solving differential equation

Discussion in 'C Programming' started by greg.smith, Sep 22, 2003.

  1. greg.smith

    greg.smith Guest

    Hi there,

    I've been trying to solve this different via numerical methods(C
    programming) and it's
    driving me crazy....

    I can't post my code because it's simply one big mess and doesn't work.

    d^2y/dx^2 - 1/(L^2)*y .... where L is just some arbitrary number......L =
    0.2045....

    I cannot figure this out, I was thinking matlab might be easier but I really
    want to learn to do this via the C language. I just want to solve for
    values of Y, and plot them against x.

    Any help is appreciated,

    gs
    greg.smith, Sep 22, 2003
    #1
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  2. greg.smith

    -berlin.de Guest

    greg.smith <> wrote:
    > I've been trying to solve this different via numerical methods(C
    > programming) and it's driving me crazy....


    > I can't post my code because it's simply one big mess and doesn't work.


    > d^2y/dx^2 - 1/(L^2)*y .... where L is just some arbitrary number......L =
    > 0.2045....


    That's not an equation, just a sum of two terms (or do you mean that's
    the left hand side and the other side is 0), and your also not telling
    about the boundary conditions or initial values for y and y' which you
    will need for a numerical solution.

    > I cannot figure this out, I was thinking matlab might be easier but I really
    > want to learn to do this via the C language. I just want to solve for
    > values of Y, and plot them against x.


    You don't show any code and you don't tell what method you try to use
    (there are quite a number of methods of varying quality) so what kind
    of help are you expecting? Have you been looking at "Numerical Recipes
    in C" by Press et al.? While the code in the book is quite horrible,
    chapter 16 and 17 give a useful introduction to numerically solving DEs
    (and more information than you can expect from Usenet postings). You
    can download the chapters from e.g.

    http://www.library.cornell.edu/nr/bookcpdf.html.

    Regards, Jens
    --
    _ _____ _____
    | ||_ _||_ _| -berlin.de
    _ | | | | | |
    | |_| | | | | | http://www.physik.fu-berlin.de/~toerring
    \___/ens|_|homs|_|oerring
    -berlin.de, Sep 22, 2003
    #2
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  3. greg.smith wrote:

    > Hi there,
    >
    > I've been trying to solve this different via numerical methods(C
    > programming) and it's
    > driving me crazy....
    >
    > I can't post my code because it's simply one big mess and doesn't work.
    >
    > d^2y/dx^2 - 1/(L^2)*y .... where L is just some arbitrary number......L =
    > 0.2045....
    >
    > I cannot figure this out, I was thinking matlab might be easier but I really
    > want to learn to do this via the C language. I just want to solve for
    > values of Y, and plot them against x.
    >
    > Any help is appreciated,


    Using D to stand for the second derivative of y with respect ot x,
    whether the form is

    D = 1/(L*L*y)
    or
    D = y/(L*L)

    the answer is trivial as a closed form exercise. When you can do something
    trivially in closed form, it is an *error* to use numerical methods.





    --
    Martin Ambuhl
    Martin Ambuhl, Sep 22, 2003
    #3
  4. greg.smith

    Bigdakine Guest

    >Subject: solving differential equation
    >From: "greg.smith"
    >Date: 9/21/03 6:58 PM Hawaiian Standard Time
    >Message-id: <bpvbb.10880$>
    >
    >Hi there,
    >
    >I've been trying to solve this different via numerical methods(C
    >programming) and it's
    >driving me crazy....
    >
    >I can't post my code because it's simply one big mess and doesn't work.
    >
    >d^2y/dx^2 - 1/(L^2)*y .... where L is just some arbitrary number......L =
    >0.2045....
    >
    >I cannot figure this out, I was thinking matlab might be easier but I really
    >want to learn to do this via the C language. I just want to solve for
    >values of Y, and plot them against x.


    Well you should be able to get the solution to that one with just pencil and
    paper.


    Stuart
    Dr. Stuart A. Weinstein
    Ewa Beach Institute of Tectonics
    "To err is human, but to really foul things up
    requires a creationist"
    Bigdakine, Sep 24, 2003
    #4
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