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Stirlings approx/CoinFlips/Gamma function

  1. Feb 17, 2014 #1
    1. The problem statement, all variables and given/known data
    Hi I actually have three questions that I am posting here, help in all of them would be greatly appreciated!

    1) Prove that ln(n!) ≈ nln(n)-n+ln(2*pi*n)/2 for large n

    2)Supposed you flip 1000 coins, what is the probability of getting exactly 500 heads

    3) Show that n! = [itex]\int x^{n}exp(-x)dx[/itex] where n is an integrer and the injtegral is from 0 to infinity. (The gamma function extends this definition of factorial to include non-integrers, z).

    2. Relevant equations


    3. The attempt at a solution

    1) I am pretty lost here. This is a third year thermodynamics course (not been heavily math based so far) so the solution shouldn't be something above that level. I don't even know where to start.

    2) This one I thought would be really easy (and it should be). This is what I did.

    I started off by ignoring the last term of stirlings approximation (this is what the textbook has and my prof did). the probability is

    P[itex]_{N}[/itex](n) = [itex]\frac{N!}{(N-n)!n!}[/itex]*2[itex]^{-N}[/itex]
    by using stirlings approximation, (first taking the natural log of both sides) and simplifying I got (and my professor)

    P = exp(NlnN - (N-n)ln(N-n)-nln(n)-Nln(2))

    but when I plug in 1000 for N and 500 for n I keep getting 1 which I don't think is right. What am I doing wrong?

    3)I tried to do an integration by parts on the right side but it leads to some undefined things (i.e. 0*infinity) so I am lost yet again. I thought that was just a definition, how do I show that?

  2. jcsd
  3. Feb 17, 2014 #2
    Please ignore number two, it required that the terms left off to be added. Still need help with 1 and 3
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