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## Re: polyfit weirdness

 From: Paul Kienzle Subject: Re: polyfit weirdness Date: Wed, 12 Feb 2003 18:49:15 -0500 User-agent: Mozilla/5.0 (Windows; U; Win 9x 4.90; en-US; rv:1.3a) Gecko/20021212

```Miquel Cabanas wrote:

```
```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

```
```Could this be a property of newer versions of LAPACK?

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```