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Question regarding Power Series

  1. May 21, 2015 #1
    1. The problem statement, all variables and given/known data
    It is stated in my textbook that the sum ## \sum_{0}^{\infty} 8^{-n}(x^2-1)^n ## is not a power series but can be turned into one using he substitution ##y=x^2-1## which then becomes the power series ##\sum_{0}^{\infty} 8^{-n}y^n ## They aren't offering any explanation as to why and I have evaluated the interval on which the series converges and it gives the same result regardless of whether or not the substitution is used. I guess what I'm wondering is why isn't the first sum a power series?

    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. May 21, 2015 #2
    The x2 inside the parenthesis makes it not a power series. A power series must have x1 inside the parenthesis.
     
  4. May 21, 2015 #3
    Okay thanks for the information.
     
  5. May 22, 2015 #4

    Orodruin

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    Well, technically it is a power series in ##x^2-1##.
     
  6. May 22, 2015 #5

    Ray Vickson

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    No. A series of the form ##\sum c_n x^{2n}## IS a power series. The reason the series ##\sum_n 8^{-n}(x^2-1)^n## is not a power series (in ##x##) is that it takes powers of a multi-term polynomial of ##x##, rather than of a monomial in ##x##. That is, the function ##x^2-1## is not a monomial.
     
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