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Homework Help: Power series expansion of a function of x

  1. Feb 19, 2009 #1
    1. The problem statement, all variables and given/known data

    [Directions to problem]
    Show that the function of x gives a power series expansion on some interval centered at the origin. Find the expansion and give its interval of validity.

    [tex] \int_0^x e^{-t^2} dt [/tex]

    2. Relevant equations

    3. The attempt at a solution

    I have that, [tex]e^{-t^2} = \sum_0^{\infty} \frac{(-1)^n(t^2)^n}{n!} [/tex]

    I now am wondering whether I can take the integral of this series as follows,

    [tex]\int_0^x \sum_0^{\infty} \frac{(-1)^n(t^2)^n}{n!} dt = \sum_0^{\infty} \frac{(-1)^n(x^{2n+1})}{n!(2n+1)} [/tex]

    Am I allowed to do that and if so, what is the justication?


    I performed the ratio test on the result and the limit as n approached 0 was 0, and I therefore concluded that the series converges for all x in R.
  2. jcsd
  3. Feb 19, 2009 #2


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    Science Advisor

    I think you mean "prove the function has a power series" since the integration itself does not directly "give" the power series.

    Yes, as long as you are inside the radius of convergence, you can integrate a power series term by term.
  4. Feb 19, 2009 #3


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    Gold Member

    Also the series expansion of [tex]e^{x}[/tex] is valid for all x
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