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Power Series

  1. Mar 29, 2015 #1

    I've been solving power series problems where the question asks to find 2 power series solutions.

    I can solve it almost all, I can find the recurrence solution... however while checking the solutions, I see some answer solutions they used c0=1 and c1=0 to find the 2 solutions, and some answer solutions they did not use that.

    am I missing something? I don't even understand why they used c0=1 c1=0, and when should i use or not.
  2. jcsd
  3. Mar 29, 2015 #2


    Staff: Mentor

    Could it have something to do with the function being even or odd?
  4. Mar 30, 2015 #3


    User Avatar
    Science Advisor

    It's not very clear what question you are asking! I think you are talking about a second order linear differential equation and you should understand that such equations will have two independent solutions such that all solutions can be written as a linear combination of those two.

    In particular, given second order linear differential equation p(x)y''+ q(x)y'+ r(x)y= 0. In order to get a specific solution you would have to be give two additional conditions and the simplest would be the two initial value conditions, y(0)= 0, y'(0)= 1 and y(0)= 1, y'(0)= 0.

    Of course, if we write the solutions in terms of a power series, [itex]y(x)= a_0+ a_1x+ a_2x^2+ \cdot\cdot\cdot[/itex] then [itex]y'(x)= a_1+ 2a_2x+ 3a_3x^2+ \cdot\cdot\cdot[/itex] so that [itex]y(0)= a_0[/itex] and [itex]y'(0)= a_1[/itex]. So the simplest way to find two independent solutions, as power series, is to require that, in one, [itex]a_0= 1[/itex] and [itex]a_1= 0[/itex] while in the other [itex]a_0= 0[/itex] and [itex]a_1= 1[/itex].
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