Your Favorite Math/Logic Riddles? 1965
shma asks: "Whether you're involved in the Sciences, Mathematics, or Engineering, you undoubtedly enjoy finding simple solutions to seemingly difficult problems. I'm sure you all have a favorite mind-bender, and who better to share it with than the Slashdot community? Post your own problems and try to solve others. Just one request: If you have figured out the solution, link to it in a post, rather than write it out where anyone can see it." What brain benders tickle your fancy?
"Here's a sample to consider: You're in a dark room with 50 quarters, 18 of which are heads up. You are allowed to move around the coins or flip some or all of them, if you wish. Problem is, it's too dark to tell what you're moving or flipping (no, you can't figure it out by touch either). Your job is to split the coins into two groups, each of which has the same number of heads up coins. How do you accomplish this?"
Soduku (Score:4, Interesting)
-B
Keeping my skills fresh (Score:4, Interesting)
Sequence (Score:2, Interesting)
Fork in the road (Score:1, Interesting)
Petals of the Rose (Score:5, Interesting)
Bill Gates is said to have solved the problem by memorizing the combinations first [borrett.id.au], the brute force approach.
It ones of those that requires a knack for seeing the simple things
Re:Infinity (Score:4, Interesting)
Re:easy one (Score:5, Interesting)
What do you get if you multiply 6 by 9? (Score:3, Interesting)
Solution (Score:4, Interesting)
Jugs (Score:3, Interesting)
An original brain teaser (Score:2, Interesting)
it...
In the following sequence:
1, 4, 8, 13, 21, 30, 36, 44...
What is the next number and why:
A. 48
B. 50
C. 53
D. 57
E. 61
F. There is no pattern
Re:thrice-plus-one-or-half (Score:3, Interesting)
This is called the "Collatz Conjecture [wikipedia.org]": given a positive integer a_1 = n, let a_i = a_{i-1}/2 if a_i is even, and a_i = 3a_{i-1}+1 if n is odd. Repeat. In other words, take a number, divide by two if it's even and take three times it plus one if it's odd, and repeat ad nauseum. Try a few integers, and you'll find that they eventually end up cycling: 1, 2, 4, 1,
This problem fascinated me through high school, and I eventually ended up going into mathematics partly because of the fun I had exploring its ins and outs.
Re:Oldie but goodie... (Score:3, Interesting)
How many cans can a canner can, if a canner can can cans?
Another online version (Score:5, Interesting)
A True/False Oldie but Goodie (Score:3, Interesting)
Epimenides was a Cretan who made one immortal statement: "All Cretans are liars."
"The Epimenides paradox [wikipedia.org] is a problem in logic. This problem is named after the Cretan philosopher Epimenides of Knossos (flourished circa 600 BC), who stated , "Cretans, always liars". There is no single statement of the problem; a typical variation is given in the book Gödel, Escher, Bach (page 17), by Douglas R. Hofstadter.
Lightbulb problem (Score:5, Interesting)
I like this problem because people are ordinarily good at logic have so much trouble with it. I once had the pleasure of meeting Donald Knuth and stumped him with this puzzle.
Re:Petals of the Rose (Score:3, Interesting)
Unfotunately, one rumor says that the smarter you are, the longer it takes to figure out.
Because smart people often fall for complex solutions.
Algebraic proof: 2=1 (Score:2, Interesting)
x=y
x^2=xy
x^2-y^2=xy-y^2
(x+y)(x-y)=y(x-y)
x+y=y
2y=y
2=1
Every step uses perfectly valid algebra, yet something is obviously very wrong somewhere.
Enjoy...
Re:Divisible by 3 or 6? (Score:3, Interesting)
Any ABCABC number is divisible by 13... (Score:2, Interesting)
Take any six-digit number that's of the form ABCABC where A,B,C are any integers (yes, they can be the same, yes they can be zero, although that might make it less than six-digits if A, or A and B, are zero), and that number is guaranteed to be divisible by 13.
Violation of angular momentum (Score:2, Interesting)
2) To rotate an object one needs to give it angular velocity, hence angular momentum.
3) To have finite angular momentum, an object needs torque applied to it (or a force applied away from the center of moment).
