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Homework Help: Proving stirlings formula

  1. Nov 6, 2012 #1
    1. The problem statement, all variables and given/known data

    Prove that

    lim[itex]_{n \rightarrow ∞}[/itex] [itex]\frac{n! e^{n}}{n^{n+\frac{1}{2}}}[/itex] = [itex]\sqrt{2π}[/itex]

    2. Relevant equations

    3. The attempt at a solution
    Alright so for this problem I noticed it looked kind of similar to the integral formula for a normal distribution from statistics with the 1/[itex]\sqrt{2π}[/itex] in it, but I'm not really sure what to do. I imagine there's a sine and cosine somewhere in there but I'm not exactly sure how to bring it in, maybe via taylor polynomials through the terms given?

    I've yet no definitive solution but I've got some basic outlines of ideas.. I'm right now in Calc 2 so i expect there to be series and sequences involved..
  2. jcsd
  3. Nov 6, 2012 #2

    Ray Vickson

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    Google is your friend.

  4. Nov 6, 2012 #3
    Thanks but it doesn't really give me a decent method I can follow. Some of the methods make little sense to me
  5. Nov 6, 2012 #4


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    My first question is this, how rigourous do you want this to be? It can be very long and in depth proof or it can be a paragraph. If this is homework, I'm assuming the short proof without a lot of detail is preferred?

    First, step, is look at the log(n!). Transform this into something useful and think about if it's a decreasing and increasing function. From there determine an inequality that is always true.
    Last edited: Nov 6, 2012
  6. Nov 7, 2012 #5

    Ray Vickson

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    There are several web pages that present several approaches. If you don't like one of them , go to another. All of them require some calculus and some hard work.

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