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#1 2012-05-20 21:30:00
- Agnishom
- Super Member
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The Wrong Hats Problem
It has been many days since I last posted. Feeling 
to post again
Anyway,
There had been eight men and eight hats. They were so much drunk that they could not recognise their own hats. So they picked them up in a random manner. Now what is the chance that no man will choose his own hat?
Of Course the first step is to find the factorial of 8 which is equal to the number of ways they can pick up the hats in. But it is such a large number that it would take years to write all of them down?
Can You give me the answer????
I know the answer but I dont know how to derive it. So a help would be nice
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#2 2012-05-20 21:41:21
- bobbym
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Re: The Wrong Hats Problem
Hi;
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#3 2012-05-20 21:47:48
- Agnishom
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Re: The Wrong Hats Problem
How do you get it?
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#4 2012-05-20 21:49:32
- anonimnystefy
- Real Member
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Re: The Wrong Hats Problem
If I am correct,than this is the derangement problem for 8 elements. I think you can find it on Wikipedia.
The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“A secret's worth depends on the people from whom it must be kept.” ― Carlos Ruiz Zafón
#5 2012-05-20 21:51:26
- bobbym
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Re: The Wrong Hats Problem
Yes, this is a derangement problem. You can first use the approximation of
There are many formulas to generate the number of derangements.
http://en.wikipedia.org/wiki/Derangement
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#6 2012-05-20 21:58:22
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
Thnks I will read the article but will you explain what is approximation of 1/e?
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#7 2012-05-20 21:59:19
- bobbym
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Re: The Wrong Hats Problem
As n approaches infinity the probability of a derangement approaches 1 / e.
http://en.wikipedia.org/wiki/Derangement
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#8 2012-05-20 22:10:15
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
http://upload.wikimedia.org/wikipedia/en/math/3/f/6/3f619a61786028a59a524136391f52f0.png
How do you prove the above?
Please explain step by step
Can't Understand
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#9 2012-05-20 22:17:02
- bobbym
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Re: The Wrong Hats Problem
Hi;
I am sorry but I can not see that picture clearly.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#10 2012-05-20 22:20:24
- anonimnystefy
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Re: The Wrong Hats Problem
Hi bobbym
It says !n=(n-1)*(!(n-1)+!(n-2))
Last edited by anonimnystefy (2012-05-20 22:20:43)
The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“A secret's worth depends on the people from whom it must be kept.” ― Carlos Ruiz Zafón
#11 2012-05-20 22:21:22
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
The formula is:
!n=(n-1) (!(n-1) + !(n-2))
Last edited by Agnishom (2012-05-20 22:21:58)
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#12 2012-05-20 22:25:28
- bobbym
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Re: The Wrong Hats Problem
Hi Agnishom;
That is the recurrence relation for the number of derangements. Did you read this:
http://en.wikipedia.org/wiki/Derangement
About half way down the page they will describe the process. It is difficult.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#13 2012-05-20 22:34:12
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
They wrote:
Suppose that there are n persons numbered 1, 2, ..., n. Let there be n hats also numbered 1, 2, ..., n. We have to find the number of ways in which no one gets the hat having same number as his/her number. Let us assume that first person takes the hat i. There are n − 1 ways for the first person to choose the number i. Now there are 2 options:
1. Person i does not take the hat 1. This case is equivalent to solving the problem with n − 1 persons n − 1 hats: each of the remaining n − 1 people has precisely 1 forbidden choice from among the remaining n − 1 hats (i's forbidden choice is hat 1).
2. Person i takes the hat of 1. Now the problem reduces to n − 2 persons and n − 2 hats.
From this, the following relation is derived:
!n = (n - 1) (!(n-1) + !(n-2)).\,
Now would you please explain point 1 and 2
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#14 2012-05-20 22:38:55
- bobbym
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Re: The Wrong Hats Problem
Hi Agnishom;
That is a standard book way of getting a recurrence form. I know it as East-West analysis from a famous book written by the late Herbert S. Wilf. Though I have read it many times I am unable to do it or even explain it. I am sorry about that, wish I could do more here.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#15 2012-05-20 22:46:04
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
Dear Bobbym,
Do you own a copy of Amusement in Mathematics by Henry. E. Dudeney?
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#16 2012-05-20 22:49:05
- bobbym
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Re: The Wrong Hats Problem
Hi;
Yes.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#17 2012-06-25 12:32:09
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
Actually I don't understand these two lines:
1. Person i does not take the hat 1. This case is equivalent to solving the problem with n − 1 persons n − 1 hats: each of the remaining n − 1 people has precisely 1 forbidden choice from among the remaining n − 1 hats (i's forbidden choice is hat 1).
2. Person i takes the hat of 1. Now the problem reduces to n − 2 persons and n − 2 hats.
and why hat 1? Why dont we extend the list with Hat 2 and Hat 3 and forever???
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#18 2012-06-25 12:46:30
- anonimnystefy
- Real Member
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Re: The Wrong Hats Problem
You could've said that we start from Person 2 for the casework, but that wouldn't change much except a few indexes. The point of that casework is to see what are the values weget for each case that exists.
The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“A secret's worth depends on the people from whom it must be kept.” ― Carlos Ruiz Zafón
#19 2012-06-28 14:43:01
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
Actually I still don't get it
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#20 2012-07-05 19:20:23
- biffboy
- Novice
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Re: The Wrong Hats Problem
Agnishom wrote:
It has been many days since I last posted. Feeling
Anyway,
There had been eight men and eight hats. They were so much drunk that they could not recognise their own hats. So they picked them up in a random manner. Now what is the chance that no man will choose his own hat?
Of Course the first step is to find the factorial of 8 which is equal to the number of ways they can pick up the hats in. But it is such a large number that it would take years to write all of them down?
Can You give me the answer????
I know the answer but I dont know how to derive it. So a help would be nice
I would have thought the answer was simply 7/8*6/7*5/6*4/5*.3/4*2/3*1/2*1/1=3/4
#21 2012-07-05 20:36:48
- bobbym
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Re: The Wrong Hats Problem
Hi biffboy;
Welcome to the forum! Please check the link in post #12.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.
#22 2012-07-10 11:48:55
- Agnishom
- Super Member
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Re: The Wrong Hats Problem
Hi biffboy
Why do you calculate that way?
Pls Explain
'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'The whole person changes, why can't a habit?' -65
#23 2012-07-10 12:07:18
- anonimnystefy
- Real Member
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Re: The Wrong Hats Problem
Hi Agnishom
biffboy's solution isn't correct.
The limit operator is just an excuse for doing something you know you can't.
“It's the subject that nobody knows anything about that we can all talk about!” ― Richard Feynman
“A secret's worth depends on the people from whom it must be kept.” ― Carlos Ruiz Zafón
#24 2012-07-10 18:48:00
- bobbym
- Administrator
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Re: The Wrong Hats Problem
Hi Agnishom;
The calculating part is a derangement problem. biffboy's solution is not correct.
In mathematics, you don't understand things. You just get used to them.
Probability is the most important concept in modern science, especially as nobody has the slightest notion what it means.
90% of mathematicians do not understand 90% of currently published mathematics.