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I am trying to fit some experimental data and I am trying to use NMinimize function to calculate the parameters that fit the experimental dataset. I am also using ParametricNDSolve and I can't make it to work together.

This is a toy example:

sol0 = ParametricNDSolve[{x'[t] == k*x[t], x[0] == x0}, {x[t]}, {t, 0, 100}, {k, x0}]

So I have to find k and x0 that best fit the exp data

The data is:

data = RandomReal[{0.95 #,  1.05 #}] & /@ ((x[t][1, 10] /. sol0 /. t -> #) & /@ Range[0, 10])
{9.59679, 27.9723, 73.9852, 190.905, 534.717, 1421.69, 4178.41, 11296.8, 30648.2, 79970.6, 229304.}

I create an objective function:

objfn[k_, x0_] := 
 Norm[((  x[t][k, x0] /. sol0 /. t -> #  ) & /@ 
     Table[i, {i, 0, 10}]) - data]

Now I try to find the paramters:

NMinimize[{objfn[k, x0], k > 0, x0 > 0}, {k, x0}]

I've got a relative long error message that starts with

NMinimize::nnum.

Unfortunately, I can't figure out the changes that I have to introduce to make this work. I would really appreciate if any could help me.

Thank you very much in advance Fernando

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    Commented Sep 3, 2015 at 11:10
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    – rhermans
    Commented Sep 3, 2015 at 12:39
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    $\begingroup$ @rhermans I noticed that you have started using the welcome comment above, and I agree with your recommendations overall, especially with the one regarding waiting a while to accept an answer. I wonder, though: should one really wait as long as 24 hours before voting up deserving answers? $\endgroup$
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    – rhermans
    Commented Sep 3, 2015 at 16:23

1 Answer 1

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Notice the details of the error:

NMinimize::nnum: The function value [...] is not a number at {k,x0} = {1.91862,1.66351}. >>

You need to be sure that objfn doesn't evaluate until is given a Real value as argument. For that, change objfn[k_, x0_] to objfn[k_Real, x0_Real].

You can also avoid the substitution Rule and evaluate the functions directly.

objfn[k_Real, x0_Real] := 
 Norm[Table[sol0[[1, 2]][k, x0], {t, 0, 10}] - data]

Now this definition of objfn can be used with NMinimize

NMinimize[{objfn[k, x0], k > 0, x0 > 0}, {k, x0}]
{527.089, {k -> 0.990979, x0 -> 10.6917}}
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  • $\begingroup$ Hi @rhermans, Many thanks for the prompt answer. $\endgroup$
    – Ferran
    Commented Sep 3, 2015 at 12:48
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    $\begingroup$ My pleasure! You can return the favor, As you receive help you can give back. Stay in the community, earn reputation and badges, learn and share what you have learned. $\endgroup$
    – rhermans
    Commented Sep 3, 2015 at 12:54

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