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Thread: Ivpag

  1. #1
    Junior Member
    Join Date
    Aug 2012
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    2

    Ivpag

    Hello people

    I'm using IVPAG to solve quite a large system of stiff ODEs and thus am using Gear's method. In the subroutine that is called by IVPAG I am calculating a derivative of a continuously increasing function, thus the derivative should always be positive. However, IVPAG gives me non-physical results, i.e. sometimes the derivative becomes negative. The thing is, that the derivative I am calculating is a function of a variable that is integrated by IVPAG. I am using the derivative in other subroutines of my code, and if I get wrong signs of the derivative, I get an error that propagates and messes up the results.

    I know this is a vague description, but I don't want to bother you with details. My question is, how does IVPAG proceed when integrating stiff problems? I suspect that when an abrupt change occurs (discontinuity), IVPAG calculates 'too far' (i.e. the error increases) and then goes back and reduces the step size.

    I'd be happy with any suggestions!

    Btw, I checked most of the options that can be adjusted in IVPAG, but up to now nothing I tried seemed to do the trick.

    Thx,
    riehen
    Last edited by riehen; 08-24-2012 at 06:37 AM.

  2. #2
    Senior Member
    Join Date
    Sep 2005
    Location
    Albuquerque, NM USA
    Posts
    140
    Without seeing the behavior with numerical results it is difficult to suggest what might solve your problem. But for your sake please do not use IVPAG any longer. The code DAESL will integrate stiff systems and has many other features that can be brought up as opportunities and options. This IVPAG code is mainly regarded as of historical interest.So start over with DAESL and let us begin from there. If you do not have DAESL in the library version that you use, switch to version 7.
    Dickie T. Bird

  3. #3
    Junior Member
    Join Date
    Aug 2012
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    2
    Richard,
    thx a lot for your suggestion using DAESL. After a quick look at the options available for DAESL, Im optimistic that it will do the trick. Ill give it a shot after changing to IMSL 7.
    cheers

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