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Frobenius method indical roots differing by a postive integer

  1. Sep 21, 2010 #1
    1. The problem statement, all variables and given/known data

    For what values of the constant K does the differential equation
    y"-(1/4 +K/x)y=0 (0<X< infinity)

    have a nontrival solution vanishing at x=0 and x= infinity ?

    2. Relevant equations

    Hints that were suggested for my prof were:
    For large x assume K/x is small and drop this term to obtain and approximate solution valid for large X. One solution found will be singular at infinity (cast out the solution with the singularity at infinity. Obtain a 2nd solution using reduction of order and then solve the resulting equation using a power series. One series for the larger root will yield a series that can be truncated for certain values of K

    3. The attempt at a solution

    I followed his hints and assumed X was larger and solved the resulting 2nd order ode with constant coffeicents. I obtained

    y= C1e^-x/2 and C2 e^x/2


    I then cast out the solution e^x/2 and used the reduction or order method
    y=v(x)P(x) where P(x) = e^-x/2
    after finding y' y'' I plugged them into the original 2nd order ode and after performing all the algebraic manipulation I ended up with a new 2nd order ode

    v"-v'-(K/X)v=0

    I then used the frobenius method to solve this diff. eq.

    I ended up with indical roots of s=0; s=1

    And the following recursion relationships

    S=0: Cn+1=(n+k)/(n+1)(n) Cn for n=0,1,2....

    S=1: Cn+1= (n+1+k)/(n+2)(n+1) Cn for n=0,1,2....

    for the S=0 relationship, when I plug in n=0 I am dividing by zero.

    I have read in other resources that their is a problem at occurs when your indical roots differ by ... I want to say postive Interger and I realize that the frobenious method only guaranties one solution, but I am not what the next step is in order to find the 2nd solution.



    *(This is my first time posting so on a side note if anyone that is familiar could provide me with a reference on using the latex equation editor it would be greatly appreciated)
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
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