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My data is:

d = {{0, 3.889}, {0.04, 3.941}, {0.06, 5.036}, {0.08, 12.589}, {0.1, 16.972}, {0.2, 13.200}, {0.4, 12.063}};

I need to fit a function as: $\log_b a + \log_b [x (1 - x)]$ and obtain the appropriate $a$ and $b$. Also, $0 < x < 1$.

So, I have:

FindFit[d, Log[b, a] + Log[b, x (1 - x)], {a, b}, x]

But I get several errors, such as: "Indeterminate expression 0. ComplexInfinity encountered."

How to overcome these errors?

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    – bbgodfrey
    Commented Nov 9, 2021 at 11:37
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    $\begingroup$ Your data dshould include the x-values! $\endgroup$
    – Ulrich Neumann
    Commented Nov 9, 2021 at 11:49

1 Answer 1

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I changed your model a little bit. First data point cann't be used because of Log[0]

Try

mod = NonlinearModelFit[Rest[d] , {a + b Log[ x (1 - x)], a > 0 }, {a, b }, x,Method -> "NMinimize"] //Quiet
Show[{Plot[mod[x], {x, 0, .4}], ListPlot[d]}]

enter image description here

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  • $\begingroup$ Why did you restrict on logarithmic model? $\endgroup$
    – Ulrich Neumann
    Commented Nov 9, 2021 at 12:42
  • $\begingroup$ You're welcome! $\endgroup$
    – Ulrich Neumann
    Commented Nov 9, 2021 at 13:15
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    $\begingroup$ @UlrichNeumann shouldn't you impose restrictions like $b\neq 1$ and $b>0$? $\endgroup$
    – rhermans
    Commented Nov 9, 2021 at 13:39
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    $\begingroup$ @rhermans The original model from question would require these constraints, the model in my answer doesn't need these constraints. $\endgroup$
    – Ulrich Neumann
    Commented Nov 9, 2021 at 13:43

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