4) Gravity acts on the center of moment and does not result in torque on any free falling object.
5) Cats dropped feet up manage to land on their feet.
6) Does this mean cats violate conservation of angular momentum; no wonder Egyptians worshiped them.
What is wrong with this discussion; no math involved from my Classical dynamics class.
Sticky Triangles (Score:5, Interesting)
If I give you 3 sticks, you can make one triangle. If I give you 2 more sticks (5), you can make 2 triangles. If I give you another stick (6), how can you make 4 triangles?
The King and the Chalice (only for Experts!) (Score:3, Interesting)
The king sits in his central room and the n prisoners are all locked in their sound proof cells. In the king's central chamber is a table with a single chalice sitting atop it. Now, the king opens up a door to one of the prisoners' rooms and lets him into the room, but always only one prisoner at a time! So he lets in just one of the prisoners, any one he chooses, and then asks him a question, "Since I first locked you and the other prisoners into your rooms, have all of you been in this room yet?" The prisoner only has two possible answers. "Yes," or, "I'm not sure." If any prisoner answers "yes" but is wrong, they all will be beheaded. If a prisoner answers "yes," however, and is correct, all prisoners are granted full pardons and freed. After being asked that question and answering, the prisoner is then given an opportunity to turn the chalice upside down or right side up. If when he enters the room it is right side up, he can choose to leave it right side up or to turn it upside down, it's his choice. The same thing goes for if it is upside down when he enters the room. He can either choose to turn it upright or to leave it upside down. After the prisoner manipulates the chalice (or not, by his choice), he is sent back to his own cell and securely locked in.
The king will call the prisoners in any order he pleases, and he can call and recall each prisoner as many times as he wants, as many times in a row as he wants. The only rule the king has to obey is that eventually he has to call every prisoner in an arbitrary number of times. So maybe he will call the first prisoner in a million times before ever calling in the second prisoner twice, we just don't know. But eventually we may be certain that each prisoner will be called in ten times, or twenty times, or any number you choose.
Here's one last monkey wrench to toss in the gears, though. The king is allowed to manipulate the cup himself, k times, out of the view of any of the prisoners. That means the king may turn an upright cup upside down or vice versa up to k times, as he chooses, without the prisoners knowing about it. This does not mean the king must manipulate the cup any number of times at all, only that he may.
Assume that both the king and the prisoners have a complete understanding of the game as I have just explained it to you, and that the prisoners were given time beforehand to come up with a strategy. The king was able to hear the prisoners discuss, however, so also assume that if there is a way to foil a strategy, the king will know it and exploit the weakness. The prisoners must utilize a strategy that works in absolutely every single possible case.
Now you must figure out not only how to keep the prisoners alive, but how to also ensure their eventual freedom. When can any one of them be certain they've all been in the central chamber of the dungeon at least once? And how? Don't try to imagine any trickery like scratching messages in the soft gold of the chalice. The problem is as simple as it sounds. The prisoners have absolutely no way of communicating with each other except through the two orientations of the chalice. If any of them attempts any trickery at all they will all be beheaded. All the prisoners can do is turn the chalice upside down or right side up, as they choose, whenever they are called into the chamber.
(written by a former roomate)
hats (Score:3, Interesting)
you have five hats (two red, three black) and three people. you queue the people up in order of height and have them face the same way (this way the tallest person can see the two people in front of him/her, the middle person can see the shortest person, and the shortest person can't see anyone). you put a hat on each person's head and instruct them that they are not allowed to take the hat off or turn around. you then ask them to tell you what color their hat is. after a while, the person at the front of the line correctly announces the color of his/her hat. how did the person at the front of the line know and what were the other hat colors?
Light Bulb (Score:1, Interesting)
next to a door are 3 light switches. one of the switches are connected to an incandescant light bulb in the windowless room on the other side of the door. you may not open the door and flip the switches. you may only enter the room once. which switch lights up the room?
Solution [mooder.org]
Monty Hall (Score:2, Interesting)
Knight's reward (Score:2, Interesting)
Once upon a time, in a land far far away, there lived a knight who had
just rescued his first damsel in distress. The knight was called before
the king to receive a reward. The king told the knight that he had
written an amount of gold on a piece of paper and twice the amount of
gold on another piece of paper. He placed the two pieces of paper face
down in front of the knight, and told him he could chose either one.
The king would give the knight the amount of gold on the paper as a
reward. Or, the knight could opt to get the amount of gold on the other
paper instead.
This was the knight's first reward, so he had no idea what he was likely
to get. But the knight reasoned that no matter what amount he saw on the
paper he chose, he would take the other one because he had more to gain
than lose. For example, if the paper he chose had 16. He might win
another 16, and at worst he only loses 8!
The damsel points out that if the knight is going to end up with the amount
on the other piece of paper anyway, why not just choose it first and not
switch. His reward will be the same.
Is the damsel correct? Or is the knight's plan sound?
Refute the argument you disagree with. (Refuting the incorrect argument
is the challenge.)
you're given a globe (Score:3, Interesting)
One I like (Score:2, Interesting)
As you walk along a path, you come to a fork. In the fork are two men, of which you know little, except that they must have come from one of the villages on the other islands nearby.
There are three villages--the Marqetteres always lie, the panguons always tell the truth, and the Shie'ep always do what everybody else is doing.
You may ask one question to one of the men. What do you do?
Answer(ROT-13):
Fvzcyr. Lbh vtaber obgu zra, naq jnyx fgenvtug gb gur zvyr uvtu cyngrnh, juvpu jbhyq or ivfvoyr sebz nal cbvag ba n bar-nper qrfreg vfynaq.
Spacial geometry one (Score:2, Interesting)
Assume Earth is a perfect sphere.
Q1) Where can you stand such that if you go 1km North, then 1km East, then 1km South, you're back where you started?
A1 rot13'ed) gur fbhgu cbyr. pregnvayl abg gur abegu cbyr, nf lbh pna'g tb abegu sebz gurer. naq vs lbh fnvq 1xz fbhgu bs gur abegu cbyr v'q fnl ab gbb, nf lbh pna'g tb rnfg sebz gur abegu cbyr, bayl fbhgu.
Q2) OK smarty. Where ELSE can you do it from, on the Earth's surface? No tricks are involved either, just a bit of thinking.
A2) n ovg bire bar xz fbhgu bs gur abegu cbyr: nsgre jnyxvat gur 1xz abegu, n 1xz jnyx rnfg pbzcyrgryl pvepyrf gur abegu cbyr, zrnavat lbh'ir qbar n ebhaq gevc. 1xz fbhgu gura ergheaf lbh gb gur vavgvny cbfvgvba. n srj crbcyr pbzcynva nobhg guvf bar, nf lbh nera'g jnyxvat va n fgenvtug yvar, rira gubhtu lbh'er nyjnlf urnqvat rnfg. lbh pna erzvaq gurz gung gurl jrera'g tbvat va n fgenvtug yvar va n1 rvgure. naq nfx gurz gb qrsvar 'rnfg' vs gurl fgvyy nera'g unccl.
Q3) You really think you're good don't you? OK, I want to know where ELSE!
(read this when you think you have it, before you read the real answer: gur nafjre vf abg nabgure cbfvgvba ba gur rnegu'f fhesnpr qhr rnfg (be jrfg) bs gur nafjre gb d2. jryy vg vf, ohg vg'f abg tbbq rabhtu, gurer'f fbzrjurer ryfr.)
A3) guvf nafjre vf nyzbfg gur fnzr nf gur ynfg, ohg vafgrnq bs cynpvat lbhefrys fb gung gur bar xz rnfgreyl jnyx vf n pbzcyrgr ybbc, lbh'er rira pybfre gb gur abegu cbyr, naq znantr gjb ybbcf! be, sbe gung znggre, lbh pna zbir rira pybfre, naq nf lbh nccebnpu gur '1xz fbhgu' cbvag sebz gur abegu cbyr lbh jvyy svaq zber naq zber fbyhgvbaf.
Enjoy.
