Solving two simple ordinary differential equations with odesolve. Why the second one fails?

[-MFra-]

Hi all!

Maybe I'm losing in a glass of water, but I can not solve the second differential equation with odesolve, in the problem hereunder exposed. What's wrong?

[vlehner]

Hi F.M.

I'm missing the equation Fy(x) in the solver.

(or is it F0y(x)?) if yes, i got this solution:

is this right?

Best Regards, Volker

[-MFra-]

Thanks Volker,

This is a basic case and I need to check out the results.

In the general case, the analytical solution of the first equation is not known. It is used in the second where the problem arises.

Greetings

Frank

[vlehner]

Hi Frank,

And the answer is sooooooo easy with the choice of Adaptive mode.

You can keep your sheet like it was in the origin and change only to adaptive and it works perfectly.

Viktor is absolutely right.

Man lernt nie aus....

Greetings, Volker

[ViktorKorobov · Accepted solution]

Dear F.M.,

Choose the method for solving the ODE by right-clicking on Odesolve: Adaptive instead AdamsBDF.

Viktor

[-MFra-]

Hi Viktor,

thank you very much.

Greetings

Frank

[ViktorKorobov]

Frank, I was glad to help. But I do not know why Adams/BDF-method not work here.

Viktor

[-MFra-]

This is not clear to me as well. Let's wait and see if there is someone who will tell us about it.

Frank

[Werner_E]

At least I can explain whats going on (but not exactly why its going on 😉

Both odesolve-block should solve for x from 10^-6 up to 4*10^-6.

But the second block calls the first and already  in the 15th try the adamsBDF feeds x=4.34*10^-6 inF.y(x) which makes the first solve block fail because that value is grater than the upper limit 4*10^-6.

Attached are the x-values used by AdamsBDF and Adaptive (the latter is truncated as i did not want to wait for all the writing to finish).

If you trap that error, you can also use AdamsBDF in your second solve block:

[-MFra-]

Thank you very much, Werner.

Franc