Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Eigenfunction equation

  1. Dec 18, 2006 #1
    I've been going round in circles with this problem for days:

    Find the eigenvalues and associated normalised eigenfunctions of the operator L:

    [tex]L_y = x^2 y'' + 2 xy' + \frac{y}{4}[/tex]

    Boundary conditions [tex]y(1)=y(e)=0[/tex]

    So what I've done:
    substitute [tex]x = \exp(t)[/tex]

    Then [tex]L_y = \frac{d^2y}{dt^2} + \frac{dy}{dt} + \frac{y}{4}[/tex] where the differentials are now y wrt t

    The eigenvalue equation is [tex]L_y = X_n y[/tex] where [tex]X_n[/tex] are my eigenvalues

    Then I solve [tex]y'' + y' + (1/4 - X_n)y = 0[/tex]

    and get [tex]y = \exp (-1/2 t) (A \exp(+\sqrt(X_n)t) + B \exp(-\sqrt(X_n)t)[/tex]

    Applying the boundary problems I find B = - A

    and that [tex]X_n == 0[/tex]. Which doesn't seem right!

    Am I on the right track here or have I missed the point totally!
    Last edited: Dec 18, 2006
  2. jcsd
  3. Dec 20, 2006 #2
    Can anyone help? Please say if i need to give more info on the problem
  4. Dec 21, 2006 #3


    User Avatar
    Science Advisor

    And your boundary conditions are now y(0)= y(1)= 0.

    You're doing fine except that you are ASSUMING that Xn is not negative (since you are taking its square root). What happens if Xn is negative?
    Last edited by a moderator: Dec 22, 2006
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook