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Prob with NDSolve in Mathematica 
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#1
Feb1412, 09:28 AM

P: 61

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 ? 


#2
Feb1412, 11:13 AM

PF Gold
P: 162

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?



#3
Feb1412, 12:25 PM

P: 61

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. 


#4
Feb1412, 12:40 PM

P: 2,195

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



#5
Feb1412, 12:56 PM

P: 61

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 :



#6
Feb1412, 03:27 PM

PF Gold
P: 162

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



#7
Feb1412, 04:27 PM

P: 61




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