help-octave
[Top][All Lists]

## Re: Problem of polyfit

 From: Tetsuro KURITA Subject: Re: Problem of polyfit Date: Thu, 6 Oct 2005 19:34:51 +0900

```
p = inv(X'*X)*X'*y
= inv(X)*inv(X')*X'*y
= inv(X)*I*y
= inv(X)*y
= X \ y

```
wpolyfit does the same with weighting on y, except that it uses QR decomposition to solve X \ y.
```
In generaly,

inv(a*b) \= inv(b)*inv(a)

```
If both of a and b are nonsingular matrixes, the following equation is right.
```inv(a*b) = inv(b)*inv(a)

Acutually, in some case, polyfit.m give bad result.

=======================================================
Tetsuro KURITA
=======================================================

-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------

```