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I am trying to compare an approximation formula for a particular nonlinear ODE with different right hand sides. The code I use is:

sol = NDSolve[{w''[t] + Exp[w[t]] == Sin[t], w[0] == 0, 
w'[0] == 0}, w, {t, 0, 10}, Method->"StifnessSwitching"];
wsol[t_] := Evaluate[w[t] /. sol]
wBlue[s_, t_] := 
 1/s NIntegrate[
   Log[1 - Tanh[(t - t1)/Sqrt[2]]^2] Sin[t1], {t1, 0, t}]

However, NDSolve gives

NDSolve::ndnum: Encountered non-numerical value for a derivative at t == 0.`.

I guess it is because of the right hand side f[t]. Is there any way to get rid of this and plot wsol and wBlue against t?

Thank you for your time.

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    $\begingroup$ You miss typed "StiffnessSwitching" - but you do not need that in the first place. $\endgroup$
    – user21
    Feb 1, 2018 at 11:08
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    $\begingroup$ Thanks! There was a minor typo, that's all. $\endgroup$ Feb 1, 2018 at 11:21

1 Answer 1

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You have to correct the definition f[t]:

f[t_] := Sin[t]

sol = NDSolve[{w''[t] + Exp[w[t]] == f[t], w[0] == 0, w'[0] == 0},w, {t, 0,10}]
(* {{w -> InterpolatingFunction[{{0., 10.}}, <>]}} *)
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  • $\begingroup$ It was a typo in the code. The NDSolve still gives the same error. I tried several built in methods like BDF, MethodOfLines, ExplicitRungeKutta, etc. $\endgroup$ Feb 1, 2018 at 10:32
  • $\begingroup$ Before(!) you execute NDSolve f[t] must be defined. Otherwise NDSolve gives the error NDSolve::ndnum: $\endgroup$ Feb 1, 2018 at 10:38
  • $\begingroup$ Dear Ulrich, I have already fixed that issue. $\endgroup$ Feb 1, 2018 at 10:41
  • $\begingroup$ f[t_] := Sin[t] should be the first line in your code. If so your code run's without error warning! $\endgroup$ Feb 1, 2018 at 10:45
  • $\begingroup$ Can you run it successfully on your machine? I get the same error even when I write Sin[t] directly in the NDSolve. $\endgroup$ Feb 1, 2018 at 10:51

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