon as the guru announces having seen more blue-eyed people than you’ve seen yourself, then you’ll know your eyes must be blue too, so you’ll leave that night, as will all the other blue-eyed people. Hence our theorem obviously holds in this simpler puzzle.
But this “simpler” puzzle is actually perfectly equivalent to the original puzzle. If there were just one blue-eyed person, she would leave on the first night, so if nobody leaves on the first night, then everybody will know there are at least two blue-eyed people, so there’s no need for the guru to announce this on the second day. Similarly, if there were just two blue-eyed people, they’d then recognize this and leave on the second night, so if nobody leaves on the second night, then there must be a third blue-eyed person inspiring them to stay, so there’s no need for the guru to announce this on the third day. And so on… The guru’s announcements on the later days just tell people things they already could have figured out on their own.
