Jacobian Calculation Crashes for System of ODEs

[JohnRudnicki]

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?

[Werner_E]

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

[JohnRudnicki]

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.

[Werner_E · Accepted solution]

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.

[JohnRudnicki]

Thanks. Happy Holidays.