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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

Flip a coin 2N times, where N is large. Let P(x) be the probability of obtaining

exactly N + x heads. Show that P(x) = e^((-x^2)/N) divides by sqrt of pi times N

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

Hi;

Is this what you want

to prove?

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,954

I don't think that would be correct, then.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

I do not think so either.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,954

It does seem to work without the minus sign, though.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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How do you do that?

'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'

'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

Hi;

Do what?

The answer is

that I can prove.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,954

That is what the original problem is asking for. But I cannot get it. I am using the limit definition and Stirling's formula.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

Post 7 is what I need proven/

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

Can you prove it then?

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,954

Hi Shivamcoder3013

Have you tried taking the limit as N goes to infinity of the ratio of the exact answer and the approximate one and proving it equals 1?

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

Hi;

The paper I am looking at "Gaussian and Coins."

Using Stirlings:

Notice the approximately equal sign that is because you are approximated a discrete distribution ( binomial ) with the Normal distribution.

1) is an approximation for 2) which the above steps prove. Even for large N it is still an approximation. When N approaches infinity 1) = 2).

To prove that you might need the limit but maybe since Stirlings formula is asymptotic to the factorial it might be implied in step 3.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

I am not getting how you get that...

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

Hi Shivamcoder3013;

I am not getting much of the derivation either. It is a lot of algebra and undoubtedly was done with the help of a package. I put it down so you would have something.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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**ShivamS****Member**- Registered: 2011-02-07
- Posts: 3,646

Ok, I will try to think about it a bit. Thanks a lot.

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**anonimnystefy****Real Member**- From: Harlan's World
- Registered: 2011-05-23
- Posts: 15,954

Hi bobbym

Have you tried getting the limit of the ratio of the two expressions (the exact one and the approximate one)? It does not approach 1.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

The knowledge of some things as a function of age is a delta function.

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**bobbym****Administrator**- From: Bumpkinland
- Registered: 2009-04-12
- Posts: 98,081

yields 0.001079819330263761

The exact answer is:

yields 0.0010798643294

Seems pretty good. Try for larger n with x small in comparison to convince yourself numerically.

anonimnystefy wrote:

Have you tried getting the limit of the ratio of the two expressions (the exact one and the approximate one)? It does not approach 1.

I think the limit is 1.

According to M that is true. Why do you think the limit is not 1?

bobbym wrote:

To prove that you might need the limit but maybe since Stirlings formula is asymptotic to the factorial it might be implied in step 3.

Stirlings is an asymptotic form for the factorial. The limit of the ratio of Stirlings and the factorial is 1. The fact that he use Stirlings in his proof guarantees the above limit.

**In mathematics, you don't understand things. You just get used to them.**

**If it ain't broke, fix it until it is.**

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