The Dilema (Score:2, Interesting)
Re:hats (Score:3, Interesting)
Knowing this, the second person looks at the hat in front of him. If he sees a red hat, he knows his is black. Since he does not see a red hat, his is either red or black, he doesn't know. But knowing this, the first guy can deduce that his hat is black.
I don't think you can know what the other colors were from this, but the first person can correctly deduce his hat color. Interesting puzzle. I've heard it before, but never took the time to analyze it.
Re:Soduku (Score:2, Interesting)
Regards,
Steve
To get past the lameness filter I had to encode the file. To decode it, copy the text into a file like "sudoku.endcoded", remove the spaces that slashdot inserts at character 51, run "uudecode sudoku.encoded", and then run "uncompress main.py". Ugh thats a lot of work for a damn file, but hey it works
begin 664 main.py.Z
M'YV0(T*\J#-'S@LQ:=R\@).'#IHW;A282<.F#(@>(-[`*>,&
M!1T\=$2D4-`F#!TY:?!<!+&EBP(Y;^[,K*E`P0@04LJ$(0,B
MS!LY+>FD@0@BC!NB<\K089I&:A@V(*+"Q"/Q*0@V"2T:G5C1
M10H="A(XE0-B#)HP<L*,H5.&KU&Y9?`F2)#&3-^_@0<7OHA1
M#)R-5U%LB<%"!HL9+&BPJ,'"!HL;+'"PR-%EY>(R:!W[!2R8
MAHT;.')@SJLY9^?/',F(3D@'Q>[(OE/4SBLVYG/0TC?;WKS3
MCD6'%KN3W0M"#!NK:YB"P!/8S9DR*.1P%P@C4(%&&NJI0=!6
M5&W0Q4)]][F1'X(@U.$&0660D1=]Y!G%GU7_!7@7<_2-P5X=
M8XY5U)%\6VS6Q18NL@&C&[51!D(,(-R8P!AUR/&6&W1\8<=7
MC#_"8--BB]&7AGXG^@>@C<R!^=,0%0$V(1H6;9;F8HT5!4((
ME%,*J19P/*;Q8Y!#=I'DG(L5>NB61+I51AMON&==DT\"2F49
M7Y`R6N>8>)(J9(Q57=47IG]*N:E^6.;THZ&+>MJH2V/U:.L6
MH)TN!JI0HL)YUAMC?(4AM/FA,&J9_ZD'F$5AG-45'16%]091
M?UN-:A^UR>IEEHMDT@A@"]9!6L(,*5R(+Z(Q[IO""OPJ*2]?
MOPWG)##!*^J*(P@@1'PG1C!DQ!>]J<*00JM\P@KEL572RFM,
M<"^[J*C!NY:+1\L@O)RPS$/V3&H7PE8*8+&Q!EKE2B3FI++.
FAVV(,9E1`+MQ(QPP00D"UEK+P0)S8%-W&1<I@7!V`FF++8((
`
end
Re:What do you get if you multiply 6 by 9? (Score:3, Interesting)
#define NINE 8+1
printf("TATLTUAE = %d, SIX * NINE);
Re:The King and the Chalice (only for Experts!) (Score:3, Interesting)
Obviously, the most devious thing that the king can do is to always make sure the chalice is rightside up. Therefore, it will always be rightside up. Therefore, the chalice can provide no information. Therefore, it is a red-herring.
With all other avenues of gathering information forbidden, there seems no information left to base an answer on.
Re:The King and the Chalice (only for Experts!) (Score:1, Interesting)
Am I close?
Comment removed (Score:3, Interesting)
Re:Truth vs. Lies (Score:2, Interesting)
Three Salesmen (Score:1, Interesting)
When the bell boy gives the cash to the night manager, the manager says "No, no, no no. This is not right - The room is only $25 dollars - not $30. Here are five $1 notes. Please give them back to the guests".
On the way back back, the bell boy thinks, "I have five dollars and three guests. I can't divide this evenly. So I'll just keep two dollars for myself."
The salesmen take their cash and turn in for the night.
So.......
Each Saleman has effectively paid nine dollars (ten dollars minus one returned).
The bell boy has two dollars in his back pocket.
$9 + $9 + $9 + $2 = $29.
Where has the last dollar gone???
