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Help with infinite sum.

  1. Nov 14, 2011 #1
    Could someone suggest how I could go about calculating this infinite sum?

    [tex]\sum_{n=1}^{\infty}n^{2}x^{n}[/tex]

    I'm having some trouble as this series does not have a constant ratio, difference or matches any standard series I can think of at the moment.

    Thanks
     
  2. jcsd
  3. Nov 14, 2011 #2

    Dick

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    You derive something like that by looking at derivatives of the geometric series. You can sum x^n, n=0 to infinity, right? Take a derivative of both sides. What's sum n*x^n, k=0 to infinity? Now take another derivative.
     
  4. Nov 14, 2011 #3
    So, take the derivative, do the sum then take the new derivative?

    So the first sum would be the derivative of [itex] nx^{n-1} [/itex]

    Or just look at the what derivatives I can solve that give me [itex]n^{2}x^{n}[/itex]?
     
  5. Nov 14, 2011 #4

    Dick

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    So did you get the sum of n*x^(n-1)? What is it?
     
    Last edited: Nov 14, 2011
  6. Nov 14, 2011 #5
    It's the sum of increasing derivatives of [itex]x^{n}[/itex].

    i.e. [tex] \frac{d1}{dx}+\frac{dx}{dx}+\frac{d^{2}x^{2}}{dx^{2}} ...\frac{d^{n}x^{n}}{dx^{n}} [/tex]
     
  7. Nov 14, 2011 #6

    Dick

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    No, that's not what I mean and it's not right. sum x^n=1/(1-x), right? Differentiate BOTH sides of the equation.
     
  8. Nov 14, 2011 #7
    As so:

    [tex] \frac{d}{dx}\sum{x^{n}}=\sum \frac{d}{dx}x^n = \frac{d}{dx} \frac{1}{1-x} = \frac{1}{(1-x)^{2}} [/tex]
     
  9. Nov 14, 2011 #8

    Dick

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    Ok, so sum n=0 to infinity n*x^(n-1)=1/(1-x)^2. Right? Now take a second derivative.
     
  10. Nov 14, 2011 #9
    You mean

    [tex] \sum n(n-1)x^{n-2} = \frac{-2}{(1-x)^{3}} [/tex]

    Then do I remove the extra factor of x^{n-2} by adding on both sides to get my final answer?
     
  11. Nov 15, 2011 #10

    Dick

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    You've got a sign error in that derivative. If you know sum n(n-1)x^(n-2) then you can find sum n(n-1)x^n, right? They differ by a factor of x^2. Now n(n-1)*x^n=n^2*x^n-n*x^n. That's the general strategy. You'll need to pay some attention to the details of the limits to get the right answer.
     
  12. Nov 15, 2011 #11
    I think I get the idea, but I'm getting something messy out.

    [tex] \frac{2x^{2}}{(1-x)^{3}}+\frac{1}{(1-x)} [/tex]

    To take care of the fact that the sum is between 1 and infinity not 0 and infinity can I just subtract the n=0 term from my calculated sum?
     
  13. Nov 15, 2011 #12

    Ray Vickson

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    Well, just look at the n=0 term in sum n*x^(n-1), etc. You really can answer your questions for yourself.

    RGV
     
  14. Nov 15, 2011 #13
    Yes. I realise that came out a bit more stupid than I intended it to. My apologies for not being specific enough.


    However, I was mostly thinking in general terms. I have a few more sums to work out, and I hope to use this strategy in more situations. To subtract the n=0 term (should it be non-zero) from a the result of the standard infinite to get the sum between n=1 and n=infinite makes sense to me, but I've often been wrong about these things before.:smile:
     
  15. Nov 15, 2011 #14

    Dick

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    You are getting pretty close. sum n^2*x^n-sum n*x^n=2x^2/(1-x)^3, right? But I think you've got the sum n*x^n part wrong. Can you go back and fix that?
     
  16. Nov 15, 2011 #15

    Ray Vickson

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    If you think you will be having trouble at small n, just write out the first few terms explicitly; for example, [itex]S =\sum_{n=0}^\infty x^n = 1 + x + x^2 + \sum_{n=3}^\infty x^n. [/itex] Now it is perfectly clear what the first few terms of [itex] dS/dx \mbox{ and } d^2S/dx^2 [/itex] are.

    RGV
     
  17. Nov 15, 2011 #16
    Sorry, I realise now I wrote down my sum wrong on my paper.

    Using
    [tex] \sum nx^{n-1} = \frac{1}{(1-x)^{2}} [/tex]

    [tex] \sum nx^{n}= \frac{x}{(1-x)^{2}}[/tex]

    Thus my answer is:

    [tex]\frac{2x^{2}}{(1-x)^{3}}+\frac{x^{3}}{(1-x)^{2}} [/tex]
     
    Last edited: Nov 15, 2011
  18. Nov 15, 2011 #17

    Dick

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    Closer and closer. Now you've got sum n*x^n ok. But you don't really mean x^3, do you? Keep checking.
     
  19. Nov 15, 2011 #18
    No, the x^2s cancel. This is becoming a vivid reminder of how I shouldn't try and rush maths in my head and should sit down with a pencil and shut up :)
     
  20. Nov 15, 2011 #19

    Dick

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    Maybe, so what's your final answer? I'm dying to know.
     
  21. Nov 16, 2011 #20
    [tex]\frac{2x^{2}}{(1-x)^{3}}+\frac{x}{(1-x)^{2}} [/tex]

    Which can be simplified to:

    [tex] \frac{x(1+x)}{(1-x)^{3}}[/tex]

    Which makes sense in the problem I'm doing, and maple agrees :)
     
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