# Prob with NDSolve in Mathematica

• Mathematica
How can I show the regular part of the solution of a differential equation, numerically solved with NDSolve, if there's a singularity on the curve ?

I know how to use NDSolve and show its solution, but Mathematica gives a bad curve after some point (singularity jumping). I don't want to show this part, just the regular curve BEFORE the singularity (which is occuring at t = %\$&*).

More precisely, the curve function should be strictly positive : a[t] > 0. The NDSolve should stop the resolution if a <= 0. I added the command StoppingTest -> (a[t] < 0.001) or StoppingTest -> (a[t] <= 0) but it doesn't work. I'm still getting wrong curve parts with a[t] < 0.

Any idea ?

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kai_sikorski
Gold Member
Why don't you just solve over the interval you're solving over now, but only plot it over the interval where it's positive?

Duh ! Because I don't know in advance what are the singularities !

There are two singularities on the curve, and I need to plot the regular part between them. There's no way I can know in advance the exact values of the singularities.

phyzguy
Try using Sow and Reap inside the NDSolve, and only Sow when the answer meets your criterion, as in the attached notebook.

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Try using Sow and Reap inside the NDSolve, and only Sow when the answer meets your criterion, as in the attached notebook.
Very good trick ! Thanks a lot for that info !

However, I found the right solution to my problem : I just have to define the x values at which the curve y[x] blows away, like this :

Code:
Xmin := (y /. Curve)[[1]][[1]][[1]][[1]]
Xmax := (y /. Curve)[[1]][[1]][[1]][[2]]
Then, I plot the curve between these two values.

kai_sikorski
Gold Member
I don't see how that's different than what I suggested.. but glad it worked for you

I don't see how that's different than what I suggested.. but glad it worked for you
Actually, it's exactly what you suggested. I just wasn't able to see how to implement it at first.