Suck on that, my fellow brains-on-stilts.
Heh heh heh.
Re:Math and science are obsolete (Score:2, Interesting)
*sigh*
I'm trying to explain to you the practical consequences of what you're proposing. I was just describing aspects of reality. You'll notice I never said anything was good or bad, just what will or won't happen. If you can find the place where I was dismissive of the poor, I'd really like to know where it is.
Now, I already explained enough so you could understand the difference between cracking down on the rich vs. the poor. The rich can easily scurry away and/or stop producing. The poor can neither easily scurry away nor stop producing. Again, this is not to say anything is "good" or "bad", just that it "will" or "won't" raise tax revenues. Contrary to your staunch refusal to dispassionately analyze the topic, there really are relevant practical considerations in raising taxes.
In fact, I'd like nothing more than to test out your ideas. Check out the link in my sig. I submitted an idea to a policy site. The idea is that basically, in one state, we do what you propose: high taxes on the rich, high minimum wage, good workers protections and workplace safety requirements, etc. In the other state, do the oppose: no min. wage, low taxes on the rich, no safety requirements, etc. If you're really serious about your views, you'd leap at the chance to do this and see who's right based upon which state people flock to.
You do think you're right, right?
The BART Ticket Puzzle (Score:2, Interesting)
Re:Solution (Score:5, Interesting)
Disprufe(TM) by contradiction:
1. Suppose sqrt(2) ^ sqrt(2) ^ sqrt(2) ^
2. Then, sqrt(2) ^ (sqrt(2) ^ sqrt(2) ^
3. Hence, sqrt(2) ^ n = n.
4. Therefore, n obviously equals 4, because sqrt(2) ^ 4 = 4.
5. Hence, sqrt(2) ^ sqrt(2) ^ sqrt(2) ^
What's wrong with this logic?
That's a dumb response (Score:1, Interesting)
How is it that you have any solution more "real" than the solutions he just provided? Could it be that you haven't really thought through your riddle so well?
Re:Riddle (Score:3, Interesting)
Your married-with-two-kids co-worker invites you over to dinner. When you arrive a son of the coworker answers the door. What is the probability that the other child is a girl?
Followup:
The co-workers oldest child, a son, answers the door. What is the probability that the other child is a girl?
Most who have gone through a formal stats class have seen this one before, but it is always fun to try and wrap your head around it the first time.
Re:Ok, here's mine (Score:3, Interesting)
1
12
1112
3112
132112
1113122112
311311222112
what's the next line?
Re:One possible solution: (Score:5, Interesting)
I was once a judge at a "Phyics Olympics" where there was one puzzle in which students had to figure out the wiring if a circuit consisting of a couple of light bulbs and a couple of switches. They were "supposed" to solve the puzzle by flipping the switches, noting what lights were on and off, and inferring the circuit.
One team took the apparatus apart and inspected the wiring.
I gave 'em full marks.
The head judge went spare.
Science is not a game, and there aren't any rules according to which you are "supposed" to solve the problem. Alexander the Great was demonstrating the practice of experimental science when he unravelled the Gordian knot, and Feyrabend was onto something when he said, "Anything goes."
Puzzles set by humans have more to do with communication between the puzzle-setter and the puzzle-solver than anything else. Some people even decry computer-generated puzzles because of this--they say that the pleasure they get from solving puzzles comes from the feeling of interaction with another mind.
Re:Algebraic proof: 2=1 (Score:2, Interesting)
The defining property of the largest integer is that there is no larger integer. But if you take any integer and add one, you get a larger integer. So the largest integer must fulfill x = x + 1, and taking it one step farther for good measure, x = x + 2.
x = x + 2
Square both sides.
x^2 = x^2 + 4x + 4
Subtract x^2.
0 = 4x + 4
Subtract 4x.
-4x = 4
Divide by -4.
x = -1
Checking this result, -1 is indeed an integer and nothing is one more than -1. QED.
