NonlinearRegress and numerical function

• To: mathgroup at smc.vnet.net
• Subject: [mg20162] NonlinearRegress and numerical function
• From: Fredrik <fredrik at ic.chalmers.se>
• Date: Sun, 3 Oct 1999 21:07:33 -0400
• Organization: Chalmers University of Technology
• Sender: owner-wri-mathgroup at wolfram.com

```Hi

I've got a big problem making Mathematica realize that my function is
numerical. The following problem illustrates it in a rather simplified
way:

g[t1_] := y[t] /. DSolve[{y'[t] == -y[t] 3, y[0] == 6}, y[t], t][[1]] /.

t -> t1

data = {0, 10, 20, 100};

fit = Transpose[{data, g[data]}];

f[t1_, a_,b_] := (y[t] /.NDSolve[{y'[t] == -y[t] a, y[0] == b}, y[t],
{t, 0, 100}][[1]]) /.t -> t1

NonlinearRegress[data, f[t, a, 6], {t}, {a, {2.9, 3.2}, 2, 4}]

It should be  rather straightforward to solve this, but NonlinearRegress

seem to evaluate the function 'f' first and the results:

NDSolve::"ndnum": "Encountered non-numerical value for a derivative at \

\!\(t\) == \!\(2.680466916142905`*^-274\)."

etc....

I've tried to use Unevaluated and Hold but nothing works!

Any help would be greatly appreciated!

Regards

_________________________________________________
loke at ic.chalmers.se
Phone: +46 (0)31 7721867
Fax:   +46 (0)31 7723622
Chalmers University of Technology
Solid State Electronics Laboratory
Department of Microelectronics ED
S-412 96 Guteborg, SWEDEN
_________________________________________________

```

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