Re: polyfit weirdness

[Miquel Cabanas]

hi,

On Tue, Feb 11, 2003 at 02:08:37AM -0600, Scott Lamb wrote:
> 
> So first I'm calling polyfit to get the trendline:
> 
>     l1 = 1e-9 * [ 578.45 545.88 435.87 404.71 365.26 ]
>     v1 =        [   0.66   0.79   1.40   1.58   1.92 ]
>     f1 = 299792458 ./ l1
>     [p1,cv1] = polyfit(f1,v1,1)
> 
> ...and it doesn't return what I'd expect at all:

I think we can narrow the problem to be related to the
left-division in polyfit, could you try running (this is
what polyfit does),

   n = 1;
   l = 5;

   x = [ 5.1827e+14  5.4919e+14  6.8780e+14  7.4076e+14 ...
   8.2076e+14 ]';

   y = [ 0.66000  0.79000  1.40000  1.58000  1.92000 ]';

   X = (x * ones (1, n+1)) .^ (ones (l, 1) * (0 : n));
   p = X \ y;
   yf = X * p;

In my working Octave I get

   p =

      -1.4948e+00
       4.1680e-15
       
and (estimated y-values)

   yf =

     0.66529
     0.79416
     1.37188
     1.59262
     1.92605

Also, what are eps, realmax and realmin in your versions? Again,
mines are,

   eps =  2.2204e-16
   realmax =  1.7977e+308
   realmin =  2.2251e-308


Miquel

-- 
Miquel E Cabanas ------------------------------------------------------
SeRMN, Universitat Autonoma de Barcelona (address@hidden)
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