The Dot-town suicide

Question

Each resident of Dot-town carries a red or blue dot on his (or her) forehead, but if he ever figures out what color it is he kills himself. Each day the residents gather; one day a stranger comes and tells them something — anything — non-trivial about the number of blue dots. Prove that eventually every resident kills himself, no matter what the stranger said.

Comment: “Non-trivial” means here that there is some number of blue dots for which the statement would not have been true. Thus we have a frighteningly general version of classical problems involving knowledge about knowledge.

Answer 1

The minimal statement, that suffices is I think:

There are not $n$ blue dots. This removes only one option for the blue dots and thus it seems minimal in terms of information

From there on:

If there are $n+1$ blue dots. After one day, every blue-dotter has realized that he is also a blue-dotter (after seeing the $n$ blue dots) and kills himself.
If there are $n+2$ blue dots. After one day, none of the blue-dotters have killed themselves, because they have seen $n+1$ blue dots. They however realize that they also have blue dots, so they kill themselves on the next day.
This is generalized for $n+x$. After all blue-dotters have died, the red-dotters kill themselves on the next day.

But what if?

If there are $n-x$ blue dots? Things play out identically for the red dots as they now know that there are not $p-n$ red dots where $p$ is the population size. In the mean time the blue dots are happily waiting, as they don't gain information until the red ones are dead.

The only flaw I see is that the statement may not be minimal, so I would love to see your comments.

Answer 2

Let's say there are $n$ residents, $b$ of which have blue dots.

The stranger's statement is something like

"The number of blue dots is in the set $B$".

where $B$ is a subset of $\{1,2,\dots,n\}$ which is neither empty nor the whole set. We will assume the statement is true, namely, that $b\in B$.

Let's investigate which values of $b$ will cause a resident to kill themself on their first day.

• If a resident has a blue dot, they will see $b-1$ blue dots, so they know the number of blue dots is $b-1$ or $b$. If $b-1\notin B$, the stranger's statement eliminates the $b-1$ case, so the resident learns their eye color is blue and kills themself.

• Similarly, a red-dotted resident will suicide if $b+1\notin B$.

This means that a suicide will happen if $b$ is "near the boundary" of $B$. Below is an example, where the $\color{blue}{\text{blue}}$ numbers are in $B$, and the underlined values are the ones which result in a suicide: $$ {\color{blue}{0}}\,\,\color{blue}1\,\, \color{blue}2\,\,\underline{\color{blue}3}\,\,{4}\,\,5\,\,{6}\,\,\underline{\color{blue}7}\,\,\color{blue}8\,\,\color{blue}9\,\,\color{blue}{10}\,\,\color{blue}{11}\,\,\underline{\color{blue}{12}}\,\,{13}\,\,\underline{\color{blue}{14}}\,\,\underline{\color{blue}{15}}\,\,16\,\ \underline{\color{blue}{17}} $$

If there are no suicides, then none of the underlined values are possible, so the set of possible values of $b$ shrinks. On the next day, there will be a suicide if $b$ is near the boundary of this shrunken set, and no suicide causes the set to shrink again.

Eventually, the boundary will shrink to be next to the true value, $b$, causing one of the colors to suicide, and the other color to suicide on the next day.

Answer 3

When someone says they have at least one blue dot: From everyone's PoV, there are two possibilities re: how many blue dots they all have. When someone reduces it to 1, it means (s)he's figured out his/her dot color. They can also make guesses about others' PoV, guess their guesses... If someone sees x blue dots and assumes he's blue-dotted too, (s)he has to assume (s)he knows how many blue dots someone else sees, but that that person might think there are two possibilities re: how many blue dots they all have. Same for when the first person assumes (s)he has a red dot.

It can be represented with a tree. A non-trivial comment would imply at least one is red/blue at worst, so if no blue-dotted people assume they're blue-dotted, the number of seen/existing blue dots in the tree will fall to 0 too early, which they can't assume. A blue-dotted person will eventually figure out (s)he's one, and since it doesn't matter who it is, it would be every blue-dotted person. If the red-dotted ones survive, they'll realize that's because they're red-dotted and they'll die too.

If the stranger includes 0 but excludes some other numbers, it eliminates them and the numbers less than them, including 0. The reasoning will be similar to the classic case above.

Answer 4

Even ignoring the midnight issue, with a little bit of lateral thinking, this breaks down. It only works if we assume the stranger must make a statement that covers the entire population. If we allow statements involving subdivisions among the villagers, he can say "At least one left-handed villager has a blue dot," and only the left-handed villagers will kill themselves.

In fact, It's possible to say something about the entire population that won't cause any suicides:

"Dividing the villagers into men, women, and children under 18, there is at least one pair of villagers who are in different groups and have the same color dot."

Because:

As long as no group is empty, there is no possible arrangement of dots that will result in a suicide on the first day. WLOG, consider a man. If he sees that all women and children have the same color dot, then there's the pair right there. Otherwise, there's at least one woman or child with a red dot and one with a blue dot, so even if the men are monochrome there's a valid pair with one of them.

Answer 5

I think all he has to say is "The number of red dots is greater than or less than the number of blue dots."

If there are 600 blue dots and no red dots, everyone panics and says "oh crap I'm the red dot!" But then they don't instantly kill themselves, and think "Phew, that means I'm not a red dot. oh shiiii" and die.

If there are an equal number of each, the same logic applies. Everyone sees the number of other dots, and concludes either truly or falsely about their dot coloration. if true, they suicide, if not, they still suicide.

Cletus wonders what it is he said.

Edit 1: You know, come to think of it, he can say anything at all.

If he says, truthfully or falsely, anything about the number of blue dots, then both the red dot people and the blue dot people suddenly become aware that they are one or the other and not some other color like green.

The first day (or in this case, infinitely tiny fraction of a second) they either come to the correct conclusion about their dot coloration and die, or they come to the false conclusion, and then realize they are incorrect, and that their dot coloration is the opposite. Then they die.

The only thing necessary to begin this catastrophe is for the people to be aware of the binary nature of the dot coloration and for someone to introduce them to the idea that other people now know something about each other's dot coloration.

Answer 6

Couldn't it be as simple as understanding that there is no specified population of Dot-town. There could just be two.

If the stranger says there are two blue dots, both residents know. If the stranger says there is only one blue dot, both residents know. If the stranger says there are no blue dots, both residents know.

Everybody dies.

Answer 7

The dim-witted traveler turns up to Dot-town and says after seeing only 4 townsfolk:

Hey guys, why do all the women have red dots and the men have blue dots? (I believe this stipulates as non-trivial?)

Then:

All the women with red dots and the men with blue dots kill themselves.

Which causes:

All the surviving town members to realise they must have the opposite colour, and therefore kill themselves. The dim-witted traveler then goes "Woohoo! I own a whole town!"

Generalized:

Having a red (or blue) dot is a boolean value, therefore you know what you have when you know what you don't have. I'm surprised the townsfolk were able to live long enough to create a town in the first place ;).

Answer 8

The stranger said..

"There are NO blue dots."
So, everyone realized that they all have red dots on their forehead and killed themselves!

The end~