My all-time favorite logic puzzle (Score:5, Interesting)
But anyway, logic puzzles. This logic puzzle is excellent. I've had it up on my site (http://www.xkcd.com/blue_eyes.html [xkcd.com]), and after I got boingboing'ed I got a lot of email about it, so I've been able to tweak the wording to get rid of most of the confusing stuff, leaving only the logic. It's extremely subtle; I've never seen anything like it.
Here's the puzzle:
A group of people live on an island. They are all perfect logicians -- if a conclusion can be logically deduced, they will do it instantly. No one knows the color of their eyes. Every night at midnight, a ferry stops at the island. If anyone has figured out the color of their own eyes, they [must] leave the island that midnight.
On this island live 100 blue-eyed people, 100 brown-eyed people, and the Guru. The Guru has green eyes, and does not know her own eye color either. Everyone on the island knows the rules and is constantly aware of everyone else's eye color, and keeps a constant count of the total number of each (excluding themselves). However, they cannot otherwise communicate. So any given blue-eyed person can see 100 people with brown eyes and 99 people with blue eyes, but that does not tell them their own eye color; it could be 101 brown and 99 blue. Or 100 brown, 99 blue, and the one could have red eyes.
The Guru speaks only once (let's say at noon), on one day in all their endless years on the island. Standing before the islanders, she says the following:
"I can see someone with blue eyes."
Who leaves the island, and on what night?
There are no mirrors or reflecting surfaces, nothing dumb, It is not a trick question, and the answer is logical. It doesn't depend on tricky wording, and it doesn't involve people doing something silly like creating a sign language or doing genetics. The Guru is not making eye contact with anyone in particular; she's simply saying "I count at least one blue-eyed person on this island who isn't me."
And lastly, the answer is not "no one leaves."
Re:Math and science are obsolete (Score:1, Interesting)
Low taxes, no expensive workers protection, no minimum wage will move business to the 2nd coutry.
Business in the first country will not be able to compete with business from the 2nd country. Since workers protection
is good they will be afraid to hire people since it will be costly to fire them.
This will lead to much higher unemployment in the first country.
Now the answer depends on the unemployment benefits. If they are low - people will flock to the 2nd country faster.
If they are high - it will create another drain on the 1st country resources. In 100 years it will be significantly poorer than
2nd country and people will flock to the 2nd country then.
It is already happenning: Europeans moving to the US, I even know a few Europeans who moved to China.
Just compare France and the US.
My fav math puzzle is: (Score:1, Interesting)
There are two mathematicians in a room. The product of two integers >=2 is given to the first mathematician, and the sum of the same two integers is given to the second one. Hence, the first mathematician only knows the product and the second only knows the sum of the two integers. However, both are aware that the first knows the product and the second knows the sum of the two integers.
The first mathematician is asked whether he can determine the two numbers, and he answers no.
The second mathematician is then asked whether he can determine the two numbers, and he too answers no.
The first mathematician is then asked once again whether he can determine the two numbers and this time the answer is yes!
What are these two numbers?
Re:Petals of the Rose (Score:2, Interesting)
1. Bears appear only in pairs.
2. Bears live in holes. No holes - no bear.
That's all the information given. After the dice are rolled, you have to tell the number of holes and the number of bears. To make it harder, he did not always roll five dice, but varied from 2 to 5 dice. We tried like crazy for 45 minutes without getting a solution. Note that in this variation of the puzzle, the name does not give away the solution.
It impressed me so much, that I haven't forgotten this lesson even after being out of school for more than 20 years.
Take a Break - 8 Ways to 15 (Score:3, Interesting)
He wasn't really using brute force. (Score:3, Interesting)
Anyway I read that story and it didn't appear to me that he was trying to solve it by memorization, but rather that after an hour, seeing hundreds of rolls, he remembered many of them, which isn't all that surprising. What I got out of the story, is that he persistently kept at the problem trying many different ideas until he finally got it, even after everyone else in the group had solved it or quit.
I don't think that how quickly someone solves any one particular problem is much of an indicator of how smart they are. We were doing brain teasers on an ACM trip my freshman year of college, and I was one of the first to get most of them. Then there was this one that everyone got right away and I couldn't get it. When I finally did a day later I was kicking myself because it was so obvious - I just wasn't looking at it the right way.
