RE : Re: refining log to ]0; +Inf[

[gro gro]

Well, thanks for the tip!

Have a nice day,
Regards,

Giansolo


Przemek Klosowski <address@hidden> a écrit :
>  
>    I'm currently running into trouble using leasqr function. In fact i'm making a non linear regression on the p vector using the function : 
>    y = (1/p(2))*(log((x-p(3))/EQF) - p(1));
>
>    as p(3) varies trought the leasqr algorithm and may produce negative values (x-p(3)<0), i may recover complex values.
>
> Well, when the argument of the log() is near zero, the function is
> obviously very sensitive to the value of p(3), and introducing large
> discontinuities will throw off the convergence. 
>
> It'll probably be better if you simply limited the range of p(3) to
> the range which guarantees that your objective function y is
> well-defined for all values of your
>  x
>
>   p(3) < min(x)
>
> One way to do that is by variable substitution p(3) -> Xmin-exp(p(3)),
> so that you'll fit:
>   
>    y = (1/p(2))*(log(((x-Xmin)+exp(p(3)))/EQF) - p(1));
>
> your original p(3) can be then calculated from the fitted value.

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