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I learned of the “Blue Eyes” puzzle for the first time a couple of weeks ago, and I’ve been playing around with some of the logical concepts behind it since then. For anyone unfamiliar with this puzzle, I suggest you read and solve it before trying this one.

My shenanigans led me to come up with the following puzzle (which I have, by now, already solved, but I have a feeling the MSE community will have fun with it):

An island has one blue-eyed person and one brown-eyed person, who can see each others’ eyes but not their own and cannot communicate. Every night, a ferry comes to take away any people who already know their own eye color. Both people are perfect logicians.

Prove that, for any $n\in\mathbb Z^+$, there exists a statement that you can announce to both of them so that they will both leave on night $n$ (no earlier and no later).

When I say “statement,” I mean this very generally. The method I used to prove this and construct such a statement for arbitrary $n$ was very messy and for large $n$ results in a “statement” that is very cumbersome to read (but logically coherent nonetheless).

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Franklin Pezzuti Dyer
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  • @automaticallyGenerated No, they don’t know anything about the distribution of eye-colors. For instance, if I see a blue-eyed person, I could have brown eyes, or red eyes, or purple eyes, or any color eyes for all I (eye?) know. – Franklin Pezzuti Dyer Sep 03 '19 at 00:14
  • Well, assuming the islanders trust you to tell the truth (and if they don't nothing you say could do anything) you could tell them "Either I told the other islander something that will let thim figure out his eye colors in $n-1$ days or your eyes are x." – fleablood Sep 03 '19 at 00:23
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    @fleablood Sorry, I should clarify something. You are allowed to announce one statement to both of them, and cannot say anything to only one of them. More precisely, the statement is Common Knowledge. – Franklin Pezzuti Dyer Sep 03 '19 at 00:35
  • @WillO Nope, that doesn’t work. If you tell them that, they know that there’s no reason for either of them to leave on day n-1, so they will both leave immediately knowing that their eye colors are {blue, brown}. – Franklin Pezzuti Dyer Sep 03 '19 at 00:51
  • @WillO No, there is just one moment each day when people can leave. What I’m saying is this: the statement you proposed doesn’t work because both people know that “you will both know your eye colors and leave on day n-1” is false, and would therefore know that their eye colors are {blue, brown} (since you told them that one or the other is true). – Franklin Pezzuti Dyer Sep 03 '19 at 01:00
  • @FranklinPezzutiDyer : Yes, you are right. My bad. I'll delete my wrong comments in a few minutes. – WillO Sep 03 '19 at 01:33
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    WillO's answer won't work as they will know that there will be no information that will come that will let them leave on day n-1 so they will know imediately that that option makes no sense. They have no reason to wait n-1 days. You need a reason they can wait. You could tell them that their eyes are either brown/blue, green/brown, or yellow/blue. They'll figure there eyes are brown or yellow/blue or green and if they are are yellow/green the other knows immediately so it will take two days. I bet you can build on that. – fleablood Sep 03 '19 at 04:53

2 Answers2

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I'll put my solution in a spoiler, not to ruin it for anybody. It assumes that there are a countably infinite number of eye colours, though...

Announcement for $n=1$:

For $n=1$, announce sentence $\varphi_1:=$ "there is one person with blue eyes, and one person with brown eyes." Both islanders will leave with the ferry the next night, for obvious reasons.

Announcement for $n=2$:

For $n=2$, announce: $\varphi_2:=$"if one of you has green eyes, the other has blue eyes, and if one of you has grey eyes, the other has brown eyes, and if neither of you have green or grey eyes, then $\varphi_1$."

Suppose I'm one of the islanders, and I can see blue eyes, then I know that I have either green or brown eyes. However, if I had green eyes, the other islander would leave the next night due to them knowing they have blue eyes. Therefore, since they didn't leave on the first night, I learn I must have brown eyes and will leave on night $2$. The same reasoning holds for the other islander.

Announcement for $n=3$:

For $n=3$, announce $\varphi_3:=$ "if one of you has purple eyes, the other has green eyes, and if one of you has red eyes, the other has grey eyes, and if neither of you have purple or red eyes, then $\varphi_2$."

Again, suppose I am the islander who sees blue eyes, then I know that neither of the colours can be purple or red (since then the other would be green or grey), thus $\varphi_2$ must hold.

From this, I know that I have either green eyes or brown eyes. In case I have green eyes, the other islander will think they have blue eyes or purple eyes, but does not know which, thus they can't leave on the first night. However, if the other islander had purple eyes, I would leave on the first night, which I don't. Hence on the second day, the other islander would know they did not have purple eyes and conclude they have blue eyes.

Therefore, if I had green eyes, the other islander would leave on the second night. Now, since the other islander didn't leave on the second night (for the same reason I didn't leave on the second night), I don't have green eyes, so I must have brown eyes, and hence I will leave on the third night.

Announcement for general $n\geq 2$:

We can keep doing this. Let $c_n$ and $c_n'$ be two colours that have not been used yet (so $c_1= blue$ and $c_1'=brown$, $c_2= green$ and $c_2'=grey$, $c_3=purple$ and $c_3'=red$). Then announce $\varphi_n:=$ "If one of you has $c_n$ eyes, the other has $c_{n-1}$ eyes, and if one of you has $c_n'$ eyes, the other has $c_{n-1}'$ eyes, and if neither of you have $c_n$ or $c_n'$ eyes, then $\varphi_{n-1}$."

Greg Martin
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Vsotvep
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Here is another, more concise solution to this puzzle. First, the $n=4$ version:

In the sequence "yellow, grey, red, blue, brown, green, purple, orange", the eye colors that the two of you have appear consecutively.

Here's the explanation:

Initially, in the "yellow & grey" and "purple & orange" hypotheticals, this tells the person who sees yellow/orange that their own eyes are grey/purple respectively, so they leave the island on night $1$. Therefore in the "grey & red" and "green & purple" hypotheticals, seeing this not happen tells the person who sees grey/purple that their eyes are not yellow/orange, so they leave the island on night $2$. That means that in the "red & blue" and "brown & green" hypotheticals, seeing this not happen tells the person who sees red/green that their eyes are not grey/purple, so they leave the island on night $3$. That means that in reality, seeing this not happen tells the brown-eyed person that their eyes are not red, and the blue-eyed person that their eyes are not green; both leave the island on night $4$.

We solve the general problem with a similar statement: the statement

In the sequence "$c_1$, $c_2$, ..., $c_{n-1}$, blue, brown, $c_n$, $c_{n+1}$, ..., $c_{2n-2}$", the eye colors that the two of you have appear consecutively. (Here, $c_1, \dots, c_{2n-2}$ can be any choice of $2n-2$ different colors other than blue and brown.)

Misha Lavrov
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