From help-octave-request at bevo dot che dot wisc dot edu Wed Dec 17 14:12:24 2003 Subject: Octave QR factorization From: Bart Vandewoestyne To: help-octave at bevo dot che dot wisc dot edu Date: Wed, 17 Dec 2003 14:04:21 -0600 Hello Octave-list, While I was checking the correctness of an exercise given to my students during one of my algebra exercise sessions, i found the following curiosity: The purpose of the exercise was for students to calculate the QR factorization of a given matrix A. Now all I wanted to do is enter the factorization we found by hand into Octave and see if the product of Q and R is again the original matrix A. What I found is that when I multiply the matrices Q and R we've found by hand and then check for Q*R==A, Octave tells me that Q*R is not equal to A, most probably due to rounding errors I guess... If I run the same script in Matlab, Matlab *does* give me that Q*R=A... so it doesn't experience the rounding errors as much as Octave does??? So now my question is: why does Matlab gives different results than Octave? Is there a big difference in the implementation of QR between Matlab and Octave? Is there a way to decrease the rounding errors made by Octave so we get the same accuracy as Matlab? The script I used can be found at http://www.cs.kuleuven.ac.be/~bartv/p370_oef1_6_17.m When replying, please also reply to me personally, as I am not subscribed to the list... Thanks, Bart -- Bart Vandewoestyne Bart.Vandewoestyne_at_pandora.be Naamsesteenweg 328 bus 201 GSM: +32 (0)478 397 697 B-3001 Heverlee http://osswin.sourceforge.net ---------------------------------------------------------------------- "Theory is the general; experiments are the soldiers." (Leonardo da Vinci) ------------------------------------------------------------- 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 -------------------------------------------------------------