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Homework Help: Find the series solution,power series

  1. Jul 3, 2011 #1
    1. The problem statement, all variables and given/known data
    2xy''-x(x-1)y'-y=0 about x=0
    what are the roots of the indicial equation and for the roots find the recurrence relation that defines the the coef an

    2. Relevant equations
    2xy''-x(x-1)y'-y=0 about x=0

    assuming the solution has the form y=[itex]\Sigma[/itex]anxn+r
    y'=[itex]\Sigma[/itex](n+r)anxn+r-1
    y''=[itex]\Sigma[/itex](n+r)(n+r-1)anxn+r-2

    3. The attempt at a solution
    after plugging into the solution I get
    2[itex]\Sigma[/itex](n+r)(n+r-1)anxn+r-1-[itex]\Sigma[/itex](n+r)anxn+r+1-[itex]\Sigma[/itex](n+r)anxn+r-1-[itex]\Sigma[/itex]anxn+r
    then I attempt to make all the x's the same and and make the sigma's equal so after doing that I get
    2[itex]\Sigma[/itex](n+r+1)(n+r)an+1xn+r-[itex]\Sigma[/itex](n+r-1)an-1xn+r-[itex]\Sigma[/itex](n+r+1)an+1xn+r-[itex]\Sigma[/itex]anxn+r
    I know that I need to replace the 0 under the sigma's with a (-1) on terms 1,3 but term 2 is whats throwing me off any help would be great
     
  2. jcsd
  3. Jul 4, 2011 #2

    vela

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    The third sum should be positive. Explicitly writing in the limits, you have
    [tex]2\sum_{n=0}^\infty (n+r)(n+r-1)a_n x^{n+r-1}
    -\sum_{n=0}^\infty (n+r) a_n x^{n+r+1}
    +\sum_{n=0}^\infty (n+r)a_n x^{n+r-1}
    -\sum_{n=0}^\infty a_n x^{n+r} = 0[/tex]
    As you noted, only the first and third sums give you a [itex]x^{r-1}[/itex] term, and the second sum doesn't give you an [itex]x^r[/itex] term. Separating those terms out, you have
    \begin{eqnarray*}
    &&[2r(r-1) + r]a_0x^{r-1} + \\
    &&[2(r+1)r a_1 + (r+1)a_1 - a_0]x^r + \\
    &&2\sum_{n=1}^\infty (n+r+1)(n+r)a_{n+1}x^{n+r}
    -\sum_{n=1}^\infty (n+r-1)a_{n-1}x^{n+r}
    +\sum_{n=1}^\infty (n+r+1)a_{n+1}x^{n+r}
    -\sum_{n=1}^\infty a_n x^{n+r} = 0
    \end{eqnarray*}
    By assumption, a0 isn't equal to 0, so you must have 2r(r-1)+r=0. That's your indicial equation.
     
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