This story showed my that Bill Gates is a very persistent and determined person which is probably a big reason that he was so successful. That and he needed to get a girlfriend at that point, as do I apparently
All horses are the same color (by induction) (Score:1, Interesting)
An excellent collection of puzzles (Score:1, Interesting)
For example:
Bigger or Smaller: Alice chooses two distinct real numbers between 0 and 1, writes them onto two chits of papers and places the chits in a jar. Bob gets to select one of the chits randomly and open it. He then has to declare whether the number he sees is the bigger or smaller of the two. Is there any way he can be correct more than half the times Alice plays this game with him?
f(f(x)) == -x? Is it possible to write a function int f(int x) in C that satisfies f(f(x)) == -x? Without globals and static variables? Is it possible to construct a function f mapping rationals to rationals such that f(f(x)) = 1/x?
30 Coins: 30 coins of arbitrary denominations are laid out in a row. Ram and Maya alternately pick one of the two coins at the ends of the row. Could Maya ever collect more money than Ram?
Excellent book of CS/Math puzzles by Peter Winkler (Score:1, Interesting)
Re:My all-time favorite logic puzzle (Score:3, Interesting)
I think this stipulation is also necessary:
1) That everyone with blue eyes (at least) is wholly involved in figuring out if they have blue eyes and should comply (bear with me, this is different than you think)
Without this specification, there can be no implicit communication as to the understanding of others.
But to be fair, this is hardly the end of the specifications, and is why I so detest logic puzzles. An earlier poster had it right when they said that a logic puzzle is hardly about logic, but about communication between the puzzle maker and tester.
I think there is an issue with your stipulation about the islanders not "otherwise" communicating with each other because I think communication other than the exact count of islanders is exactly what happens.
To rectify this I could, for instance, instead demand there be such stipulations as:
2) The islanders don't tell each other what their eye colors are
3) Or otherwise sign
and because we're tired and want to stop this progression from escaping us we'll try a catch all...
4) or intentionally let each others know
Perhaps you might say someone demanding that there could be such a "outside mode of communication" isn't "playing along", but this is precisely what I mean. There must be a communication of what the solution might be for a guesser to play along with what might get him there. I would like to especially point out that one can get into the same trouble with rule 4 as you did with your stipulation (and for the same reason, and that this is unavoidable). I would say that an islander following stipulation 1 is implicitly breaking this or any such rule.
But we're hardly done yet... I want to examine rule 1 a little closer and to do that I need to outline the "solution".
1 blue eyed islander:
In the case of the 1 blue eyed islander, he sees that everyone else has brown eyes (and the guru green) so knows he must have blue eyes. He leaves. The others do not leave because they see him leave.
2 blue eyed islanders:
The two blue eyed islanders see that there is another blue eyed islander and so don't leave on the first day. They then both leave on the second day, knowing that the other must have not left because they expected themselves to leave the first day*. No one else leaves, because they knew there were at least two others, and so waited for the third night**.
And so on for n blue eyed islanders.
So, first onto the problem with condition 1), and then onto those little stars.
Condition 1) at first seems as if it is a simple restatement of the condition that islanders know the count of other's people eye color at all times. It is not. In addition, it also signifies that every islander (or at least ever islander with blue eyes) must be *carrying out* the logical processes and understand its implications.
But then, for any islander to have any certainty of what the other islanders know, he must also have the guarantee that every other islander is carrying out these processes. This leads us to:
5) Every islander knows that every (blue eyed) islander is wholly involved in deciding if they have blue eyes.
It's actually a bit more involved and also involves knowing all other stipulations are also in effect, but I would like to keep this somewhat comprehensible.
And now that we realize this is a stipulation for the valid reasoning of an islander, an islander also needs to be assured of 5). Thus:
6) Every islander knows that every (blue eyed) islander knows that every (blue eyed) islander is wholly involved in deciding if they have blue eyes.
And so on. It is only important that every blue eyed islander has correctly proceeded for one to decide how to proceed.
Those stars:
*Conveniently, this directly extends from the previous statement, so it should flow nicely. For an islander with blue eyes to know that the others know he has blue eyes, he must assume that the islanders "correctly proceeded". Thi