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MathGroup Archive 2007

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recording singularities of NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg80500] recording singularities of NDSolve
  • From: "Judith Bunder" <bunder at phys.cts.nthu.edu.tw>
  • Date: Thu, 23 Aug 2007 06:23:37 -0400 (EDT)

Hi,

I have to solve a system of simultaneous equations and I have to solve them
several times with varying initial conditions. I use something like this:

Do[{c1 = i/imax, c2 = j /jmax,
    sol[i, j] = First@NDSolve[eqns, coup, {t, 0,
     100}]}, {j, 1, 5}, {i, 1, 5}]

So, I solve the the problem 25 times with 5 different j values and 5 different
i values. The variables c1 and c2 are the variable initial conditions. I have
no problems with this part. 

My problem is, sometimes the solution diverges. This is good, I'm looking for
divergent points. But, I want some method of recording where the solution
diverges. I get the warning messages:

NDSolve::ndsz At t=18.28484703670573` step size is effectively zero;
singularity or stiff system suspected. 

so I could manually record this value of t. But, this isn't practical as
several of my 25 solutions are usually divergent (and I also need to record
the value of i and j). Is there any way to define something like t-div[i,j]
which will record all these points where my solution diverges?

Thanks in advance,
Judy  


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