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JohnRudnicki15-Moonstone
15-Moonstone
December 20, 2020
Solved

Jacobian Calculation Crashes for System of ODEs

  • December 20, 2020
  • 1 reply
  • 1975 views

I have a program that works (PPPWorks_12_19_2020) for solving a system of odes using Radau with the Jacobian (I am using this because the solutions have peaks which are slightly better resolved with this choice). I made what I thought was an innocuous change (PPPNoWork_12_19_2020) adding the equation dA=0 so that I could set the value of A in the initial conditions. Now the calculation for the Jacobian grinds away for a while, then crashes MathCad. What did I do wrong? Maybe I now have two many equations d(...)= 0 for setting parameters in the IC? Any suggestions?

    Best answer by Werner_E

    A:=A makes the variable undefined for symbolic calculations.

    If you delete the assignment A:=0, the variable is undefined anyway, so A:=A is not necessary.

    Werner_E_1-1608595154615.png

     

     

    1 reply

    W
    Werner_E25-Diamond I
    25-Diamond I
    December 20, 2020

    Shouldn't you enable the region "A:=A" ?

    J
    JohnRudnicki15-MoonstoneAuthor
    15-Moonstone
    December 21, 2020

    Thanks for the clue, but the problem was that I had already set A:=0 a few lines above and did not remove it when I tried to put A in the initial conditions. It worked still leaving A:=A disabled. BTW do you know why it is necessary to set a:=a, etc. I modified this program from one Alvaro posted a few years ago in reponse to another question i had. I copied this but never understood it.

    W
    Werner_E25-Diamond IAnswer
    25-Diamond I
    December 21, 2020

    A:=A makes the variable undefined for symbolic calculations.

    If you delete the assignment A:=0, the variable is undefined anyway, so A:=A is not necessary.

    Werner_E_1-1608595154615.png

     

     

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